THE PROXIMATE CONSTITUENTS OF PLANTS.
The substances absorbed by the plant, which are of simple composition,
and contain only two elements, are elaborated within it, and converted
into the many complicated compounds of which its mass is composed. Some
of these, as, for example, the colouring matters of madder and indigo,
the narcotic principle of the poppy, &c., are confined to a single
species, or small group of plants, while others are found in all plants,
and form the main bulk of their tissues. The latter are the only
substances which claim notice in a treatise like the present. They have
been divided into three great classes, of widely different properties,
composition, and functions.
_1st. The Saccharine and Amylaceous Constituents._--These substances are
compounds of carbon, hydrogen, and oxygen, and all possess a certain
degree of similarity in composition, the quantities of hydrogen and
oxygen they contain being always in the proportion required to form
water, so that they may be considered as compounds of carbon and water;
not that it can be asserted that they actually do contain water, as
such, for of that there is no evidence, but only that its elements are
present in the proportion to form it.
_Cellulose._--This substance forms the fundamental part of all plants.
It is the principal constituent of woody fibre, and is found in a state
of purity in the fibre of cotton and flax, and in the pith of plants;
but in wood it is generally contaminated with another substance, which
has received the name incrusting matter, because it is deposited in and
around the cells of which the plant is in part composed. Cellulose is
insoluble in all menstrua, but, when boiled for a long time with
sulphuric acid, is converted into a substance called dextrine. Cellulose
consists of--
From pith of Elder-tree. Spongioles of roots.
Carbon 43.37 43.00
Hydrogen 6.04 6.18
Oxygen 50.59 50.82
------- -------
100.00 100.00
It is represented chemically by the formula, C_{24}H_{21}O_{21}, which
shows it to be a compound of 24 atoms of carbon with 21 of hydrogen and
21 of oxygen.
_Incrusting matter._--Large quantities of this substance enter into the
composition of all plants. Of its chemical nature little is known, as it
cannot be obtained separate from cellulose, but it is analogous to that
substance in its composition, and probably contains hydrogen and oxygen
in the proportion to form water.
_Starch._--Starch is one of the most abundant constituents of plants,
and is found in most seeds, as those of the cereals and the leguminous
plants; in the tubers of the potatoe, the bulbs of tulips, &c. &c. It is
obtained by placing a quantity of wheat flour in a bag, and kneading it
under a gentle stream of water. When the water is allowed to stand, it
deposits the starch as a fine white powder, which, when examined by the
microscope, is found to be composed of minute grains, formed of
concentric layers deposited on one another. These grains vary
considerably in size and structure in different plants; but in the same
plant they are generally so much alike as to admit of their recognition
by a practised observer. They were formerly believed to be composed of
an external coating of a substance insoluble in water, and containing in
their interior a soluble kernel; but this opinion has been refuted, and
distinct evidence been brought to show that the exterior and interior of
the globules are identical in chemical properties. Starch is insoluble
in cold water, but by boiling, it dissolves, forming a thick paste. By
long continued boiling with water containing a small quantity of acid,
it is completely dissolved and converted into dextrine, and eventually
into sugar. The same change is produced by the action of fermenting
substances, such as the extract of malt; when heated in the dry state to
a temperature of about 390 Fahr., it becomes soluble in cold water. It
is distinguished by giving a brilliant blue compound with iodine. Starch
contains--
Carbon 44.47
Hydrogen 6.28
Oxygen 49.25
------
100.00
and its composition is represented by the formula C_{12}H_{10}O_{10}, so
that it differs but little from cellulose in composition, although its
chemical functions in the plant are extremely different. It is connected
with some of the most important changes which occur in the growing
plants, and by a series of remarkable transformations is converted into
sugar and other important compounds.
_Lichen Starch_ is found in most species of lichens, and is
distinguished from common starch by producing a green colour with
iodine. Its composition is the same as that of ordinary starch.
_Inuline._--The species of starch to which this name is given is
characterised by its dissolving in boiling water, and giving a white
pulverulent deposit in cooling. It is found in the tuber of the dahlia,
in the dandelion, and some other plants. Its composition is identical
with that of cellulose, and its formula is C_{24}H_{21}O_{21}.
_Gum_ is excreted from various plants as a thick fluid, which dries up
into transparent masses. Its composition is identical with that of
starch. It dissolves readily in cold water, and is converted into sugar
by long continued boiling with acids. Its properties are best marked in
gum arabic, which is obtained from various species of acacia; that from
other plants differs to some extent, although its chemical composition
is the same.
_Dextrine._--When starch is exposed to a heat of about 400 deg., or when
treated with sulphuric acid, or with a substance extracted from malt
called _diastase_, it is converted into dextrine. It may also be
obtained from cellulose by a similar treatment. The dextrine so obtained
has the same composition as the starch from which it is produced, but
its properties more nearly resemble those of gum. It plays a very
important part in the process of germination, and may be converted into
sugar on the one hand, and apparently also into starch on the other.
_Sugar._--Under this name are included four or five distinct substances,
of which the most important are, cane sugar, grape sugar, and the
uncrystallisable sugar found in many plants.
_Cane Sugar._--This variety of sugar, as its name implies, is found most
abundantly in the sugar cane, but it occurs also in the maple,
beet-root, and various species of palms, from all of which it is
extracted on the large scale. It is extremely soluble in water, and can
be obtained in large transparent prismatic crystals, as in common
sugar-candy. It swells up, and is converted into a brown substance
called caramel, when heated, and by contact with fermenting substances,
yields alcohol and carbonic acid. It contains--
Carbon 42.22
Hydrogen 6.60
Oxygen 51.18
------
100.00
and its chemical formula is C_{12}H_{11}O_{11}.
_Grape Sugar_ is met with in the grape, and most other fruits, as well
as in honey. It is produced artificially when starch is boiled for a
long time with sulphuric acid, or treated with a large quantity of
diastase. It is less soluble in water than cane sugar, and crystallises
in small round grains. Its composition, when dried at 284 deg., is--
Carbon 40.00
Hydrogen 6.66
Oxygen 53.34
------
100.00
and its formula is C_{12}H_{12}O_{12}; but when crystallised it contains
two equivalents of water, and is then represented by the formula
C_{12}H_{12}O_{12} + 2H_{2}O.
The uncrystallisable sugar of plants is closely allied to grape sugar,
and, so far as at present known, has the same composition, although,
from the difficulty of obtaining it quite free from crystallised sugar,
this is still uncertain.
_Mucilage_ is the name applied to the substance existing in linseed,
and in many other seeds, and which communicates to them the property of
swelling up and becoming gelatinous when treated with water. It is found
in a state of considerable purity in gum tragacanth and some other gums.
Its composition is not known with absolute certainty, but it is either
C_{24}H_{19}O_{19}, or C_{12}H_{10}O_{10}; and in the latter case it
must be identical with starch and gum.
It will be observed that all the substances belonging to this class are
very closely related in chemical composition, some of them, as starch
and gum, though easily distinguished by their properties, being
identical in constitution, while others only differ in the quantity of
water, or of its elements which they contain. In fact, they may all be
considered as compounds of carbon and water, and their relations are,
perhaps, more distinctly seen when their formulae are written so as to
show this, as is done in the following table, in the second column of
which those containing twelve equivalents of carbon are doubled, so as
to make them comparable with cellulose:--
Water.
Grape sugar, C_{12}H_{12}O_{12} C_{24}H_{24}O_{24} C_{24} + 24
Cane sugar, C_{12}H_{11}O_{11} C_{24}H_{22}O_{22} C_{24} + 22
Cellulose, C_{24}H_{21}O_{21} C_{24}H_{21}O_{21} C_{24} + 21
Inuline, C_{24}H_{21}O_{21} C_{24}H_{21}O_{21} C_{24} + 21
Starch, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Dextrine, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Gum, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Mucilage, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
The relation between these substances being so close, it is not
difficult to understand how one may be converted into another by the
addition or subtraction of water. Thus, cellulose has only to absorb an
equivalent of water to become grape sugar, or to lose an equivalent in
order to be converted into starch, and we shall afterwards see that
such changes do actually occur in the plant during the process of
germination.
_Pectine and Pectic Acid._--These substances are met with in many fruits
and roots, as, for instance, in the apple, the carrot, and the turnip.
They differ from the starch group in containing more oxygen than is
required to form water along with their hydrogen; but their exact
composition is still uncertain, and they undergo numerous changes during
the ripening of the fruit.
_2d. Oily or Fatty Matters._--The oily constituents of plants form a
rather extensive group of substances all closely allied, but
distinguished by minor differences in properties and constitution. Some
of them are very widely distributed throughout the vegetable kingdom,
but others are almost peculiar to individual plants. They are all
compounds of carbon, hydrogen, and oxygen, and are at once distinguished
from the preceding class, by containing much less oxygen than is
required to form water with their hydrogen. The principal constituents
of the fatty matters and oils of plants are three substances, called
stearine, margarine, and oleine, the two former solids, the latter a
fluid; and they rarely, if ever, occur alone, but are mixed together in
variable proportions, and the fluidity of the oils is due principally to
the quantity of the last which they contain. If olive oil be exposed to
cold, it is seen to become partially solid; and if it be then pressed, a
fluid flows out, and a crystalline substance remains; the former is
oleine, though not absolutely pure, and the latter margarine. The
perfect separation of these substances involves a variety of troublesome
chemical processes; and when it has been effected, it is found that each
of them is a compound of a peculiar acid, with another substance having
a sweet taste, and which has received the name of glycerine, or the
sweet principle of oil. Glycerine, as it exists in the fats, appears to
be a compound of C_{3}H_{2}O, and its properties are the same from
whatever source it is obtained. The acids separated from it are known by
the names of margaric, stearic, and oleic acids.
_Margaric Acid_ is best obtained pure by boiling olive oil with an
alkali until it is saponified, and decomposing the soap with an acid,
expressing the margaric acid, which separates, and crystallising it from
alcohol. It is a white crystalline fusible solid, insoluble in water,
but soluble in alcohol and in solutions of the alkalies. Its composition
is--
Carbon 75.56
Hydrogen 12.59
Oxygen 11.85
------
100.00
and its formula C_{34}H_{34}O_{4}.
_Stearic Acid._--Although this acid exists in many plants, it is most
conveniently extracted from lard. It is a crystalline solid less fusible
than margaric acid, but closely resembling it in its other properties.
Its formula is C_{36}H_{36}O_{4}.
_Oleic Acid._--Under this name two different substances appear to be
included. It has been applied generally to the fluid acids of all oils,
while it would appear that the drying and non-drying oils actually
contain substances of different composition. The acid extracted from
olive oil appears to have the formula C_{36}H_{34}O_{4}, while that from
linseed oil is C_{46}H_{38}O_{6}, but this is still doubtful.
Other fatty acids have been detected in palm oil, cocoa-nut oil, &c.
&c., which so closely resemble margaric and stearic acids as to be
easily confounded with them. Though presenting many points of interest,
it is unnecessary to describe them in detail here.
_Wax_ is a substance closely allied to the oils. It consists of two
substances, cerine and myricine, which are separated from one another by
boiling alcohol, in which the former is more soluble. They are extremely
complex in composition, the former consisting principally of an acid
similar to the fatty acids, called cerotic acid, and containing
C_{54}H_{54}O_{4}. The latter has the formula C_{92}H_{92}O_{4}. The wax
found in the leaves of the lilac and other plants appears to consist of
myricine, while that extracted from the sugar-cane is said to be
different, and to have the formula C_{48}H_{50}O_{2}. It is probable
that other plants contain different sorts of wax, but their
investigation is still so incomplete, that nothing definite can be said
regarding them. Wax and fats appear to be produced in the plant from
starch and sugar; at least it is unquestionable that the bee is capable
of producing the former from sugar, and we shall afterwards see that a
similar change is most probably produced in the plant. The fatty matters
contained in animals are identical with those of plants.
_3d. Nitrogenous or Albuminous Constituents of Plants and Animals._--The
nitrogenous constituents of plants and animals are so closely allied,
both in properties and composition, that they may be most advantageously
considered together.
_Albumen._--Vegetable albumen is found dissolved in the juices of most
plants, and is abundant in that of the potato, the turnip, and wheat. In
these juices it exists in a soluble state, but when its solution is
heated to about 150 deg., it coagulates into a flocky insoluble substance.
It is also thrown down by acids and alcohol. Coagulated albumen is
soluble in alkalies and in nitric acid. Animal albumen exists in the
white of eggs, the serum of blood, and the juice of flesh; and from all
these sources is scarcely distinguishable in its properties from
vegetable albumen.
It is a substance of very complicated composition, and chemists are not
agreed as to the formula by which its constitution is to be expressed, a
difficulty which occurs also with most of the other nitrogenous
compounds. The results of the analyses of albumen from different sources
are however quite identical, as may be seen from those subjoined--
From From From From
Wheat. Potatoes. Blood. White of Egg.
Carbon 53.7 53.1 53.4 53.0
Hydrogen 7.1 7.2 7.0 7.1
Nitrogen 15.6 ... 15.5 15.6
Oxygen } { ... 22.1 22.9
Sulphur } 23.6 { 0.97 1.6 1.1
Phosphorus } { ... 0.4 0.3
----- ----- -----
100.0 100.0 100.0
Closely allied to vegetable albumen is the substance known by the name
of _glutin_, which is obtained by boiling the gluten of wheat with
alcohol. It appears to be a sort of coagulated albumen, with which its
composition completely agrees.
_Vegetable Fibrine._--If a quantity of wheat flour be tied up in a piece
of cloth, and kneaded for some time under water, the starch it contains
is gradually washed out, and there remains a quantity of a glutinous
substance called gluten. When this is boiled with alcohol, the _glutin_
above referred to is extracted, and vegetable fibrine is left. It
dissolves in dilute potash, and on the addition of acetic acid is
deposited in a pure state. Treated with hydrochloric acid, diluted with
ten times its weight of water, it swells up into a jelly-like mass.
When boiled or preserved for a long time under water, it cannot be
distinguished from coagulated albumen.
_Animal Fibrine_ exists in the blood and the muscles, and agrees in all
its characters and composition with vegetable fibrine, as is shown by
the subjoined analyses--
Wheat Flour. Blood. Flesh.
Carbon 53.1 52.5 53.3
Hydrogen 7.0 6.9 7.1
Nitrogen 15.6 15.5 15.3
Oxygen 23.2 24.0 23.1
Sulphur 1.1 1.1 1.2
----- ----- -----
100.0 100.0 100.0
_Caseine._--Vegetable caseine exists abundantly in most plants,
especially in the seeds, and remains in the juice after albumen has been
precipitated by heat, from which it may be separated in flocks by the
addition of an acid. It has been obtained for chemical examination,
principally from peas and beans, and from the almond and oats. When
prepared from the pea it has been called _legumine,_ from almonds
_emulsine_, and from oats _avenine_; but they are all three identical in
their properties, although formerly believed to be different, and
distinguished by these names. Vegetable caseine is best obtained by
treating peas or beans with hot water, and straining the fluid. On
standing, the starch held in suspension is deposited, and the caseine is
retained in solution in the alkaline fluid; by the addition of an acid
it is precipitated as a thick curd. Caseine is insoluble in water, but
dissolves readily in alkalies; its solution is not coagulated by heat,
but, on evaporation, becomes covered with a thin pellicle, which is
renewed as often as it is removed.
_Animal Caseine_ is the principal constituent of milk, and is obtained
by the cautious addition of an acid to skimmed milk, by which it is
precipitated as a thick white curd. It is also obtained by the use of
rennet, and the process of curding milk is simply the coagulation of its
caseine. It is soluble in alkalies, and precipitated from its solution
by acids, and in all other respects agrees with vegetable caseine.
The composition of animal caseine has been well ascertained, but
considerable doubt still exists as to that of vegetable caseine, owing
to the difficulty of obtaining it absolutely pure. The analyses of
different chemists give rather discordant results, but we have given
those which appear most trustworthy--
From Peas.
Carbon 50.6 50.7
Hydrogen 6.8 6.6
Nitrogen 16.5 15.8
Oxygen 25.6 23.8
Sulphur 0.5 0.8
Phosphorus ... 2.3
----- -----
100.0 100.0
Other results differ considerably from these, and some observers have
even obtained as much as eighteen per cent of nitrogen and fifty-three
of carbon.
The composition of animal caseine differs from this principally in the
amount of carbon. Its composition is--
Carbon 53.6
Hydrogen 7.1
Nitrogen 15.8
Oxygen 22.5
Sulphur 1.0
-----
100.0
The most cursory examination of these analytical numbers is sufficient
to show that a very close relation subsists between the different
substances just described. Indeed, with the exception of vegetable
caseine, they may be said all to present the same composition; and, as
already mentioned, there are analyses of it which would class it
completely with the others. While, however, the quantities of carbon,
hydrogen, nitrogen, and oxygen are the same, differences exist in the
sulphur and phosphorus they contain, and which, though very small in
quantity, are indubitably essential to them. Much importance has been
attributed to these constituents by various chemists, and especially by
Mulder, who has endeavoured to make out that all the albuminous
substances are compounds of a substance to which he has given the name
of _proteine_, with different quantities of sulphur and phosphorus. The
composition of proteine, according to his newest experiments, is--
Carbon 54.0
Hydrogen 7.1
Nitrogen 16.0
Oxygen 21.4
Sulphur 1.5
-----
100.0
and is exactly the same from whatever albuminous compound it is
obtained. Although the importance of proteine is probably not so great
as Mulder supposed, it affords an important illustration of the close
similarity of the different substances from which it is obtained, the
more especially as there is every reason to believe that the different
albuminous compounds are capable of changing into one another, just as
starch and sugar are mutually convertible; and the possibility of this
change throws much light on many of the phenomena of nutrition in plants
and animals. Indeed, it would seem probable that these compounds are
formed from their elements by plants only, and are merely assimilated by
animals to produce the nitrogenous constituents they contain.
_Diastase_ is the name applied to a substance existing in malt, and
obtained by macerating that substance with cold water, and adding a
quantity of alcohol to the fluid, when the diastase is immediately
precipitated in white flocks. It is produced during the malting process,
and is not found in the unmalted barley. Its chemical composition is
unknown, but it is nitrogenous, and is believed to be produced by the
decomposition of gluten. If a very small quantity of diastase be mixed
with starch suspended in hot water, the starch is found gradually to
dissolve, and to pass first into the state of dextrine, then into that
of sugar. The change thus effected takes place also in a precisely
similar manner in the plant, diastase being produced during the process
of germination of all seeds and tubers, for the purpose of effecting
this change, and to fulfil other functions less understood, but no doubt
equally important. Diastase is found in the seeds only during the period
when the starch they contain is passing into sugar; as soon as that
change has taken place, its function is ended, and it disappears.