CHAPTER XVIII--ADAPTATIONS

General discussion; adaptations, accommodations, and adjustments;

chromatic adaptations; morphological adaptations; accommodations;

concluding statements 249-258

INDEX 259-268

INTRODUCTION

The history of biological science shows that the conceptions which men have

held concerning the nature of plant and animal growth have undergone a

series of revolutionary changes as the technique of, and facilities for,

scientific study have developed and improved. For a long time, it was

thought that life processes were essentially different in character than

those which take place in inanimate matter, and that the physical sciences

had nothing to do with living changes. Then, too, earlier students had only

vague notions of the actual structure of a living organism. Beginning with

the earliest idea that a plant or an animal exists as a unit organism, to

be studied as such, biological science progressed, first to the recognition

and study of the individual organs which are contained within the organism;

then to the tissues which make up these organs; then (with the coming into

use of the microscope as an aid to these investigations) to the cells of

which the tissues are composed; then to the protoplasm which constitutes

the cell contents; and finally to the doctrine of organic evolution as the

explanation of the genealogy of plants and animals, and the study of the

relation of the principles of the physical sciences to the evolutionary

process. The ultimate material into which organisms are resolved by this

process of biological analysis is the cell protoplasm. But protoplasm is

itself made up of a complex system of definite chemical compounds, which

react and interact according to the laws of physical science. Hence, any

study of the chemistry of plant growth is essentially a study of the

chemical and physical changes which take place in the cell protoplasm.

Protoplasm differs from non-living matter in three respects. These are (1)

its chemical composition; (2) its power of waste and repair and of growth;

and (3) its reproductive power. From the standpoint of chemical

composition, protoplasm is the most complex material in the universe. It

not only contains a greater variety of chemical elements, united into

molecules of enormous size and complexity, but also a greater variety of

definite chemical compounds than exist in any other known mixture, either

mineral or organic in type. One of the first problems in the study of

protoplasm is, therefore, to bring this great variety of complex compounds

into some orderly classification and to become familiar with their

compositions and properties. Again, living matter is continually undergoing

a process of breaking down as a result of its energetic activities and of

simultaneously making good this loss by the manufacture of new protoplasm

out of simple food materials. It also has the power of growth by the

production of surplus protoplasm which fills new cells, which in turn

produce new tissues and so increase the size and weight of individual

organs and of the organism as a whole. Hence, a second field of study

includes the chemical changes whereby new protoplasm and new

tissue-building material are elaborated. Finally, living material not only

repairs its own waste and produces new material of like character to it,

but it also produces new masses of living matter, which when detached from

the parent mass, eventually begin a separate existence and growth.

Furthermore, the plant organism has acquired, by the process of evolution,

the ability not only to produce an embryo for a successive generation but

also to store up, in the tissues adjacent to it, reserve food material for

the use of the young seedling until it shall have developed the ability to

absorb and make use of its own external sources of food material. So that,

finally, every study of plant chemistry must take into consideration the

stored food material and the germinative process whereby this becomes

available to the new organism of the next generation. Also, the chemistry

of fertilization of the ovum, so that a new embryo will be produced, and

the other stimuli which serve to induce the growth phenomena, must be

brought under observation and study.

A further step in the development of biological science has been to

separate the study of living things into the two sciences of botany and

zoology. From the standpoint of the chemistry of the processes involved

this segregation is unfortunate. It has resulted in the devotion of most of

the study which has been given to life processes and living things to

animal chemistry, or "physiological chemistry." As a consequence,

biochemistry, which deals with the living processes of both plants and

animals, is yet in its infancy; while phytochemistry is almost a new

science, yet its relation to the study of plants can scarcely be less

vital than is that of physiological chemistry to studies of animal life.

The common conception that plant life and animal life are antithetical or

complementary to each other has much to justify it. Animals breathe in

oxygen and exhale carbon dioxide; while plants use the carbon dioxide of

the air as a part of the raw material for photosynthesis and exhale oxygen.

Plants absorb simple gases and mineral compounds as raw food materials and

build these up into complex carbohydrates, proteins, fats, etc.; while

animals use these complex compounds of plant origin as food, transforming

parts of them into various other forms of structural material, but in the

end breaking them down again into the simple gases and mineral compounds,

which are expelled from the body through the excretory organs. Thus it

would seem that the study of the chemistry of plant life and of animal life

must necessarily deal with opposite types of phenomena.

But one cannot advance far into the study of the biochemistry of plants and

animals before he discovers marked similarities in the chemical principles

involved. Many of the compounds are identical in structure, undergo similar

changes, and are acted upon by similar catalysts. Plant cells exhibit

respiratory activities, using oxygen and giving off carbon dioxide, in

exactly the same way that animal organisms do. The constructive

photosynthetic processes of green plants are regulated and controlled by a

pigment, chlorophyll, which is almost identical with the blood pigment,

hæmatin, which regulates the vital activities in the animal organism,

differing from the latter only in the mineral element which links the

characteristic structural units together in the molecule. Many other points

of similarity in the chemistry of the life processes of plants and animals

will become apparent as the study progresses. It is sufficient now to call

attention to the fact that these vital processes, in either plants or

animals, are essentially chemical in character, and subject to study by the

usual methods of biochemical investigations.

The protoplasm of the cell is the laboratory in which all the changes which

constitute the vital activities of the plant take place. All of the

processes which constitute these activities--assimilation, translocation,

metabolism, and respiration--involve definite chemical changes. In so far

as it is possible to study each of these activities independently of the

others, they have been found to obey the ordinary laws of chemical

reactions. Thus, the effect of the variations in intensity of light upon

photosynthesis causes increase in the rate of this activity which may be

represented by the ordinary responses of reaction velocities to external

stimuli. Similarly, the effect of rises in temperature upon the rate of

assimilation and upon respiration are precisely the same as their effect

upon the velocity of any ordinary chemical reaction. Within certain

definite ranges of temperature, the same statement holds true with

reference to the rate of growth of the plant, although the range of

temperature within which protoplasm lives and maintains its delicate

adjustment to the four vital processes of life is limited; beyond a certain

point, further rise in temperature does not produce more growth but rather

throws the protoplasmic adjustment out of balance and growth either slows

up markedly or stops altogether.

Hence, we may say that the methods by which the plant machine (protoplasm)

accomplishes its results are essentially and definitely chemical in

character and may be studied purely from the standpoint of chemical

reactions, but the maintenance of the machine itself in proper working

order is a vital phenomenon which is largely dependent upon the external

environmental conditions under which the plant exists. A study of the

phenomena resulting from the colloidal condition of matter is throwing a

flood of light upon the mechanism by which protoplasm accomplishes its

control of vital activities. But we are, as yet, a long way from a complete

understanding of how colloidal protoplasm acquires and maintains its unique

ability of self-regulation of the conditions necessary to preserve its

colloidal properties and of how it elaborates the enzymes which control the

velocity of the chemical reactions which take place within the protoplasm

itself and which constitute the various processes of vital activity.

The object of this study of the chemistry of plant growth is to acquire a

knowledge of the constitution of the compounds involved and of the

conditions under which they will undergo the chemical changes which, taken

all together, constitute the vital processes of cell protoplasm.

CHEMISTRY OF PLANT LIFE

CHAPTER I

PLANT NUTRIENTS

There is some confusion in the use of the terms "nutrient," "plant food,"

etc., as applied to the nutrition and growth of plants. Strictly speaking,

these terms ought probably to be limited in their application to the

organized compounds within the plant which it uses as sources of energy and

of metabolizable material for the development of new cells and organs

during its growth. Botanists quite commonly use the terms in this way. But

students of the problems involved in the relation of soil elements to the

growth of plants, including such practical questions as are involved in the

maintenance of soil productivity and the use of commercial fertilizers for

the growing of economic plants, or crops, are accustomed to use the terms

"plant foods," or "mineral nutrients," to designate the chemical elements

and simple gaseous compounds which are supplied to the plant as the raw

material from which its food and tissue-building materials are synthetized.

Common usage limits these terms to the soil elements; but there is no

logical reason for segregating the raw materials derived from the soil from

those derived from the atmosphere.

The essential difference between these raw materials for plant syntheses

and the organic compounds which are produced within the plants and used by

them, and by animals, as food, is that the former are inorganic and can

furnish only materials but no energy to the organism; while the latter are

organic and supply both materials and potential energy. It would probably

be the best practice to confine the use of the word "food" to materials of

the latter type, and several attempts have been made to limit its use in

this way and to apply some such term as "intake" to the simple raw

materials which are taken into the organism and utilized by it in its

synthetic processes. But the custom of using the words "food," or

"nutrient," to represent anything that is taken into the organism and in

any way utilized by it for its nourishment has been followed so long and

the newer terms are themselves so subject to criticism that they have not

yet generally supplanted the loosely used word "food."

If such use is permitted, however, it is necessary to recognize that only

the green parts of green plants can use this inorganic "food," and that the

colorless plants must have organic food.

To avoid this confusion, the suggestion has recently been made that all of

the intake of plants and animals shall be considered as food, but that

those forms which supply both materials and potential energy to the

organism shall be designated as _synergic foods_, while those which contain

no potential energy shall be known as _anergic foods_. On this basis,

practically all of the food of animals, excepting the mineral salts and

water, and all of the organic compounds which are synthetized by plants and

later used by them for further metabolic changes, are synergic foods; while

practically all of the intake of green plants is anergic food.

It is with the latter type of food materials that this chapter is to deal;

while the following and all subsequent chapters deal with the organic

compounds which are synthetized by plants and contain potential energy and

are, therefore, capable of use as synergic food by either the plants

themselves or by animals. It will be understood, therefore, that in this

chapter the word "food" is used to mean the anergic food materials which

are taken into and used by green plants as the raw materials for the

synthesis of organic compounds, with the aid of solar energy, or that of

previously produced synergic foods. In all later chapters, the term "food"

will be used to mean the organic compounds which serve as the synergic food

for the green parts of green plants and as the sole supply of nutrient

material for the colorless parts of green plants and for parasitic or

saprophytic forms (see page 16).

PLANT FOOD ELEMENTS

The raw materials from which the food and tissue-building compounds of

plants are synthetized include carbon dioxide, oxygen, water, nitrogen,

phosphorus, sulfur, potassium, calcium, magnesium, and iron. The two gases

first mentioned are derived directly from the air, through the respiratory

organs of the plant. Water is taken into the plant chiefly from the soil,

through its fibrous roots. All the other elements in the list are taken

from the soil, nitrogen being derived from decaying organic matter (the

original source of the nitrogen is, however, the atmosphere, from which the

initial supply of nitrogen is obtained by direct assimilation by certain

bacteria and perhaps other low forms of plant life), and the remaining ones

from the mineral compounds of the soil.

Carbon dioxide and oxygen, being derived from the air, are always available

to the leaves and stems of growing plants in unlimited supply; but the

supply available to a seed when germinating in the soil, or to the roots of

a growing farm crop, may sometimes become inadequate, especially in soils

of a very compact texture, or "water-logged" soils. In such cases, the

deficiency of these gaseous food elements may become a limiting factor in

plant growth.

Water is often a limiting factor in plant growth. Experiments which have

been repeated many times and under widely varying conditions show that when

water is supplied to a plant in varying amounts, by increasing the

percentage of water in the soil in which the plant is growing by regular

increments up to the saturation point, the growth of the plant, or yield of

the crop, increases up to a certain point and then falls off because the

excess of water reduces the supply of air which is available to the plant

roots. Hence, abundance of water is, in general, a most essential factor in

plant growth.

Under normal conditions of air and moisture supply, however, the plant food

elements which may be considered to be the limiting factors in the

nutrition and growth of plants are the chemical elements mentioned in the

list above.

AVAILABLE AND UNAVAILABLE FORMS

The plant food materials which are taken from the soil by a growing plant

must enter it by osmosis through the semi-permeable membranes which

constitute the epidermis of the root-hairs, and circulate through the plant

either carried in solution in the sap or by osmosis from cell to cell.

Hence, they must be in water-soluble form before they can be utilized by

plants. Obviously, therefore, only those compounds of these elements in the

soil which are soluble in the soil water are _available_ as plant food. The

greater proportion of the soil elements are present there in the form of

compounds which are so slightly soluble in water as to be _unavailable_ to

plants. The processes by which these practically insoluble compounds become

gradually changed into soluble forms are chiefly the "weathering" action of

air and water (particularly if the latter contains carbonic acid) and the

action of the organic acids resulting from decaying animal or vegetable

matter or secreted by living plants.

THE VALUE OF THE SOIL ELEMENTS AS PLANT FOOD

Analyses of the tissues of plants show that they contain all of the

elements that are to be found in the soil on which they grew. Any of these

elements which are present in the soil in soluble form are carried into the

plants with the soil water in which they are dissolved, whether they are

needed by the plant for its nutrition or not. But in the case of those

elements which are not taken out of the sap to be used by the plant cells

in their activities, the total amount taken from the soil is much less than

is that of the elements which are used in the synthetic processes of the

plant. Hence, much larger proportions of some elements than of others are

taken from the soil by plants. The proportions of the different elements

which are used by plants as raw materials for the manufacture of the

products needed for their growth varies with the different species; but a

certain amount of each of the so-called "essential elements" (see below) is

necessary to every plant, because each such element has a definite rôle

which it performs in the plant's growth. A plant cannot grow to maturity

unless a sufficient supply of each essential element comes to it from the

soil.

From the standpoint of their relative value as raw materials for plant

food, the elements which are present in the soil may be divided into three

classes; namely, the _non-essential_, the _essential and abundant_, and the

_critical_ elements.

The first class includes silicon, aluminium, sodium, manganese, and certain

other rarer elements which sometimes are found in soils of some special

type, or unusual origin. These elements seem to have no rôle to play in

the nutrition of plants; although silicon is always present in plant ash

and sodium salts are found in small quantities in all parts of practically

all plants. Nearly all species of plants can be grown to full maturity in

the entire absence of these elements from their culture medium. Occasional

exceptions to this statement in the case of special types of plants are

known, and are of interest in special studies of plant adaptations, but

need not be considered here.

The second group includes iron, calcium, magnesium, and, generally, sulfur.

All of these elements are essential for plant growth, but are usually

present in the soil in ample quantities to insure a sufficient supply in

available form for all plant needs. Recent investigations have shown,

however, that there are many soils in which sulfur is present in such

limited quantities that many agricultural crops, when grown on these soils,

respond favorably to the application of sulfur-containing fertilizers. In

such cases, sulfur is a "critical" element.

The "critical" elements are those which are essential to the growth of all

plants and which are present in most soils in relatively small proportions

and any one may, therefore, be the limiting factor in plant growth so far

as plant food is concerned. These are nitrogen, phosphorus, potassium, and

(possibly) sulfur.

RÔLE OF PLANT FOOD ELEMENTS IN PLANT GROWTH

The use which a plant makes of the elements which come to it from the soil

has been studied with great persistency and care by many plant

physiologists and chemists. Many of the reactions which take place in a

plant cell are extremely complicated, and the relation of the different

chemical elements to these is not easily ascertained. It is probable that

the same element may play a somewhat different rôle in different species of

plants, in different organs of the same plant, or at different stages of

the plant's development. But the usual and most important offices of each

element are now fairly well understood, and are briefly summarized in the

following paragraphs. It should be understood that a thorough and detailed

discussion of these matters, such as would be included in an advanced study

of plant nutrition, would reveal other functions than those which are

presented here and would require a more careful and more exact method of

statement than is suitable here. However, the general principles of the

utilization of soil elements by plants for their nutrition and growth may

be fairly well understood from the following statements.

=Nitrogen= is a constituent of all proteins (see Chapter XIII). Proteins

are apparently the active chemical components of protoplasm. Since it is in

the protoplasm of the green portions, usually foliage, of plants that the

photosynthesis of carbohydrates and the synthesis of most, or all, of the

other tissue-building materials and reserve food substances of the plant

takes place, the importance of nitrogen as a plant food can hardly be

over-emphasized. Nitrogen starvation produces marked changes in the growth

of a plant. Leaves are stunted in growth and a marked yellowing of the

entire foliage takes place; in fact, the whole plant takes on a stunted or

starved appearance. Abundance of nitrogen, on the other hand, produces a

rank growth of foliage of a deep rich color and a luxuriant development of

tissue, and retards the ripening process. In the early stages of growth,

the nitrogen is present most largely in the leaves; but when the seeds

develop, rapid translocation of protein material into the seeds takes

place, until finally a large proportion of the total supply is deposited in

them.

Nitrates are the normal form of nitrogen in the soil which is available to

plants. During germination and early growth, the young seedling uses

amino-acids, etc., derived from the proteins stored in the seed, as its

source of nitrogen; and experiments have shown that similar forms of

soluble organic nitrogen compounds can be successfully fed to the seedling

as an external food supply. Soluble ammonium salts can be utilized as

sources of nitrogen by most plants during later periods of growth,

particularly by the legumes. But for most, if not all, of the common farm

crops whose possibilities in these respects have been studied, it has been

found that a unit of nitrogen taken up as a nitrate is very much more

effective in promoting growth, etc., than is the same unit of nitrogen in

the form of ammonium salts.

While the proteins are finally stored up largely in the seeds, or other

storage organs, they are actively at work during the growing period in the

cells of the foliage parts of the plant. Hence, the popular statement that

"nitrogen makes foliage" is a fairly accurate expression of its rôle.

Inordinate production of straw in cereal crops and of leaves in root crops

often results from liberal supplies of available nitrogen in the soil early

in the growing season. If the crops develop to normal maturity, this

excessive foliage growth has no harmful results, as the surplus material

which has been elaborated is properly translocated into the desired storage

organs; but, unfortunately, the retarding effect of the surplus nitrogen

supply upon the date of maturing of the crop is often associated with

premature ripening of the plants from other causes, with the consequence

that too large a proportion of the valuable food material is left in the

refuse foliage material of the crop. Crops which are grown solely for their

leaves, such as hay crops, lettuce, cabbage, etc., profit greatly by

abundant supplies of available nitrogen; although when foliage growth is

stimulated in this way the tissue is likely to be thin-walled and soft

rather than firm and solid.

=Phosphorus= is likewise an extremely important element in plant nutrition.

But phosphorus starvation produces no such striking visible effects upon

the growth of the plant as does lack of nitrogen. Abundance of available

phosphorus early in the plant's life greatly stimulates root growth, and

later on it undoubtedly hastens the ripening process; hence, this element

seems to act as the exact antithesis of nitrogen.

The rôle of phosphorus, or of phosphates, in the physiological processes of

the cell seems to be difficult to discover. The element itself is a

constituent of some protein complexes and of the lecithin-like bodies (see

page 141) which are supposed by some investigators to play an important

part in determining the rate of chemical changes which take place in the

cell and the movement of materials into and out of it. It is an essential

constituent of the nucleus, and a meager supply of phosphorus retards, or

inhibits, mitotic cell-division. Photosynthesis of sugars and the

condensing of these into starch or cellulose takes place in plants in the

absence of available phosphorus; but the change of these insoluble

carbohydrates back again into soluble and available sugar foods does not.

Phosphorus is taken from the soil by plants in the form of phosphates. Much

study has been given to the problem of the proper supply of available soil

phosphates for economic crop production. Any discussion of soil fertility

and fertilization which did not devote large attention to the conditions

under which phosphates become available as plant food would be wholly

inadequate; but such a discussion would be out of place here.

The final result of an ample supply of phosphates in hastening the ripening

process and stimulating seed production, as contrasted with that of an

over-supply of nitrogen, has led to the popular statement that "phosphates

make seeds." This statement, while not strictly accurate, is a fairly good

summary of the combined results of the rôle of phosphorus in the plant

economy. Large amounts of phosphorus are stored in the seeds. The two facts

that large amounts of these compounds are thus available to the young

seedling and that relatively large proportions of phosphates are taken from

the soil by the plant during its early stages of growth are undoubtedly

connected with the need for rapid cell-division at these periods in the

plant's life.

=Potassium.=--The popular expression that "potash makes sugars and starch"

is a surprisingly accurate description of the rôle of this element in plant

metabolism. Either the photosynthesis of starch, or the changes necessary

to its translocation (it is not yet certain which) is so dependent upon the

presence of potassium in the cell sap that the whole process stops at once

if an insufficient supply is present. The production and storage of sugar,

or starch, in such root crops as beets, potatoes, etc., diminishes in

direct proportion with a decreasing supply of potassium as plant food. The

grains of the cereal crops become shrunken as a result of potassium

starvation; and are plump and well filled with starch in the endosperm when

sufficient potassium is available for the crop's needs.

The general tone and vigor of growth of the plant is largely dependent upon

an ample potassium supply; potash-hungry plants, like those which have been

weakened by any other unfavorable conditions, have been found to be more

susceptible to injury by disease, than those which are well nourished with

this food element. But potassium-starvation does not produce any

pathological condition of the cell contents; its absence simply prevents

the possibility of the development of the necessary carbohydrates for

vigorous growth.

There is no known difference in the availability, or effectiveness, of

potassium from the different forms of compounds containing it which may be

present in the soil. Apparently, the only essential is that the compound

shall be soluble so that it can be absorbed into the plant through the

root-hairs. Of course, the acid radical to which the basic potassium ion is

attached may, in itself, have some beneficial or deleterious influence

which gives to the compound as a whole some important effect in one case,

which might not follow in the case of another type of compound; but the

relative efficiency as plant food of a given unit of potassium seems to be

the same regardless of the nature of the compound in which it is present.

=Calcium= is an essential plant food element but its physiological use has

not yet been definitely established. It seems to stimulate

root-development, and certainly gives vigor and tone to the whole plant. It

is commonly believed that calcium is in some way connected with the

development of cell-wall material. It has been reported that the stems of

grasses and cereal plants become stiffer in the presence of ample calcium,

but this may be due to greater turgidity rather than to strengthened

cell-walls. Calcium remains in the leaves or stem as the plant ripens, but

it is not clear that this has anything to do with the stiffness or weakness

of the stem, or straw, of the plant. Experiments with algæ have shown that

in the absence of calcium salts mitotic cell division takes place, showing

that the nucleus functions properly, but the formation of the new

transverse cell-wall is retarded. This is the only direct evidence that has

been reported that calcium has any connection with cell-wall formation.

Certain species of plants, notably many legumes, require such large amounts

of calcium salts for their growth as to give to them the popular

appellation of "lime-loving plants." Other plants, known as "calciphiles,"

while not actually showing abnormally large percentages of calcium in their

ash, flourish best on soils rich in lime. On the other hand, certain other

species, known as "calcifuges," will not grow on soils which are even

moderately rich in lime; in what respect these differ in their vital

processes from others which demand large amounts of calcium, or those which

flourish on soils rich in lime, has not been determined, however.

The beneficial effect of alkaline calcium compounds in the soil, in

correcting injurious acidity, in improving the texture of clay soils, and

in promoting the proper conditions for bacterial growth, is well known; but

this has no direct connection with the rôle of calcium as plant food.

Furthermore, calcium salts in the soil have a powerful influence in

overcoming the harmful, or toxic, effects of excessive amounts of soluble

salts of magnesium, sodium, or potassium, in the so-called "alkali soils"

(i.e., those which contain excessive amounts of water-soluble salts). The

probable explanation for this fact is pointed out in a later paragraph of

this chapter (see page 14); but this property of calcium probably has no

connection with its physiological uses as plant food.

=Magnesium=, like phosphorus, is finally stored up mostly in the seeds, not

remaining in the leaves and stems, as do calcium and potassium. This fact,

together with other evidence obtained from experiments in growing plants in

culture solutions containing varying amounts of this element, has led

certain investigators to the conclusion that the rôle of magnesium is to

aid in the transport of phosphorus, particularly from older to more rapidly

growing parts of the plant. More recent investigations have shown, however,

that magnesium has other rôles which are probably more specific and more

important than this one. It is now known that magnesium is a definite

constituent of the chlorophyll molecule serving, as will be shown (see

Chapter VIII), as the means of linkage between its essential component

organic groups. Because of this fact, magnesium-starvation produces

etiolated plants, which cannot function normally. Further, magnesium seems

to be necessary for the formation of fats, apparently standing in a similar

relation to fat-formation to that of potassium to carbohydrate-formation.

This view is supported by the observations that when algæ are grown in

magnesium-free solutions they contain no fat globules and that oily seeds

are richer in magnesium than are those which store up starch as their

reserve food material. Observers of the second of these phenomena

have failed to note, however, that oily seeds are likewise richer in

phosphorus than are starchy ones, and that the presence of larger

proportions of magnesium in such seeds may, perhaps, be related to

phosphorus-translocation rather than to fat-formation.

Whatever relation magnesium may have to fat-formation, or to the

translocation of phosphorus, it is evident that these are rôles quite apart

from its use as a constituent element in chlorophyll. As yet, no

explanation of how it aids in these other synthetic processes has been

advanced.

On the other hand, an excess of soluble magnesium salts in the soil

produces definite toxic effects upon plants, magnesium compounds being

known to be among the most destructive of the "alkali soil" salts. Calcium

salts are remarkably efficient in overcoming these harmful effects of

magnesium salts. On this account, a large amount of experimental study has

been given to the question of the calcium-magnesium ratio in plants.

Numerous analyses of plant ashes have established the fact that there is a

fairly definite ratio of this kind, which ratio, however, varies with the

species of plant and is not correlated with the ratio of these elements

present in the soil on which the plant grows, as was formerly believed.

Cereal plants, as a rule, contain approximately twice as much lime as

magnesia; while leafy plants (tobacco, cabbage, etc.) usually contain about

four times as much calcium oxide as magnesium oxide.

Iron is essential to chlorophyll-formation. It is not a constituent of the

chlorophyll molecule, as is magnesium; but in the absence of iron from the

culture solution, a plant fails to produce chlorophyll and a green plant

which is deprived of a supply of iron rapidly becomes etiolated. The way in

which iron is related to chlorophyll-formation is not known.

Iron is taken from the soil by plants in the smallest proportions of any of

the essential elements. Only soluble _ferric_ compounds seem to serve as a

suitable source of supply of the element; _ferrous_ compounds being usually

highly toxic to plants.

=Sulfur= is an essential element of plant food. The amounts required by

plants were supposed, until recently, to be relatively small. This was due

to the fact that earlier studies took account only of the sulfur which, on

analysis, appeared as sulfates in the ash. Improved methods of analysis,

which insure that the sulfur which is present in the plant tissue in

organic combinations is oxidized under such conditions that it is not lost

by volatilization during the combustion of the material, have shown that

the total sulfur which is present in many plants approaches the quantity of

phosphorus which is present in the same tissue. Furthermore, recent field

and pot experiments have shown that at least a considerable part of the

beneficial effects of many fertilizers, which has previously been

attributed to the calcium, potassium, or phosphorus which they contain, is

actually due to the sulfur present as sulfates in the fertilizers used.

Sulfur occurs in the organic compounds of plants, associated with

phosphorus. It seems probable that its physiological uses are similar to

those of the latter element; but there is as yet no experimental evidence

to establish its exact rôle in the economy of plant growth. It appears to

be needed in largest proportion by plants which contain high percentages of

nitrogen in their foliage, such as the legumes. There is some evidence that

sulfur has a particular rôle in promoting the growth of bacteria, and it

may be that the percentages of total sulfur which are found in the tissues

of legumes are due to the presence of the symbiotic nitrogen-gathering

bacteria in the nodules on the roots of these plants. This point has not

yet been investigated, however.

=Sodium= is probably not essential to plant growth, although it is present

in small proportions in the ash from practically all plants. In cases of

insufficient supply of potassium, sodium can apparently perform at least a

part of the rôle of the former element; but this seems not to be a normal

relationship or use.

=Chlorine= is found in small amounts in the sap and in the ash of nearly

all plants. However, it does not appear to be essential to the growth of a

plant, except possibly in the case of certain species, such as asparagus,

buckwheat, and, perhaps, turnips and some other root crops. Whether the

benefit which these crops derive from the application of common salt to the

soil in which they are growing is due to the direct food value of either

the chlorine, or the sodium, or to some indirect effect, is not yet known.

The presence of chlorine in the sap of plants is undoubtedly due to the

inevitable absorption of soluble chlorides from the soil and apparently has

no connection with the nutritional needs of the plant.

=Silicon= is always considered as a non-essential element, although it

occurs in such large proportions in some plants as to indicate that it

cannot be wholly useless. It accumulates in the stems of plants, chiefly in

the cell-wall, and has sometimes been supposed to aid in giving stiffness

to the stems. But large numbers of analyses have failed to show any direct

correlation between the stiffness of straw of cereal plants and the

percentage of silicon which they contain. Further, plants will grow to full

maturity and with erect stems when no silicon is present in the mineral

nutrients which are furnished to them. On the other hand, certain

experiments appear to indicate that silicon can perform some of the

functions of phosphorus, if soluble silicates are supplied to

phosphorus-starved plants. But under normal conditions of plant nutrition,

it seems to have no such function.

INORGANIC PLANT TOXINS AND STIMULANTS

Much study has been given during recent years to the question of the

supposed poisonous, or toxic, effects upon plants of various soil

constituents. There seems to be no doubt that certain _organic_ compounds

which are injurious to plant life are often present in the soil, either as

the normal excretions of plant roots or as products of the decomposition of

preceding plant growths. A consideration of these supposedly toxic organic

substances would be out of place in this discussion of mineral soil

nutrients. But there seems to be no doubt that there may also be mineral

substances in the soil which may sometimes exert deleterious influences

upon plant growth. In fact, most metallic salts, except those of the few

metals which are required for plant nutrition, appear to be toxic to

plants. The exact nature of the physiological effects which are produced by

these mineral toxins is not clearly understood; indeed, it is probably

different in the case of different metals. Further, it is certain that both

the stimulating and the toxic effect of metallic compounds upon low forms

of plants is quite different from the effects of the same substances upon

the more complex tissues of higher plants, a fact which is utilized to

advantage in the application of fungicides for the control of parasitic

growths on common farm crops.

Among the elements whose physiological effects upon higher plants, such as

the cereal crops, etc., when their soluble compounds are present in the

soil, have been carefully studied, there are three fairly distinct types of

injurious mineral elements. The first of these, represented by copper,

zinc, and arsenic, apparently exert their toxic effect regardless of the

proportion in which they are present in the nutrient solution which is

presented to the plant; although the degree of injury varies with the

amount of injurious substance present, of course. The second type, of which

boron and manganese are representatives, apparently exerts a definite

stimulating effect upon plants when supplied to them in concentrations

below certain clearly defined limits; but are toxic in concentrations above

these. The third includes many soluble salts of magnesium, sodium,

potassium, etc., which while either innocuous or else definite sources of

essential plant foods when in lower concentrations, become highly toxic, or

corrosive, when present in the soil solution in concentrations above the

limits of "toleration" of individual plants for these soluble salts. The

tolerance shown by the different species of plants toward these soluble

salts (the so-called "alkali" in soils) varies widely; indeed, there seems

to be considerable variation in the resistance of different individual

plants of the same species to injury from this cause.

With reference to the toxic effect of the third type of substances, i.e.,

the common soluble salts, it is known that single salts of potassium,

magnesium, sodium, or calcium, in certain concentrations, are toxic to

plants, while mixtures of the same salts in the same concentrations are

not. Thus, solutions of sodium chloride, magnesium sulfate, potassium

chloride, and calcium chloride which, when used singly, killed plants whose

roots were immersed in them for only a few minutes, formed when mixed

together a nutrient solution in which the same plants grew normally. The

remarkable remedial effect of calcium salts in overcoming the injurious

effects of other soluble salts has already been mentioned. One explanation

of these relationships between mineral soil constituents and the living

plant is that the life phenomena depend upon a balanced adjustment between

the compounds of these different mineral elements with the proteins

(producing the so-called "metal proteids") which constitute the active

material of the cell protoplasm. According to this theory, any excess or

deficiency of any one or more of these elements in the plant juices which

surround a given cell will, of course, cause an interchange with the

mineral components of the supposed "metal proteids" which upsets the

assumed essential balance between them, with disastrous results. A more

recent, and much more satisfactory, explanation of the "antagonism" between

mineral elements in their toxic effects upon plants, which has both

theoretical and experimental confirmation, is that single salts disturb the

colloidal condition (see Chapter XV) of the protoplasm of the plant cells

in such a way as to destroy its permeability to nutrient substances, while

mixtures of salts restore the proper state of colloidal dispersion and

permit the normal functioning of the protoplasm.

It is apparent from the above brief discussions that the rôle of the

different soil elements as plant food, and their relations to the complex

processes which constitute plant growth, afford an interesting and

promising field for further study.

References

BRENCHLEY, WINIFRED E.--"Inorganic Plant Poisons and Stimulants," 106

pages, 18 figs., Cambridge, 1914.

HALL, A. D.--"Fertilizers and Manures," 384 pages, 7 plates, London, 1909.

HALL, A. D.--"The Book of the Rothamsted Experiments," 294 pages, 49 figs.,

8 plates, London, 1905.

HOPKINS, C. G.--"Soil Fertility and Permanent Agriculture," 653 pages,

Chicago, 1910.

HILGARD, E. W.--"Soils," 593 pages, 89 figs., New York, 1906.

LOEW, O.--"The Physiological Rôle of Mineral Nutrients," U. S. Department

of Agriculture, Bureau of Plant Industry, _Bulletin_ No. 45, 70 pages,

Washington, D. C., 1903.

RUSSELL, E. J.--"Soil Conditions and Plant Growth," 243 pages, 13 figs.,

_Monographs_ on Biochemistry, London, 1917. (3d ed.)

WHITNEY, M.--"A Study of Crop Yields and Soil Composition in Relation to

Soil Productivity," U. S. Department of Agriculture, Bureau of Soils,

_Bulletin_ No. 57, 127 pages, 24 figs., Washington, D. C., 1909.

CHAPTER II

THE ORGANIC COMPONENTS OF PLANTS

From the standpoint of their ability to synthetize synergic foods (see page

2) from inorganic raw materials, plants may be divided into two types;

namely, the _autotrophic_, or self-nourishing, plants, and the

_heterotrophic_ plants.

Strictly speaking, only those plants whose every cell contains chlorophyll

are entirely self-nourishing; and some parts, or organs, of almost any

autotrophic plant are dependent upon the active green cells of other parts

of the plant for their synergic food. Furthermore, if the term is used in a

very wide sense, green plants are more than self-nourishing, they really

nourish all living things. But the general significance of the term

"autotrophic plants" is apparent.

"Heterotrophic plants" must, of necessity, get food, either directly or

indirectly, from some other plant which can synthetize synergic foods or,

in a few cases, from animal organic matter. If they do this by feeding upon

the organic compounds of other living organisms, they are known as

"parasites"; while if they secure their organic food from the tissues or

debris of dead organisms, they are called "saprophytes." The heterotrophic

plants are chiefly the bacteria and fungi; although a few seed-plants are

devoid of chlorophyll or have nutritive habits similar to those of the

non-green plants, and a few species are semi-parasitic or semi-saprophytic.

It is obvious that the metabolic processes of the autotrophic plants are

very different from those of the heterotrophic type of plants. These

differences constitute a most interesting field of study for plant

physiologists. But the nature of the chemical compounds themselves and of

the chemical changes involved in their transformations is not radically

different in the two types of plants, the essential difference being in the

preponderance of one kind of activities, or chemical reactions, over

another in bringing about the metabolic processes which are characteristic

of each particular species. Hence, it does not seem necessary, or

desirable, in this study of the chemistry of plant growth, to present as

detailed a consideration of the differences in metabolic activity of the

different types of plants as complete accuracy of statement in all cases

might demand. We will, instead, discuss the organic chemical components of

plant tissues and the reactions which they undergo, using the more common

type of autotrophic plants as the illustrative material in most cases.

Hence, it will be understood that in all the following discussions of plant

activities, except where specific exceptions are definitely mentioned, it

is the green, or autotrophic, plants to which reference is made in each

case.

From the standpoint of the sum total of its activities, a green plant is

essentially an absorber of solar energy and a synthetizer of organic

substances. Each individual autotrophic plant takes up certain amounts of

the anergic foods which are discussed in the preceding chapter and

manufactures from them a great variety of complex organic compounds, using

the energy of the sun's rays, absorbed by chlorophyll, as the source for

the energy necessary to accomplish these synthetic reactions. The ultimate

object of these processes is to produce seeds, each containing an embryo

and a sufficient supply of food for the young plant of the next generation

to use until it has developed its own synthetic organs; or (in the case of

perennials) to store up reserve food materials with which to start off new

growth after a period of rest and often of defoliation. To be sure, animals

and men often interfere with the completion of the life cycle of the plant,

and utilize the seeds or stored food material for their own nutrition, but

this is a biological relation which has no influence upon the nature of the

plant's own activities.

Since all of these synthetic reactions must go on at ordinary temperatures,

active catalyzers are necessary. These the plant provides in the form of

enzymes (see Chapter XIV) which are always present in active plant

protoplasm. Proper conditions for rapid chemical action are further assured

by the colloidal nature (see Chapter XV) of the protoplasm itself.

TYPES OF CHEMICAL CHANGES INVOLVED IN PLANT GROWTH

The whole cycle of chemical changes which is involved in plant growth

represents the net result of two opposite processes; the first of these is

a constructive one which has at least three different phases: namely, a

synthesis of complex organic compounds, the translocation of this

synthetized material to the centers of growth, and the building up of this

food material into tissues or reserve supplies; and the second is a

destructive process of respiration whereby carbohydrate material is broken

down, potential energy is released, and carbon dioxide is excreted.

The synthetic processes which take place in plants are of two types;

namely, photosynthesis, in which sugars are produced, and another, which

has no specific name, whereby proteins are elaborated. The translocation of

the synthetized material involves the change of insoluble compounds into

soluble ones, effected by the aid of enzymes. For storage purposes, the

soluble forms are usually, though not always, condensed again into more

complex forms, these latter changes requiring much less energy than do the

original syntheses from raw materials.

The destructive process, respiration, is characteristic of all living

matter, either plant or animal organisms. It takes place continuously

throughout the whole life of a plant. During rapid growth it is

overshadowed by the results of the synthetic process, but during the

ripening period in which the seed is matured, and during the germination of

the seed itself, growth is practically at a standstill and the respiratory,

destructive action predominates, so that the plant actually loses weight.

GROUPS OF ORGANIC COMPOUNDS FOUND IN PLANTS

As a result of their various synthetic and metabolic activities, a great

variety of organic compounds is produced by plants. Certain types of these

compounds, such as the carbohydrates and proteins, are necessary to all

plants and are elaborated by all species of autotrophic plants. Other types

of compounds are produced by many, but not all, species of plants; while

still others are found in only a few species. It is fairly easy to classify

all of these compounds into a few, well-defined groups, based upon

similarity of chemical composition. These groups are known, respectively,

as the carbohydrates and their derivatives, the glucosides and tannins; the

fats and waxes; the essential oils and resins; organic acids and their

salts; the proteins; the vegetable bases and alkaloids; and the pigments. A

consideration of these groups of compounds, as they are synthetized by

plants, constitutes the major portion of the study of the chemistry of

plant life as presented in this book. Following the discussion of the

compounds themselves, the chapters dealing with enzymes, with the colloidal

nature of protoplasm, and with the supposed accessory stimulating agencies,

aim to show how the manufacturing machine known as the plant cell

accomplishes its remarkable results, so far as the process is now

understood.

PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE

In connection with the discussion of each of the above-mentioned groups of

organic components of plants, an attempt will be made to point out what

significance these particular compounds have in the plant's life and

growth. Certain terms will be used to designate different rôles, which it

is probably necessary to define.

There may be two possible explanations of, or reasons for, the presence of

any given type of compound in the tissues of any particular species of

plant. First, it may be supposed that this particular type of compounds is

elaborated by the plant to satisfy its own physiological needs, or for the

purpose of storing it up in the seeds as synergic food for the growth of

the embryo, in order to reproduce the species. For this rôle of the various

organic food materials, etc., we will employ the term "physiological use."

On the other hand, it is often conceivable that certain types of compounds,

which have properties that make them markedly attractive (or repellent) as

a food for animals and men, or which are strongly antiseptic in character,

or which have some other definite relationship to other living organisms,

have had much to do with the survival of the particular species which

elaborates them, in the competitive struggle for existence; or have been

developed in the plant by the evolutionary process of "natural selection."

For this relation of the compound to the plant's vital needs, we will use

the term "biological significance." Such a segregation of the rôles which

the different compounds play in the plant's economy may be more or less

arbitrary in many cases; but it will be clear that when _physiological

uses_ are discussed, reference is being made to the plant's own internal

needs; while the phrase _biological significance_ will be understood to

refer to the relation of the plant to other living organisms.

PHYSIOLOGICAL USES OF THE ORGANIC COMPONENT GROUPS

From the standpoint of the rôle which each plays in the plant economy, the

several groups of organic compounds may be roughly divided into three

classes. These are: (_a_) the framework materials, including gums, pectins,

and celluloses; (_b_) synergic foods, including carbohydrates, fats, and

proteins; and (_c_) the secretions, including the glucosides, volatile

oils, alkaloids, pigments, and enzymes.

The _framework material_, as the name indicates, constitutes the cell-wall

and other skeleton substances of the plant. It is made up of carbohydrate

complexes, produced by the cell protoplasm from the simpler carbohydrates.

The _synergic foods_, or "reserve foods" as they are sometimes called,

produced by the excess of synthetized material over that needed for the

immediate use of the plant, are accumulated either in the various storage

organs, to be available for future use by the plant itself or by its

vegetative offspring, or in the seed, to be available to the young seedling

of the next generation. Proteins not only serve as reserve food materials

but also make up the body of the living organism itself. Carbohydrates and

fats serve as synergic and reserve foods.

The _secretions_ may be produced either in ordinary cells and found in

their vacuoles, or in special secretory cells and stored in cavities in the

secreting glands (as in the leaves of mints, skin of oranges, etc.), or in

special ducts (as in pines, milkweeds, etc.) or on the epidermis (as the

"bloom" of plums, cabbages, etc., the resinous coating of many leaves,

etc.). As a general rule, the glucosides, pigments, and enzymes are the

products of unspecialized cells and have some definite connection with the

metabolic processes of the plant; while the volatile oils and the alkaloids

are usually secreted by special cells and have no known rôle in

metabolism.

CHAPTER III

PHOTOSYNTHESIS

Photosynthesis is the process whereby chlorophyll-containing plants, in the

presence of sunlight, synthetize organic compounds from water and carbon

dioxide. The end-product of photosynthesis is always a carbohydrate.

Chemical compounds belonging to other groups, mentioned in the preceding

chapter, are synthetized by plants from the carbohydrates and simple raw

materials; but in such cases the energy used is not solar energy and the

process is not photosynthesis.

Under the ordinary conditions of temperature, moisture supply, etc.,

necessary to plant growth, photosynthesis will take place if the three

essential factors, chlorophyll, light, and carbon dioxide are available.

PHYSIOLOGICAL STEPS IN PHOTOSYNTHESIS

There are five successive and mutually dependent steps in the process of

photosynthesis, as follows:

(1) There must be a gas exchange between the plant tissue and the

surrounding air, by means of which the carbon dioxide of the air may reach

the protoplasm of the chlorophyll-containing cells.

(2) Radiant energy must be absorbed, normally that of sunlight, although

photosynthesis can be brought about by the energy from certain forms of

artificial light.

(3) Carbon dioxide and water must be decomposed by the energy thus

absorbed, and the nascent gases thus produced combined into some synthetic

organic compound, with a resultant storage of potential energy.

(4) This first organic synthate must be condensed into some carbohydrate

suitable for translocation and storage as reserve food.

(5) The oxygen, which is a by-product from the decomposition of the water

and carbon dioxide and the resultant synthetic process, must be returned to

the air by a gas exchange.

Of the five steps in this process, the first two and the last are

essentially purely physical phenomena, the chemical changes involved being

those of the third and fourth steps. Hence, it is only these two parts of

the process which need be taken into account in a consideration of the

chemistry of photosynthesis.

FORMALDEHYDE, THE SIMPLEST CARBOHYDRATE STRUCTURE

The simplest carbohydrates known to occur commonly in plant tissues are the

hexoses (see Chapter IV) having the formula C_{6}H_{12}O_{6}, which is just

six times that of formaldehyde, CH_{2}O. Also, it is known that

formaldehyde easily, and even spontaneously, polymerizes into more complex

forms having the general formula (CH_{2}O)_n_; trioxymethylene,

C_{3}H_{6}O_{3}, being a well-known example. Further, both trioxymethylene

and formaldehyde itself can easily be condensed into hexoses, by simple

treatment with lime water as a catalytic agent. Hence, it is commonly

believed that formaldehyde is the first synthetic product resulting from

photosynthesis, that this is immediately condensed into hexose sugars, and

that these in turn are united into the more complex carbohydrate groups

which are commonly found in plants (see Chapter IV).

There is considerable experimental confirmation of the soundness

of this view. The whole photosynthetic process takes place in

chlorophyll-containing plant tissues with astonishing rapidity, sugars,

and even starch, appearing in the tissues almost immediately after their

exposure to light in the presence of carbon dioxide. Hence, any

intermediate product, such as formaldehyde, is present in the cell for

only very brief periods and in very small amounts. But small amounts of

formaldehyde can often be detected in fresh green plant tissues and, as

will be pointed out below, the whole process of photosynthesis,

proceeding through formaldehyde as an intermediate product, can be

successfully duplicated _in vitro_ in the laboratory.

Assuming, then, that formaldehyde is the first photosynthetic product in

the process of the production of carbohydrates from water and carbon

dioxide, the simple empirical equation for this transformation would be

H_{2}O + CO_{2} = CH_{2}O + O_{2}.

It is apparent, however, that the process is not so simple as this

hypothetical reaction would indicate, as water and carbon dioxide can

hardly be conceived to react together in any such simple way as this.

Various theories as to the exact nature of the steps through which the

chemical combinations proceed have been advanced. A discussion of the

experimental evidence upon which these are based and of the conclusions

which seem to be justified from these experimental studies is presented

below. The only value which may be attached to the empirical equation just

presented is that it does accurately represent the facts that a volume of

oxygen, equal to that of the carbon dioxide consumed in the process, is

liberated and that formaldehyde is the synthetical product of the reactions

involved.

It should be noted, in this connection, that formaldehyde is a powerful

plant poison and that few, if any, plant tissues can withstand the toxic

effect of this substance when it is present in any considerable

concentration. Hence, it is necessary to this whole conception of the

relation of formaldehyde to the photosynthetic process, to assume that,

however rapidly the formaldehyde may be produced in the cell, it is

immediately converted into harmless carbohydrate forms.

THE CONDENSATION OF FORMALDEHYDE INTO SUGARS

As has been mentioned, it is easily possible to cause either formaldehyde,

or trioxymethylene, to condense into C_{6}H_{12}O_{6}, using milk of lime

as a catalyst. Of course, no such condition as this prevails in the plant

cell, and the mechanics of the protoplasmic process may be altogether

different from those of the artificial syntheses. Furthermore, the hexose

produced by the artificial condensation of these simpler compounds is, in

every case, a non-optically active compound, while all natural sugars are

optically active (see Chapter IV). Emil Fischer has succeeded, however, by

a long and round-about process which need not be discussed in detail here,

in converting the artificial hexose into glucose and fructose, the

optically-active sugars which occur naturally in plant tissues. The

condensation of formaldehyde directly into glucose and fructose in the

plant cell is brought about by some process the nature of which is not yet

understood. Probably synthetic enzymes (see Chapter XIV), whose nature and

action have not yet been discovered, come into play. It is a noteworthy

fact, however, that the mechanics of this apparently simple chemical

change, upon which the whole nutrition of the plant depends, and which

furnishes the whole animal kingdom, including the human race, with so large

a proportion of its food supplies, is as yet wholly unknown.

It is the common practice to represent the whole results of the

photosynthetic action by the empirical equation

6H_{2}O + 6CO_{2} = C_{6}H_{12}O_{6} + 6O_{2};

but here again the only value to be attached to such an algebraic

expression is that it accurately represents the gaseous exchange of carbon

dioxide and oxygen involved in the process. Certainly, it throws no light

upon the nature of the process itself.

THEORIES CONCERNING PHOTOSYNTHESIS

The many theories which have been advanced concerning the nature of the

chemical changes which are involved in photosynthesis have served as the

basis for much experimental study of the problem. The following brief

summary will serve to point out the general trend of these investigations

and the present state of knowledge concerning the chemistry of

photosynthesis.

Von Baeyer, in 1870, advanced the hypothesis that the first step in the

process is the breaking down of carbon dioxide into carbon monoxide and

oxygen and of water into hydrogen and oxygen; that the carbon monoxide and

hydrogen then unite to produce formaldehyde, which is immediately

polymerized to form a hexose. These theoretical changes may be represented

by the following equations:

{ CO_{2} = CO + O

1. {

{ H_{2}O = H_{2} + O

2. H_{2} + CO = CH_{2}O

3. 6(CH_{2}O) = C_{6}H_{12}O_{6}

In the investigations and discussions of this hypothesis, it has been

ascertained: first, that carbon monoxide has never been found in the free

form in plant tissues; second, that when _Tropaeolum_ plants were

surrounded with an atmosphere in which there was no carbon dioxide, but

which contained sufficient carbon monoxide to give a concentration of this

gas in the cell-sap equivalent to that in which CO_{2} is normally present,

the plants grew normally and apparently elaborated starch; third, other and

more extensive experiments indicated, however, that green plants in general

cannot make use of carbon monoxide gas for photosynthesis, although this

does not prove that von Baeyer's idea that CO is a step in the process is

necessarily erroneous; and finally it was shown that carbon monoxide, in

sufficient concentration to produce the results with _Tropaeolum_ mentioned

above, usually acts as a powerful anæsthetic towards most other plants.

While these considerations do not positively prove that von Baeyer's

hypothesis is incorrect, they render it so improbable that it has generally

been abandoned in favor of others which are described below.

Erlenmeyer, even before the experimental work mentioned in the preceding

paragraph had been reported, suggested that instead of assuming a separate

breaking down of the carbon dioxide and water, it is easier to conceive

that they are united in the cell-sap into carbonic acid and that this is

reduced by the chlorophyll-containing protoplasm into formic acid and then

to formaldehyde, as indicated by the following equations:

1. H_{2}CO_{3} = H_{2}CO_{2} + O

2. H_{2}CO_{2} = CH_{2}O + O

Like von Baeyer's hypothesis, this assumes that formaldehyde and oxygen are

the first products of photosynthesis.

Proceeding upon this assumption, many investigators have studied the

question as to whether formaldehyde actually is present in green leaves.

Several workers have reported successful identification of formaldehyde in

the distillate from green leaves; while others have criticized these

results and have maintained that formaldehyde can likewise be obtained by

distilling decoctions of dry hay, etc., in which the photosynthetic process

could not possibly be conceived to be at work. Other investigators, notably

Bach and Palacci, reported that they had succeeded in artificially

producing formaldehyde from water and carbon dioxide, in the presence of a

suitable catalyzer or sensitizer. Euler, however, later showed

conclusively that under the conditions described by these investigators,

formaldehyde can be obtained even if no carbon dioxide is present, being

apparently produced by the action of water upon the organic sensitizer

which was used.

These conflicting reports led Usher and Priestley, in a series of studies

reported between 1906 and 1911, to submit the whole matter to a critical

review. Briefly, these investigators showed that the photolysis of carbon

dioxide and water results in the formation of formaldehyde and hydrogen

peroxide, as represented by the equation

CO_{2} + 3H_{2}O = CH_{2}O + 2H_{2}O_{2}.

The formaldehyde is then condensed by the protoplasm into sugars, while the

hydrogen peroxide is decomposed, by an enzyme in the plant cell, into water

and oxygen. If the formaldehyde is not used up rapidly enough by the

protoplasm, it kills the enzyme and the undecomposed hydrogen peroxide

destroys the chlorophyll, which stops the whole photosynthetic process.

Usher and Priestley were able to cause the photolysis of carbon dioxide and

water into formaldehyde outside of a green plant, in the presence of a

suitable catalyzing agent which continually destroys the hydrogen peroxide

as fast as it is formed; to show the actual bleaching effect of an excess

of hydrogen peroxide in plant tissues which had been treated in such a way

as to prevent the enzyme from decomposing it; and, finally, to demonstrate

the condensation of formaldehyde into starch by the action of protoplasm

which contained no chlorophyll.

In the meantime, Fenton, in 1907, found that in the presence of magnesium

as a catalyst (it will be shown in Chapter VIII that magnesium is a

constituent of the chlorophyll molecule) formaldehyde may be obtained from

a solution of carbon dioxide in water, especially if weak bases are

present.

Further, Usher and Priestley's later results showed that radium emanations,

acting upon a solution of carbon dioxide in water, produce hydrogen

peroxide and formaldehyde, and the latter polymerizes but not up to the

point represented by the hexose sugars; also, that the ultra-violet rays

from a mercury vapor lamp are very effective in bringing about the

production of hydrogen peroxide and formaldehyde from a saturated aqueous

solution of carbon dioxide, the reaction taking place even in the absence

of any "sensitizer," but much more readily if some "optical" or "chemical"

sensitizer is present. Finally, these investigators were able to duplicate

all their results, using green plant tissues, and to show that the

temperature changes which take place in a film of chlorophyll when it is

exposed to an atmosphere of moist carbon dioxide in the sunlight are such

as would be required by the formation of formaldehyde and hydrogen peroxide

from carbonic acid.

More recently, Ewart has showed that formaldehyde can combine chemically

with chlorophyll; from which fact, Schryver deduces the theory that if for

any reason the condensation of formaldehyde into carbohydrates by the cell

protoplasm does not proceed as rapidly as the formaldehyde is produced by

photosynthesis, the excess of the latter enters into combination with the

chlorophyll, and that if condensation into sugar uses up all the free

formaldehyde which is present in the active protoplasm, the compound of

formaldehyde with chlorophyll is broken down setting free an additional

supply for further sugar manufacture. According to this conception there

are, in the chlorophyll-bearing protoplasm, not only the agencies for the

production of formaldehyde from carbon dioxide and water and for the

condensation of this into carbohydrates, but also a chemical mechanism by

means of which the amount of free formaldehyde in the reacting mass may be

regulated so that at no time will it reach the concentration which would be

injurious to the cell protoplasm or fall below the proper proportions for

sugar-formation. This explanation affords a satisfactory solution of the

difficulty which formerly confronted the students of photosynthesis,

namely, the fact that free formaldehyde is powerfully toxic to cell

protoplasm. Without some such conception, it was difficult to imagine how

the presence of formaldehyde in the cell contents, even as a transitory

intermediate product, could be otherwise than injurious.

As a result of these studies, the nature of the chemical changes which

result in the production of formaldehyde as the first product of

photosynthesis, with the liberation of a volume of oxygen equal to that of

the carbon dioxide consumed, seems to be fairly well established.

THE PRODUCTION OF SUGARS AND STARCHES

The next step in the process, the conversion of formaldehyde into sugars

and starches, is not necessarily a _photo_synthetic one, as it can be

brought about by protoplasm which contains no chlorophyll or other

energy-absorbing pigment. It is, however, a characteristic synthetic

activity of living protoplasm. There is little definite knowledge as to how

the cell protoplasm accomplishes this important task. As has been pointed

out, the polymerization of formaldehyde into a sugar-like hexose, known as

"acrose," can be easily accomplished by ordinary laboratory reactions, and

acrose can be converted into glucose or fructose by a long and difficult

series of transformations. But such processes as are employed in the

laboratory to accomplish these artificial synthesis of optically-active

sugars from formaldehyde can have no relation whatever to the methods of

condensation which are used by cell protoplasm in its easy, almost

instantaneous, and nearly continuous accomplishment of this transformation.

Furthermore, these simple hexoses are by no means the final products of

cell synthesis, even of carbohydrates alone. In many plants, starch appears

as the final, if not the first, product of formaldehyde condensation. At

least, the transformation of the simple sugars, which may be supposed to be

the first products, into starch is effected so nearly instantaneously that

it is impossible to detect measurable quantities of these sugars in the

photosynthetically active cells of such plants. Other species of plants

always show considerable quantities of simple sugars in the vegetative

tissues, and some even store up their reserve carbohydrate food material in

the form of glucose or sucrose. Attempts have been made to associate the

type of carbohydrate formed in cell synthesis with the botanical families

to which the plants belong, but with no very great success. For each

individual species, however, the form of carbohydrate produced is always

the same, at least under normal conditions of growth. For example, the

sugar beet always stores up sucrose in its roots, although under abnormal

conditions considerable quantities of raffinose are developed. Similarly,

potatoes always store up starch, but with abnormally low temperatures

considerable quantities of this may be converted into sugar, which becomes

starch again with the return to normal conditions.

While it is impossible, with our present knowledge, to even guess at the

mechanism by which protoplasm condenses formaldehyde into sugars and these,

in turn, into more complex carbohydrates, the structure and relationships

to each other of the final products of photosynthesis are well known, and

are discussed at length in the following chapter.

References

BARNES, C. R.--"Physiology" (Part II of Coulter, Barnes and Cowles'

"Textbook of Botany"), 187 pages, 18 figs., Chicago, 1910.

GANONG, W. F.--"Plant Physiology," 265 pages, 65 figs., New York, 1908 (2d

ed.).

JOST, L., trans. by GIBSON, R. J. H.--"Plant Physiology," 564 pages, 172

figs., Oxford, 1907.

MARCHLEWSKI, L.--"Die Chemie des Chlorophylls," 187 pages, 5 figs., 7

plates, Berlin, 1909.

PARKIN, JOHN.--"The Carbohydrates of the Foliage Leaf of the Snowdrop

(_Galanthus nivalis L._) and their Bearing on the First Sugar of

Photosynthesis," in _Biochemical Journal_, Vol. 6, pages 1 to 47, 1912.

PFEFFER, W., trans. by EWART, A. J.--"Physiology of Plants." Vol. I, 632

pages, 70 figs., Oxford, 1900.

CHAPTER IV

CARBOHYDRATES

These substances comprise an exceedingly important group of compounds, the

members of which constitute the major proportion of the dry matter of

plants. The name "carbohydrate" indicates the fact that these compounds

contain only carbon, hydrogen, and oxygen, the last two elements usually

being present in the same proportions as in water. As a rule, natural

carbohydrates contain six, or some multiple of six, carbon atoms and the

same number of oxygen atoms less one for each additional group of six

carbons above the first one; e.g., C_{6}H_{12}O_{6}, C_{12}H_{22}O_{11},

C_{18}H_{32}O_{16}, etc.

Carbohydrates are classed as open-chain compounds, that is, they may be

regarded as derivatives of the aliphatic hydrocarbons. From the standpoint

of the characteristic groups which they contain, they are

aldehyde-alcohols. In common with many other polyatomic open-chain

alcohols, they generally possess a characteristic sweet, or mildly

sweetish, taste. In the case of the more complex and less soluble forms,

this sweetish taste is scarcely noticeable and these compounds are commonly

called the "starches," as contrasted with the more soluble and sweeter

forms, known as "sugars."

The characteristic ending _ose_ is added to the names of the members of

this group. As systematic names, the Latin numeral indicating the number of

carbon atoms in the molecule is combined with this ending; e.g.,

C_{5}H_{10}O_{5}, pentose, C_{6}H_{12}O_{6}, hexose, etc.

In recent years, as a matter of scientific interest, many sugarlike

substances which contain from two to nine carbon atoms combined with the

proper number of hydrogen and oxygen atoms to be equivalent to the same

number of molecules of water in each case, have been artificially prepared

in the laboratory and designated as dioses, trioses, tetroses, pentoses,

hexoses, heptoses, octoses, and nonoses, respectively. Substances

corresponding in composition and properties with the artificial tetroses

and one or two derivatives of heptoses are occasionally found in plant

tissues, and a considerable number of pentoses and their condensation

products are common constituents of plant gums, etc.; but the great

majority of the natural carbohydrates are hexoses and their derivatives.

GROUPS OF CARBOHYDRATES

Since the simpler carbohydrates are sugars, i.e., they possess the

characteristic sweet taste, the name "saccharide" is used as a basis for

the classification of the entire group. The simplest natural sugars, the

hexoses, C_{6}H_{12}O_{6}, are known as _mono-saccharides_. The group of

next greater complexity, those which have the formula C_{12}H_{22}O_{11}

and may be regarded as derived from the combination of two molecules of a

hexose with the dropping out of one molecule of water at the point of

union, are known as _di-saccharides_. Compounds having the formula

C_{18}H_{32}O_{16} (i.e., three molecules of C_{6}H_{12}O_{6} minus two

molecules of H_{2}O) are _tri-saccharides_; and the still more complex

groups, having the general formula (C_{6}H_{10}O_{5})_n_, are called the

_poly-saccharides_. The mono-, di-, and tri-saccharides are generally

easily soluble in water, have a more or less pronouncedly sweet taste, and

are known as the _sugars_; while the polysaccharides are generally

insoluble in water and of a neutral taste, and are called _starches_. As

will be seen later, there are many natural plant carbohydrates belonging to

each of these groups.

In addition to these saccharide groups, there are other types, or groups,

of compounds which resemble the true carbohydrates in their chemical

composition and properties and are often considered as a part of this

general group. These are the pentoses, C_{5}H_{10}O_{5}, and their

condensation products, the pentosans (C_{5}H_{8}O_{4})_n_, and their methyl

derivatives, C_{6}H_{12}O_{5}; certain polyhydric alcohols having the

formula C_{6}H_{8}(OH)_{6}; pectose and its derivatives, pectin and pectic

acid; and lignose substances of complex composition. It is doubtful whether

these compounds are actual products of photosynthesis in plants, or have

the same physiological uses as the carbohydrates and it has seemed wise to

consider them in a separate and later chapter.

ISOMERIC FORMS OF MONOSACCHARIDES

Four sugars having the formula C_{6}H_{12}O_{6}, namely, glucose, fructose,

mannose, and galactose, occur very commonly and widely distributed in

plants. In addition to these, thirteen others having the same percentage

composition have been artificially prepared, while seven additional forms

are theoretically possible. In other words, twenty-four different

compounds, all having the same empirical formula and similar sugar-like

properties are theoretically possible. In order to arrive at a conception

of this multiplicity of isomeric forms, it is necessary to understand the

two types of isomerism which are involved. One of these is _structural_

isomerism, and the other is _space_- or _stereo_-isomerism.

=Structural Isomerism.=--This refers to an actual difference in the

characteristic groups which are present in the molecule. As has been said,

all carbohydrates, from the standpoint of the characteristic groups which

they contain, are aldehyde-alcohols. The hexoses all contain five alcoholic

groups and one primary aldehyde, or one secondary aldehyde (ketone), group.

If the aldehyde oxygen is attached to the carbon atom which is at the end

of the six-membered chain, the structural arrangement is

|

that of an aldehyde, C=O and the sugar is of the type known

|

H

as "aldoses"; whereas, if the oxygen is attached to any other

|

carbon in the chain, the ketone arrangement, C=O results and

|

the sugar is a "ketose." This difference is illustrated in the Fischer

open-chain formulas for glucose (an aldose) and fructose (a ketose) as

follows:

Glucose Fructose

CH_{2}OH CH_{2}OH

| |

CHOH CHOH

| |

CHOH CHOH

| |

CHOH CHOH

| |

CHOH C=O

| |

CHO CH_{2}OH

=Stereo-isomerism=, or space isomerism, as its name indicates, depends upon

the different arrangement of the atoms or groups in the molecule in space,

and not upon any difference in the character of the constituent groups.

This possibility depends upon the existence in the molecule of the

substance in question of one or more _asymmetric carbon atoms_ and

manifests itself in differences in the optical activity of the compound.[1]

Thus, in the formula for glucose shown above there appear four asymmetric

carbon atoms, namely, those of the four secondary alcohol groups (in the

terminal, or primary alcohol, group, carbon is united to hydrogen by two

bonds, and in the aldehyde group it is united to oxygen by two bonds).

Similarly, fructose contains three asymmetric carbon atoms.

As an example of how the presence of these asymmetric carbon atoms results

in the possibility of many different space relationships, the following

graphic illustrations of the supposed differences between dextro-glucose

and levo-glucose, and between dextro- and levo-galactose, may be cited.[2]

_d_-glucose _l_-glucose _d_-galactose _l_-galactose

CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH

| | | |

H-C-OH H-C-OH H-C-OH HO-C-H

| | | |

H-C-OH H-C-OH HO-C-H H-C-OH

| | | |

HO-C-H H-C-OH HO-C-H H-C--OH

| | | |

H-C-OH HO-C-H H-C-OH HO-C-H

| | | |

CHO CHO CHO CHO

Comparisons of the above formulas will show that the difference

between the formulas for _d_- and _l_-glucose lies in the arrangement of

the H atoms and the OH groups around the two asymmetric carbon atoms next

the aldehyde end of the chain; while the _d_- and _l_-galactoses differ in

that this arrangement is in the reverse order around all four of the

asymmetric carbons. By similar variations in the grouping around the four

asymmetric atoms, it is possible to produce the sixteen different space

arrangements shown on page 37 for the groups of an aldohexose. Sugars

corresponding to fourteen of these different forms have been discovered,

three of which are of common occurrence in plants, either as single

mono-saccharides or as constituent groups in the more complex

carbohydrates; the remaining two forms have only theoretical interest.

Similarly, for a ketohexose, which contains three asymmetric carbon atoms,

there are eight possible arrangements. Three sugars of this type are known,

only one (fructose) being common in plants; the others are of only

theoretical interest.

FOOTNOTES:

[1] It is assumed that the reader, or student, is familiar with the

theoretical and experimental evidence in support of the existence of the

so-called "asymmetric" carbon atom and its relation to the effect of the

compound which contains it, when in solution, in rotating the plane of

polarized light. For purposes of review, or of study of this most

interesting and important phenomenon, the reader is referred to any

standard text-book on Organic Chemistry.

[2] Attention should be called, at this point, to the fact that such

formulas as these cannot possibly accurately represent the actual

arrangement of the constituent groups of a carbohydrate molecule around an

asymmetric carbon atom. The limitations of a plane-surface formula prevent

any illustration of the three-dimension relationships in space.

Furthermore, there are certain facts in connection with the birotation

phenomenon and the relation of the molecular configuration to biochemical

properties (which see) that cannot be explained on the basis of the

open-chain arrangement represented by the Fischer formulas used here. A

closed-ring arrangement, showing the aldehyde oxygen as linked by its two

bonds to the first and the fourth carbon atoms of the chain, thus forming a

closed-ring of four carbon and one oxygen atoms, instead of being attached

by both bonds to a single carbon atom, as in the above formulas, is

undoubtedly a more nearly accurate representation of the actual linkage in

the molecule than are the open-chain formulas used above.

The differences in conception embodied by these two types of formulas may

be shown by the following formulas for glucose:

CH_{2}OH CH_{2}OH

| |

CHOH CHOH

| |

CHOH CH-CHOH ------O-------

| | | | |

CHOH O | or CH_{2}OH·CHOH·CH·CHOH·CHOH·CHOH

| | |

CHOH CH-CHOH

| |

CHO OH

Fischer's Closed-ring formulas

formula

It will be observed that in the closed-ring formula there are five

asymmetric carbon atoms, and the asymmetry of the terminal one forms the

basis for the explanation of the existence of the so-called [alpha] and

[beta] modification of _d_-glucose (see page 46). However, the ordinary

aldehyde reactions of the sugars are more clearly indicated by the

open-chain formula. Some investigators are inclined to be that sugars

actually exist in the open-chain arrangement when in aqueous solution, and

in the closed-ring arrangement when in alcoholic solution. The closed-ring

formulas will be used in this text in the discussions of the birotation

phenomena and of biochemical properties, but for the explanations of the

stereo-isomeric forms and similar phenomena, the open-chain formulas are

just as useful in conveying an idea of the possibilities of different space

relationships, and are so much simpler in appearance and in mechanical

preparation, that it seems desirable to use these rather than the more

accurate closed-ring formulas.

CHEMICAL CONSTITUTION OF MONOSACCHARIDES

The term "monosaccharides," as commonly used, refers to hexoses. It applies

equally well, however, to any other sugar-like substance which either

occurs naturally or results from the decomposition of more complex

carbohydrates, and which cannot be further broken down without destroying

its characteristic aldehyde-alcohol groups and sugar-like properties.

All such monosaccharides, being alcohol-aldehydes, can easily be reduced to

the corresponding polyatomic alcohols, containing the same number of carbon

atoms as the original monosaccharides, each with one OH group attached to

it. All aldose monosaccharides are converted, by gentle oxidation, into the

corresponding monobasic acid, having a COOH group in the place of the

original CHO group. Further oxidation either changes the alcoholic groups

into COOH groups, producing polybasic acids, or breaks up the chain. When

ketose monosaccharides are submitted to similar oxidation processes, they

are broken down into shorter chain compounds.

The various monosaccharides which have thus far been found as constituents

of plant tissues, or as parts of other more complex compounds which occur

in plants, are shown in the following table:

_Trioses_ (C_{3}H_{6}O_{3}) _Tetroses_ (C_{4}H_{8}O_{4})

Aldose--Glyceric aldehyde, Aldoses--_d_- and _l_-Erythrose,

or glycerose _l_-Threose

Ketose--Dioxyacetone

_Pentoses_ (C_{5}H_{10}O_{5}) _Methyl Pentoses_ (C_{6}H_{12}O_{5})

Aldoses--_d_- and _l_-Arabinose Aldoses--Rhamnose

_d_- and _l_-Xylose Fucose

_l_-Ribose Rhodeose

_l_-Lyxose Chinovose

_Hexoses_ (C_{6}H_{12}O_{6})

Mannitol series Dulcitol series

Aldoses--_d_- and _l_-Glucose _d_- and _l_-Galactose

_d_- and _l_-Mannose _d_- and _l_-Talose

_d_- and _l_-Gulose

_d_- and _l_-Idose

_d_-Altrose

_d_-Allose

Ketoses-- _d_-Fructose _d_-Tagatose

_d_-Sorbose

_Heptoses_ _Octoses_ _Nonoses_

(C_{7}H_{14}O_{7}) (C_{8}H_{16}O_{8}) (C_{9}H_{18}O_{9})

Glucoheptose Gluco-octose Glucononose

Mannoheptose Manno-octose Mannononose

Galactoheptose Galacto-octose

Persuelose

Sedoheptose

The hexoses are by far the most important group of monosaccharides. They

are undoubtedly the first products of photosynthesis, and all the other

carbohydrates may be considered to be derived from them by condensation.

Because of their biochemical significance and their immense importance as

the fundamental substances for all plant and animal energy-producing

materials, the following detailed studies of their chemical composition and

molecular configuration are fully warranted.

That all the hexoses contain five alcoholic groups is proved by the

experimental evidence that each one forms a penta-ester, by uniting with

five acid radicals, when treated with mineral or organic acids under proper

conditions. Thus, glucose penta-acetate, penta-nitrate, penta-benzoate,

etc., have all been prepared. The presence of the aldehyde group is proved

by the fact that all aldohexoses have been converted, by gentle oxidation,

into pentaoxy-monobasic acids, and the ketohexoses broken down into shorter

chain compounds by similar gentle oxidations; these reactions being

characteristic of compounds containing an aldehyde and a ketone group

respectively. This experimental evidence establishes the nature of the

characteristic groups in the molecule, in each case.

The molecular configurations illustrated in the following table are those

suggested by Emil Fischer, as a result of his exhaustive studies of the

chemical constitution of the various carbohydrates. There is, of course, no

thought that the printed formulas here presented accurately represent the

actual relationships in space of the different groups; but there is fairly

conclusive evidence that the variations in special groupings in the

different sugars are properly referable to the particular asymmetric carbon

atoms as indicated in the several formulas as presented.

1. Aldohexoses of the mannitol series:

_d_-Glucose _l_-Glucose _d_-Mannose _l_-Mannose

CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH

| | | |

H-C-OH HO-C-H H-C-OH HO-C-H

| | | |

H-C-OH HO-C-H H-C-OH HO-C-H

| | | |

HO-C-H H-C-OH HO-C-H H-C-OH

| | | |

H-C-OH HO-C-H HO-C-H H-C-OH

| | | |

CHO CHO CHO CHO

_d_-Gulose _l_-Gulose _d_-Idose _l_-Idose

CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH

| | | |

HO-C-H H-C-OH HO-C-H H-C-OH

| | | |

H-C-OH HO-C-H H-C-OH HO-C-H

| | | |

HO-C-H H-C-OH HO-C-H H-C-OH

| | | |

HO-C-H H-C-OH H-C-OH HO-C-H

| | | |

CHO CHO CHO CHO

_d_-Altrose _l_-Altrose _d_-Allose _l_-Allose

(unknown) (unknown)

CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH

| | | |

H-C-OH HO-C-H H-C-OH HO-C-H

| | | |

HO-C-H H-C-OH H-C-OH HO-C-H

| | | |

HO-C-H H-C-OH H-C-OH HO-C-H

| | | |

HO-C-H H-C-OH H-C-OH HO-C-H

| | | |

CHO CHO CHO CHO

2. Aldohexoses of the dulcitol series:

_d_-Galactose _l_-Galactose _d_-Talose _l_-Talose

CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH

| | | |

H-C-OH HO-C-H H-C-OH HO-C-H

| | | |

HO-C-H H-C-OH HO-C-H H-C-OH

| | | |

HO-C-H H-C-OH HO-C-H H-C-OH

| | | |

H-C-OH HO-C-H HO-C-H H-C-OH

| | | |

CHO CHO CHO CHO

3. Ketohexoses:

_d_-Fructose _d_-Sorbose _d_-Tagatose

CH_{2}OH CH_{2}OH CH_{2}OH

| | |

H-C-OH HO-C-H H-C-OH

| | |

H-C-OH H-C-OH HO-C-H

| | |

HO-C-H HO-C-H HO-C-H

| | |

C=O C=O C=O

| | |

CH_{2}OH CH_{2}OH CH_{2}OH

Reference will be made in subsequent paragraphs to the probable chemical

constitution of the monosaccharides other than hexoses; but the above

discussion of the structure of the hexoses will serve as a sufficient

introduction to the study of the composition of the common carbohydrates.

CHARACTERISTIC REACTIONS OF HEXOSES

=Specific Rotatory Power.=--All soluble carbohydrates, since they contain

asymmetric carbon atoms, with the consequent larger groups on one side of

the molecule than the other, rotate the plane of polarized light when it

passes through a solution of the carbohydrate in question. The amount of

the rotation depends upon the nature of the carbohydrate, the concentration

of the solution, and the length of the column of solution through which the

ray of polarized light passes. But the same definite amount of the same

sugar, dissolved in the same volume of water, and placed in a tube of the

same length, will always cause the same angular deviation, or rotation, of

the plane in which the polarized light which passes through it is vibrated.

In other words, the same number of molecules of the optically active

substance in solution will always produce the same rotatory effect. This is

called the specific rotatory power of the substance in question. It is

expressed as the number of degrees of angular deviation of the plane of

polarized light caused by a column of the solution exactly 200 mm. in

length, the concentration of the solution being 100 grams of substance in

100 cc. at a temperature of 20° C. Actual determinations of specific

rotatory power are usually made with solutions more dilute than this

standard, and the observed deviation multiplied by the proper factor to

determine the effect which would be produced by the solution of standard

concentration. If the direction of the deviation is to the right (i.e., in

the direction in which the hands of the clock move) it is spoken of as

"dextro" rotation and is indicated by the sign +, or the letter _d_; while

if in the opposite direction, it is called "levo" rotation and indicated by

the sign -, or the letter _l_. For example, the specific rotation of

ordinary glucose is +52.7°; of fructose, -92°; of sucrose, +66.5°.

=Reducing Action.=--All of the hexose sugars are active reducing agents.

This is because of the aldehyde group which they contain. Many of the

common heavy metals, when in alkaline solutions, are strongly reduced when

boiled with solutions of the hexose sugars. Alkaline copper solutions yield

a precipitate of red cuprous oxide; ammoniacal silver solutions give silver

mirrors; alkaline solutions of mercury salts are reduced to metallic

mercury, etc. Any sugar which contains a potentially active aldehyde group

will exhibit this reducing effect and is known as a "reducing sugar." In

some of the di- and tri-saccharides, the linkage of the hexose components

together is through the aldehyde group, in such a way that it loses its

reducing effect; such sugars are known as "non-reducing." Advantage is

taken of this property for both the detection and quantitative

determination of the "reducing sugars." A standard alkaline copper solution

of definite strength, known as "Fehling's solution," is added to the

solution of the sugar to be tested and the mixture boiled, when the

characteristic brick-red precipitate appears. If certain standard

conditions of volume of solutions used, length of time of boiling, etc.,

are observed, the quantity of cuprous oxide precipitated bears a definite

ratio to the amount of sugar which is present, so that if the precipitate

be filtered off and weighed under proper conditions, the weight of sugar

present in the original solution can be calculated. The proper conditions

for carrying on such a determination and tables showing the amounts of the

various "reducing sugars" which correspond to the weight of cuprous oxide

found, are given in all standard text-books dealing with the analysis of

organic compounds.

=Fermentability.=--The common hexoses are all easily fermented by yeast,

forming alcohol and carbon dioxide, according to the equation

C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}.

The importance and biochemical significance of this reaction will be

considered in detail in connection with the discussions of the relation of

molecular configuration to biochemical properties (see page 56) and the

nature of enzyme action (see page 194).

=Formation of Hydrazones and Osazones.=--Another property of the hexoses

which is due to the presence of an aldehyde group in the molecule, is that

of forming addition products with phenyl hydrazine, known as "hydrazones"

and "osazones." For example, glucose reacts with phenyl hydrazine in acetic

acid solution, in two stages. The first, which takes place even in a cold

solution may be represented by the equation

C_{6}H_{12}O_{6}+C_{6}H_{5}·NH·NH_{2} = C_{6}H_{12}O_{5}:N·NH·C_{6}H_{5}

Glucose Phenyl-hydrazine Glucose-hydrazone + H_{2}O.

The structural relationships involved may be represented as follows:

CHO H_{2}N·NH CH=N-------NH

| /\ | /\

(CHOH)_{4} + | | = (CHOH)_{4} | |

| | | | | |

CH_{2}OH \/ CH_{2}OH \/

The hydrazones of the common sugars, with the exception of the one from

mannose, are colorless compounds, easily soluble in water. Hence, they do

not serve for the separation or identification of the individual sugars.

But if the solution in which they are formed contains an excess of phenyl

hydrazine and is heated to the temperature of boiling water for some time,

the alcoholic group next to the aldehyde group (the terminal alcohol group

in ketoses) is first oxidized to an aldehyde and then a second molecule of

phenyl hydrazine is added on, as illustrated above, forming a

di-addition-product, known as an "osazone." The osazones are generally more

or less soluble in hot water, but on cooling they crystallize out in yellow

crystalline masses each with definite melting point and crystalline form.

All sugars which have active aldehyde groups in the molecule form osazones.

These afford excellent means of identification of unknown sugars, or of

distinguishing between sugars of different origin and type.

Glucose, mannose, and fructose all form identical osazones. This is because

the structure of these three sugars is identical except for the arrangement

within the two groups at the aldehyde end of the molecule (see formulas on

page 44). Since it is to these two groups that the phenyl hydrazine residue

attaches itself, it follows that the resulting osazones must be identical

in structure and properties. All other reducing sugars yield osazones of

different physical properties.

When an osazone is decomposed by boiling with strong acids, the phenyl

hydrazine groups break off, leaving a compound containing both an aldehyde

and a ketone group. Such compounds are known as "osones." The osones from

glucose, mannose, and fructose are identical. By carefully controlled

reduction, either one of the C==O groups of the osone may be changed to an

alcoholic group, producing thereby one of the original sugars again. Hence,

it is possible to start with one of these sugars, convert it into the osone

and then reduce this to another sugar, thereby accomplishing the

transformation of one sugar into another isomeric sugar.

=Formation of Glucosides.=--By treatment with a considerable variety of

different types of compounds, under proper conditions, it is possible to

replace one of the hydrogen atoms of the terminal alcoholic group of the

hexose sugars with the characteristic group of the other substance, forming

compounds known, respectively, as glucosides, fructosides, galactosides,

etc. The structural relation of methyl glucoside to glucose, for example,

may be illustrated as follows:

Glucose (C_{6}H_{12}O_{6}) Methyl Glucoside (C_{7}H_{14}O_{6})

CHO CHO

| |

(CHOH)_{4} (CHOH)_{4}

| |

CH_{2}OH CHOH

|

CH_{3}

A general formula for glucosides is R·(CHOH)_{5}·CHO; and the R may

represent a great variety of different organic radicals (see the chapters

dealing with Glucosides and with Tannins). When the glucosides are

hydrolyzed, they yield glucose and the hydroxyl compound of the radical

with which it is united. All the statements which have been made with

reference to glucosides, apply equally well with reference to fructosides,

galactosides, mannosides, etc.

It is possible, by various laboratory processes, to replace additional

hydrogen atoms in the glucose molecule with the same or other organic

radicals, thus producing glucosides containing two or more R groups; but

most of the natural glucosides contain only one other characteristic group.

=Oxidations.=--When the hexoses are oxidized they give rise to three

different types of acids, depending upon the conditions of the oxidation

and the kind of oxidizing agent used. With glucose, for example, the

relationships involved may be illustrated as follows:

CHO COOH CHO COOH

| | | |

(CHOH)_{4} (CHOH)_{4} (CHOH)_{4} (CHOH)_{4}

| | | |

CH_{2}OH CH_{2}OH COOH COOH

Glucose Gluconic acid Glucuronic acid Saccharic acid

An important property of the acids of the _gluconic_ type is that when

heated with pyridine or quinoline to 130°-150° they undergo a molecular

rearrangement whereby the acid corresponding to an isomeric sugar is

produced. For example, gluconic acid, under these conditions, becomes

mannonic acid, which can be reduced to mannose. The process is reversible;

mannose can be converted to mannonic acid, thence to gluconic acid, thence

to glucose. Similarly, galactonic acid can be converted into talonic acid,

and this to talose, and this process is reversible. These facts afford

another means of conversion of one sugar into another.

From the standpoint of physiological processes, _glucuronic acid_ is the

most interesting and important oxidation product of glucose. It is often

found in the urine of animals, as the result of the partial oxidation of

glucose in the animal tissues. Normally, glucose is oxidized in the body to

its final oxidation products, carbon dioxide and water. But when many

difficultly oxidizable substances, such as chloral, camphor, turpentine

oil, aniline, etc., are introduced into the body, the organism has the

power of combining these with glucose to form glucosides. These so-called

"paired" compounds are then oxidized to the corresponding glucuronic acid

derivatives and eliminated from the body in the urine. No phenomenon

similar to this occurs in plants, however, and glucuronic acid has never

been found in plant tissues.

=Synthesis and Degradation of Hexoses.=--Monosaccharides of any desired

number of carbon atoms can be produced from aldoses having one less carbon

atoms, by way of the familiar "nitrile" reaction. Aldoses, like all other

aldehydes, combine directly with hydrocyanic acid, forming compounds known

as nitriles, which contain one more carbon atom than was present in the

original aldehyde; the cyanogen group can easily be converted into a COOH

group; and this, in turn, reduced to an aldehyde, thus producing an aldose

with one more carbon atom than was present in the initial sugar. These

changes may be illustrated by the following equations:

(1) CHO + HCN CHOH·CN CN

| | |

(CHOH)_{3} = (CHOH)_{3} or (CHOH)_{4}

| | |

CH{2}OH CH_{2}OH CH_{2}OH

Aldopentose Nitrile

(2) CN + H_{2}O COOH + NH_{3}

| |

(CHOH)_{4} = (CHOH)_{4}

| |

CH_{2}OH CH_{2}OH

Nitrile Acid

(3) COOH - O CHO

| |

(CHOH)_{4} = (CHOH)_{4}

| |

CH_{2}OH CH_{2}OH

Acid Aldohexose

It is possible, by this process, to advance step by step from formaldehyde

to higher sugars, Emil Fischer and his students having carried the process

as far as the production of glucodecose (C_{10}H_{20}O_{10}). It usually

happens, however, that two stereo-isomers result from the "step-up" by way

of the nitrile reaction; thus, arabinose yields a mixture of glucose and

mannose, glucose yields glucoheptose and mannoheptose, etc.

The reverse process, or the so-called "degradation" of a sugar into another

containing fewer carbon atoms, may be readily accomplished in either one or

two ways. In Wohl's process, the aldehyde group of the sugar is first

converted into an _oxime_, by treatment with hydroxylamine; the oxime, on

being heated with concentrated sodium hydroxide solution, splits off water

and becomes the corresponding _nitrile_; this, on further heating, splits

off HCN and yields an aldose having one less carbon atom than the original

sugar. This process is the exact reverse of the nitrile synthesis,

described above. The second method of degradation, suggested by Ruff, makes

use of Fenton's method of oxidizing aldehyde sugars to the corresponding

monobasic acid, using hydrogen peroxide and ferrous sulfate as the

oxidizing mixture; the _aldonic acid_ thus formed is then converted into

its calcium salt, which, when further oxidized, splits off its carboxyl

group and one of the hydrogens of the adjacent alcoholic group, leaving an

aldose having one less carbon atom than the original aldose sugar.

=Enolic Forms.=--A final avenue for the interconversion of glucose,

mannose, and fructose into one another, is through the spontaneous

transformations which these undergo when dissolved in water containing

sodium hydroxide or potassium hydroxide. This change is due to the

conversion of the sugar, in the alkaline solution, into an _enol_, which is

identical for all three sugars, and which may subsequently be reconverted

into any one of the three isomeric hexoses. The relationships involved are

illustrated in the following formulas:

CHO CHO CH_{2}OH CHOH

| | | ¦

H-C-OH HO-C-H C=O C-OH

| | | |

HO-C-H HO-C-H HO-C-H HO-C-H

| | | |

H-C-OH H-C-OH H-C-OH H-C-OH

| | | |

H-C-OH H-C-OH H-C-OH H-C-OH

| | | |

CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH

Glucose Mannose Fructose Enolic Form

The preceding technical discussion of the chemical constitution and

reactions of the hexoses has been presented, not because it has any direct

connection with the occurrence or functions of these compounds in plant

tissues, but for the purpose of giving to the student a graphic conception

of the structure and properties of these simple carbohydrates, as a basis

for the understanding of the nature, properties, possible chemical

reactions, syntheses, etc., of the more complex types of carbohydrates,

which, along with these simple monosaccharides, constitute the most

important single group of organic components of plants.

THE OCCURRENCE AND PROPERTIES OF MONOSACCHARIDES

Only two monosaccharides occur as such in plants. These are glucose and

fructose. All the other hexoses, whose structure is shown on pages 37 and

38, occur in plants only as constituents of the more complex saccharides,

in glucoside-formations, or as the corresponding polyatomic alcohols.

The aldo-hexoses which occur most commonly in plants, either free or in

combination, are _d_-glucose, _d_-mannose, and _d_-galactose; while

_d_-fructose and _d_-sorbose are the common keto-hexoses.

=Glucose= (often called also dextrose, fruit sugar, or grape sugar) occurs

widely distributed in plants, most commonly in the juices of ripening

fruits, where it is usually associated with fructose and sucrose, the two

hexoses being easily derived from sucrose by hydrolysis. Glucose is also

produced by the hydrolysis of many of the more complex carbohydrates, by

the action either of enzymes or of dilute acids; lactose, maltose,

raffinose, starch, and cellulose, as well as many glucosides all yielding

glucose as one of the products of their hydrolysis. In all such cases, it

is _d_-glucose which is obtained.

Glucose is a crystalline solid (although it does not form such sharply

defined crystals as does sucrose, or "granulated sugar"), which is easily

soluble in water. It usually appears on the market in the form of thick

syrups, which are produced commercially by the hydrolysis of starch with

dilute sulfuric acid, removal of the acid after the hydrolysis is complete,

and evaporation of the resulting solution to the desired syrupy

consistency. (Since corn starch is commonly used as the raw material for

this process, these syrups are often spoken of as "corn syrup.") The

sweetness of glucose is about three-fifths that of ordinary cane sugar.

Glucose exhibits all the properties of hexoses which have been described in

general terms above. It is a reducing-sugar, and is easily fermented. The

specific rotatory power of _d_-glucose is +52.7°. But when glucose is

dissolved in water, it exhibits in a marked degree the phenomenon known as

"mutarotation"; that is, freshly made solutions exhibit a certain definite

rotatory power, but this changes rapidly until it finally reaches another

definite specific rotation. In other words, glucose is "birotatory," or

possesses two distinct specific rotatory powers, and the changing rotation

effect in aqueous solutions is due to the change from one form to the

other. When dissolved in alcohol, it does not exhibit this change in

rotatory power. In order to explain this phenomenon, it is necessary to

assume that there are two modifications of _d_-glucose, which have been

designated respectively as the [alpha] and [beta] forms. The possibility of

the existence of these two forms is explained by the assumption of the

closed-ring arrangement of the glucose molecule, as indicated in the

following formulas which represent the two possible isomeric arrangements:

HO-C-H H-C-OH

/ \ / \

/ \ / \

H-C-OH \ H-C-OH \

| O | O

HO-C-H / HO-C-H /

\ / \ /

\ / \ /

C-H C-H

| |

H-C-OH H-C-OH

| |

CH_{2}OH CH_{2}OH

[alpha]-Glucose [beta]-Glucose

It is assumed that the [alpha] modification (with its specific rotatory

power of +105°) is the normal form for crystalline glucose, but that when

dissolved in water it is changed into an _aldehydrol_, i.e., a compound

containing two additional OH groups, which later breaks down again, into

the [beta] modification (with its specific rotatory power of +22°). When

dissolved in alcohol, this change does not take place because of the

absence of the excess of water necessary to produce the intermediate

aldehydrol form.

There are other examples of the existence of the [alpha] and [beta]

modification of glucose. For example, [alpha]-methyl-glucoside and

[beta]-methyl-glucoside (specific rotatory powers, +157° and -33°,

respectively) are both known, as well as several other similar glucoside

arrangements.

=Mannose.=--This sugar does not occur as such in plants; but complex

compounds which yield _d_-mannose when hydrolyzed, known as "mannosans,"

are found in a number of tropical plant forms. The mannose which is

obtained from these by hydrolysis is very similar to glucose in its

properties, forms the same osazones as do glucose and fructose, exhibits

mutarotation, etc. Mannose may also be obtained by oxidizing mannitol, a

hexatomic alcohol, known as "mannite," which occurs in many plants,

especially in the manna-ash (_Fraxinus ornus_), the dried sap from which is

known as "manna."

=Galactose= occurs in the animal kingdom as one of the constituents of

lactose, or milk-sugar. It is also one of the constituents of raffinose, a

trisaccharide sugar found in plants, and occurs as "galactans" in many gums

and sea-weeds. The _d_-galactose, obtained by the hydrolysis of any of

these compounds, is a faintly sweet substance which resembles glucose in

many of its properties; having one characteristic difference, however, in

that it forms mucic acid instead of saccharic acid when oxidized by

concentrated nitric acid. These oxidation products are very different in

their physical properties and this difference serves to distinguish between

the two sugars from which they are derived.

=Fructose= (levulose, honey sugar, or "diabetic" sugar) occurs along with

glucose in the juices of many fruits, etc. It is a constituent of sucrose,

of raffinose, and of the polysaccharide inulin, from which it may be

obtained by hydrolysis. It is a ketose sugar, reduces Fehling's solution,

forms the same osazone as glucose, and is easily fermentable by yeast. Its

sweetness is slightly greater than that of ordinary cane sugar.

_d_-fructose (the ordinary form) is easily soluble in water, and is

strongly levorotatory, its specific rotatory power at 20° C. being -92.5°;

it is unique in the very large effect which is produced in its rotatory

power by increasing the temperature of the solution; at 87° its specific

rotatory power is reduced to -52.7°, exactly equal to but in the opposite

direction of the effect of glucose; hence, _invert sugar_, which is a

mixture of an equal number of molecules of glucose and fructose, and which

has a specific rotatory power of -19.4° at 20° C., becomes optically

inactive at 82° C.

=Sorbose= is the only other ketohexose which has any importance in plant

chemistry. It does not occur free in plants, but is the first oxidation

product from the hexatomic alcohol, sorbitol, which is present in the juice

of the berries of the mountain-ash. Sorbose is a crystalline solid, which

is not fermentable by yeast, but which otherwise closely resembles

fructose.

DISACCHARIDES

The disaccharides, having the formula C_{12}H_{22}O_{11}, may be regarded

as derived from the monosaccharides by the linking together of two hexose

groups with the dropping out of a molecule of water, in the same way that

many other organic compounds form such linkages. That this is a perfectly

correct conception, is shown by the fact that, when hydrolyzed, the

disaccharides break down into two hexose sugars, thus

C_{12}H_{22}O_{11} + H_{2}O = C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}.

With all known disaccharides, at least one of the hexoses obtained by

hydrolysis is glucose; hence all disaccharides may be regarded as

glucosides (C_{6}H_{12}O_{5}·R) in which the R is another hexose group.

Since hexoses have both alcoholic and aldehyde groups, and since either of

these types of groups may function in the linkage of the two hexoses to

form a disaccharide, it is possible for two hexoses, both of which are

reducing sugars to be linked together in three different ways: (1) through

an alcoholic group of each hexose, (2) through an alcoholic group of one

and the aldehyde group of the other, and (3) through the aldehyde group of

each hexose. Disaccharides linked in either of the first two ways will be

reducing sugars, since they still contain a potentially active aldehyde

group; but those of the third type will not be reducing sugars, since the

linkage through the aldehyde groups destroys their power of acting as

reducing agents. Examples of each of these three types of linkage are found

among the common disaccharides, as will be pointed out below.

The following table shows the general characteristics of the common

disaccharides.

_Type 1.--Aldehyde group potentially active, reducing sugars:_

Sugar Components

Maltose Glucose and glucose

Gentiobiose Glucose and glucose

Lactose Glucose and galactose

Melibiose Glucose and galactose

Turanose Glucose and fructose

_Type 2.--Non-reducing sugars:_

Sucrose Glucose and fructose

Trehalose Glucose and glucose

The disaccharides of Type 1 reduce Fehling's solution and form hydrazones

and osazones, although somewhat less readily than do the hexoses. They all

show mutarotation and exist in two modifications, indicating that the

component groups have the closed-ring arrangement.

The disaccharides of Type 2, since they contain no potentially active

aldehyde group, do not reduce Fehling's solution, nor form osazones;

neither do they exhibit mutarotation. The only disaccharides which occur as

such in plants are of this type. Disaccharides of Type 1 may be obtained by

the hydrolysis of other, more complex, carbohydrates.

All disaccharides are easily hydrolyzed into mixtures of their component

hexoses, by boiling with dilute mineral acids, or by treatment with certain

specific enzymes which are adapted to the particular disaccharide in each

case (see page 55, also Chapter XIV).

=Sucrose= (cane sugar, beet sugar, maple sugar) is the ordinary "granulated

sugar" of commerce. It occurs widely distributed in plants, where it serves

as reserve food material. It is found in largest proportions in the stalks

of sugar cane, in the roots of certain varieties of beets, and in the

spring sap of maple trees, all of which serve as industrial sources for the

sugar. In the sugar cane, and beet-roots, it constitutes from 12 to 20 per

cent of the green weight of the tissue and from 75 to 90 per cent of the

soluble solids in the juice which can be expressed from it. Its universal

use as a sweetening agent is due to the combined facts that it crystallizes

readily out of concentrated solutions and, hence, can be easily

manufactured in solid form, and that it is sweeter than any other of the

common sugars except fructose.

Sucrose is a non-reducing sugar, forms no osazone, and is not directly

fermentable by yeast, although most species of yeasts contain an enzyme

which will hydrolyze sucrose into its component hexoses, which then readily

ferment.

When hydrolyzed by acids, or by the enzyme "invertase," it yields a mixture

of equal quantities of glucose and fructose. Sucrose is dextrorotatory, but

since fructose has a greater specific rotatory action to the left than

glucose has to the right, the mixture resulting from the hydrolysis of

sucrose is levorotatory. Since the hydrolysis of sucrose changes the

rotatory effect of the solution from the right to the left, the process is

usually called the "inversion" of sucrose, and the resultant mixture of

equal parts of glucose and fructose is called "invert sugar." As has been

pointed out, solutions of invert sugar become optically inactive when

heated to 82° C., because of the reduction in the rotatory power of

fructose due to the higher temperature.

The probable linkage of the two hexoses to form sucrose, in such a way as

to produce a non-reducing sugar, is illustrated in the following formula:

------O-------

| |

CH_{2}OH·CHOH·CH·CHOH·CHOH·CH

|

O

|

CH_{2}OH·CHOH·CHOH·CH·C·CH_{2}OH

\ /

O

=Trehalose= seems to serve as the reserve food for fungi in much the same

way that sucrose does for higher plants. It is composed of two molecules of

glucose linked together through the aldehyde group of each, as trehalose is

a non-reducing sugar. This linkage is illustrated in the following formula:

------O-------

| |

CH_{2}OH·CHOH·CH·CHOH·CHOH·CH

|

O

|

CH_{2}OH·CHOH·CH·CHOH·CHOH·CH

| |

------O-------

Trehalose may be hydrolyzed into glucose by dilute acids and by the enzyme

"trehalase," which is contained in many yeasts and in several species of

fungi. It is strongly dextrorotatory (specific rotatory power, +199°). It

is not fermentable by yeast.

Trehalose appears to replace sucrose in those plants which contain no

chlorophyll and do not elaborate starch. The quantity of trehalose in such

plants reaches a maximum just before spore formation begins. Since it is

manufactured in the absence of chlorophyll, its formation must be

accomplished by some other means than photosynthesis, yet it is composed

wholly of glucose--a natural photosynthetic product.

=Maltose= rarely occurs as such in plants, although its presence in the

cell-sap of leaves has sometimes been reported. It is produced in large

quantities by the hydrolysis of starch during the germination of barley and

other grains. This hydrolysis is brought about by the enzyme "diastase,"

which is present in the sprouting grain.

Maltose is easily soluble in water, and crystallizes in masses of slender

needles. It is a reducing sugar; readily forms a characteristic osazone; is

strongly dextrorotatory (specific rotatory power +137°); and is readily

fermented by ordinary brewer's yeast, which contains both "maltase" (the

enzyme which hydrolyzes maltose to glucose) and "zymase" (the

alcohol-producing enzyme). When hydrolyzed, either by dilute acids or by

maltase, one molecule of maltose yields two molecules of glucose. Its

component hexoses are, therefore, the same as those of trehalose, a

non-reducing sugar, this difference in properties being due to the

difference in the point of linkage between the two glucose molecules, that

for maltose being such as to leave one of the aldehyde groups potentially

active, as shown in the following formula,

-------O------

| |

CH_{2}OH·CHOH·CH·CHOH·CHOH·CH

|

O

|

CHOH·CHOH·CHOH·CH·CHOH·CH_{2}

| |

----------O-----

=Isomaltose= is a synthetic sugar, obtained by Fischer, by condensing

two molecules of glucose. Its properties are quite similar to those of

maltose, but it yields a slightly different osazone and is not fermentable

by yeast. These differences are explained by the assumption that this

sugar is a glucose-[beta]-glucoside, while normal maltose is a

glucose-[alpha]-glucoside.

=Gentiobiose= is a disaccharide which results from the partial hydrolysis

of the trisaccharide _gentianose_ (see page 53). It is very similar in its

general properties to isomaltose. =Cellobiose= is a disaccharide which

results from the hydrolysis of cellulose. It is a reducing sugar, forms an

osazone, and resembles maltose.

Maltose, isomaltose, gentiobiose, and cellobiose, are all

glucose-glucosides, the difference between them being undoubtedly due to

linkage being between different alcoholic groups in the glucose molecules.

The disaccharide =lactose= is a glucose-galactoside. It is the sugar which

is present in the milk of all mammals. It has never been found in plants.

=Melibiose=, which is the corresponding vegetable glucose-galactoside, may

be obtained by the partial hydrolysis of the trisaccharide _raffinose_ (see

below). It is a reducing sugar; forms a characteristic osazone; and

exhibits mutarotation. It is not fermented by ordinary top-yeasts, but is

first hydrolyzed and then fermented by the enzymes present in

bottom-yeasts.

TRISACCHARIDES

Trisaccharides, as the name indicates, consist of three hexoses (or

monosaccharides) linked together by the dropping out of two molecules of

water. Their formula is C_{18}H_{32}O_{16}. When completely hydrolyzed,

they yield three molecules of monosaccharides; when partially hydrolyzed,

one each of a disaccharide and a monosaccharide.

One trisaccharide of the reducing sugar type, namely _rhamnose_, exists in

plants as a constituent of the glucoside xanthorhamnin. It is composed of

one molecule of glucose united to two molecules of rhamnose (methyl

pentose, C_{6}H_{12}O_{5}). It is of interest only in connection with the

properties of the glucoside in which it is present (see page 84).

Three trisaccharides which are non-reducing sugars are found in plants;

namely, raffinose, gentianose, and melizitose.

=Raffinose= occurs normally in cotton seeds, in barley grains, and in

manna; also, in small quantities in the beet root, associated with sucrose.

It is more soluble in water than is sucrose and hence remains in solution

in the molasses from beet-sugar manufacture, which constitutes the

commercial source for this sugar. Raffinose crystallizes out of

concentrated solutions, with five molecules of water of crystallization, in

clusters of glistening prisms. It is strongly dextrorotatory, the anhydrous

sugar having a specific rotatory power of +185°, and the crystalline form,

C_{18}H_{32}O_{16}, showing a specific rotation of +104.5°. It does not

reduce Fehling's solution, nor form an osazone, and in its other properties

it closely resembles sucrose.

The hydrolysis of raffinose presents several interesting possibilities. If

its structure is represented as follows:

C_{6}H_{11}O_{5}-C_{6}H_{10}O_{4}-C_{6}H_{11}O_{5}

Fructose Glucose Galactose

\______ _____/ \_______ ______/

\/ \/

Sucrose Melibiose

it is apparent that it may break down by hydrolysis in three different

ways: (1) into sucrose and galactose, (2) into fructose and melibiose, and

(3) into fructose, glucose, and galactose. As a matter of fact, it does

actually break down in these three different ways, under the influence of

different catalysts; invertase or dilute acids break it down into fructose

and melibiose, emulsin hydrolyzes it to sucrose and galactose, while strong

acids or the enzymes of bottom-yeasts break it down into the three hexoses.

=Gentianose=, a trisaccharide found in the roots of yellow gentian

(_Gentiana lutea_), is a non-reducing sugar, which when hydrolyzed yields

either fructose and gentiobiose, or fructose and two molecules of glucose.

=Melizitose=, a trisaccharide which, in crystallized form, has the formula,

C_{18}H_{32}O_{16}·2H_{2}O, occurs in the sap of _Larix europea_ and in

Persian manna, and has recently been found in considerable quantities in

the manna which collects on the twigs of Douglas fir and other conifers.

When hydrolyzed, it yields one molecule of fructose and one of turanose, a

disaccharide containing fructose and glucose linked together in a slightly

different way than they are in sucrose. Turanose itself is a reducing

sugar, but when linked with fructose to form melizitose its reducing

properties are destroyed. Melizitose is a very sweet sugar.

TETRASACCHARIDES

A complex saccharide, known as _stachyose_, which is found in the tubers of

_Stachys tuberifera_, is said by some investigators to be a tetrasaccharide

and by others to have the formula C_{36}H_{62}O_{31}·7H_{2}O (i.e., a

hexasaccharide). It is a crystalline solid, with a faintly sweetish taste,

and a specific rotatory power of +148°. When hydrolyzed it yields glucose,

fructose, and two (or more) molecules of galactose.

THE RELATION OF THE MOLECULAR CONFIGURATION OF SUGARS

TO THEIR BIOCHEMICAL PROPERTIES

As will be pointed out later (see Chapter XIV), all chemical reactions

which are involved in vital phenomena, including those of plant growth and

metabolism, are controlled by enzymes. The biochemical reactions which the

soluble carbohydrates undergo afford such excellent illustrations of the

relation of the molecular configuration of an organic compound to the

possibility of the action of an enzyme upon it, that it seems desirable to

discuss this relationship at this point, rather than to postpone it until

after the nature of enzyme action has been considered. Undoubtedly, the

student, after he has studied the nature of enzymes and their mode of

action, as presented in Chapter XIV, will find it profitable to return to

this section and review the facts here presented, as illustrating the

principles and mechanism of enzyme action. But a consideration, at this

time, of the relation of the molecular configuration of the sugars to their

biochemical reactions cannot fail to add interest to the study of these

matters from the chemical and biological standpoints.

It has been known for a long time that the dextro- and levo-isomers of a

compound which contains one or more asymmetric carbon atoms are affected

differently by biological agents, such as yeasts, moulds, bacteria, etc.

Pasteur, as early as 1850, showed that the green mould, _Penicillium

glaucum_, when growing in solutions of racemic acid (a mixture of equal

molecules of _d_- and _l_-tartaric acids) uses up only the _d_-acid,

leaving the _l_-form absolutely untouched. Later, it was found that the

same green mould attacks _l_-mandelic acid in preference to the _d_- form;

whereas the yeast, _Saccharomyces ellipsoideus_, exhibits the opposite

preference for these acids.

These observations upon some of the earlier known forms of optically active

organic acids led the way to a general study of this phenomenon as

exhibited by the optically active soluble carbohydrates. The results of

these studies may be considered in connection with the several different

types of reactions which these sugars undergo, as follows:

=Glucoside Hydrolysis.=--As was pointed out in connection with the

discussion of the mutarotation of glucose, this sugar may exist in either

the [alpha] or the [beta] modification. Glucosides of both [alpha] and

[beta] glucose are of common occurrence. The difference in molecular

configuration, in such cases, may be represented by the following formulas:

R-O-C-H H-C-O-R

/ \ / \

/ \ / \

H-C-OH \ H-C-OH \

| O | O

HO-C-H / HO-C-H /

\ / \ /

\ / \ /

C-H C-H

| |

H-C-OH H-C-OH

| |

CH_{2}OH CH_{2}OH

[alpha]-Glucoside [beta]-Glucoside

The radical represented by the R may be either a common alkyl radical (as

CH_{3}, C_{2}H_{5}, etc.), another saccharide group (as in the case of the

disaccharides, trisaccharides, etc.), or some other complex organic group

(as in the case of the natural glucosides described in Chapter VI). But, in

every case, the glucoside is easily hydrolyzed by the enzyme _maltase_ (or

[alpha]-glucase) if the molecular arrangement is that represented by the

[alpha]-attachment, or by the enzyme _emulsin_ (or [beta]-glucase) if the

glucoside is of the [beta] type; but emulsin is absolutely without effect

upon [alpha]-glucosides, and maltase does not produce the slightest change

in [beta]-glucosides. These statements hold true regardless of the nature

of the group which is represented by the R in the formulas above. Hence,

the biochemical properties of the glucosides, so far as their hydrolysis

by the enzymes which are present in many biological agents is concerned,

depends wholly upon the molecular configuration of the glucose itself.

Furthermore, neither the mannosides, which differ from glucosides only in

the arrangement of the H and OH groups attached to one of the asymmetric

carbon atoms in the hexose, nor galactosides in which two such arrangements

are different (see configuration formulas on page 57), are attacked by

either maltase or emulsin. But other enzymes specifically attack other

disaccharides, or polysaccharides, or glucoside-like complexes. For

example, _lactase_ acts energetically upon ordinary lactose and all

other [beta]-galactosides; but not upon any glucoside, mannoside, etc.

Again, neither [alpha]- nor [beta]-xylosides, which correspond with the

above-described glucosides in every particular except that the HCOH group

next the terminal CH_{2}OH group is missing, are hydrolyzed by either

emulsin or maltase.

These instances, selected from among many similar observations, clearly

prove that not only the number and kind of groups in the molecule, but also

the arrangement of the constituent groups in space, must be identical in

order that the compound may be acted upon by any given enzyme acting as a

biological hydrolytic agent.