_Laboratory exercise._--Examination of bean in pod.
Examination and identification of parts of bean seed.
_Laboratory demonstration._--Tests for the nutrients:
starch, fats or oils, protein.
_Laboratory demonstration._--Proof that such foods exist in
bean.
_Home work._--Test of various common foods for nutrients.
Tabulate results.
_Extra home work by selected pupils._--Factors necessary for
germination of bean. Demonstration of experiments to class.
_Demonstration._--Oxidation of candle in closed jar. Test
with lime water for products of oxidation.
_Demonstration._--Proof that materials are oxidized within
the human body.
_Demonstration._--Oxidation takes place in growing seeds.
Test for oxidation products. Oxygen necessary for
germination.
_Laboratory exercise._--Examination of corn on cob, the corn
grain, longitudinal sections of corn grain stained with
iodine to show that embryo is distinct from food supply.
_Demonstration._--Test for grape sugar.
_Demonstration._--Grape sugar present in growing corn grain.
_Demonstration._--The action of diastase on starch.
Conditions necessary for action of diastase.
What makes a Seed Grow.--The general problem of the pages that follow will
be to explain how the baby plant, or _embryo_, formed in the seed as the
result of the fertilization of the egg cell, is able to grow into an adult
plant. Two sets of factors are necessary for its growth: first, the
presence of food to give the young plant a start; second, certain
stimulating factors outside the young plant, such as water and heat.
[Illustration: Three views of a kidney bean, the lower one having one
cotyledon removed to show the hypocotyl and plumule.]
If we open a bean pod, we find the seeds lying along one edge of the pod,
each attached by a little stalk to the inner wall of the ovary. If we pull
a single bean from its attachment, we find that the stalk leaves a scar on
the coat of the bean; this scar is called the _hilum_. The tiny hole near
the hilum is called the _micropyle_. Turn back to the figure (page 54)
showing the ovule in the ovary. Find there the little hole through which
the pollen tube reached the embryo sac. This hole is identical with the
micropyle in the seed. The thick outer coat (the _testa_) is easily removed
from a soaked bean, the delicate coat under it easily escaping notice. The
seed separates into two parts; these are called the _cotyledons_. If you
pull apart the cotyledons very carefully, you find certain other structures
between them. The rodlike part is called the _hypocotyl_ (meaning _under
the cotyledons_). This will later form the root (and part of the stem) of
the young bean plant. The first true leaves, very tiny structures, are
folded together between the cotyledons. That part of the plant above the
cotyledons is known as the _plumule_ or _epicotyl_ (meaning _above the
cotyledons_). All the parts of the seed within the seed coats together form
the _embryo_ or young plant. A bean seed contains, then, a tiny _plant_
protected by a tough coat.
Food in the Cotyledons.--The problem now before us is to find out how the
embryo of the bean is adapted to grow into an adult plant. Up to this stage
of its existence it has had the advantage of food and protection from the
parent plant. Now it must begin the battle of life alone. We shall find in
all our work with plants and animals that the problem of food supply is
always the most important problem to be solved by the growing organism. Let
us see if the embryo is able to get a start in life (which many animals get
in the egg) from food provided for it within its own body.
Organic Nutrients.--Organic foods (those which come from living sources)
are made up of two kinds of substances, the _nutrients_ or food substances
and _wastes_ or _refuse_. An egg, for example, contains the white and the
yolk, composed of nutrients, and the shell, which is waste. The organic
nutrients are classed in three groups.
_Carbohydrates_, foods which contain carbon, hydrogen, and oxygen in a
certain fixed proportion (C{6}H{10}O{5} is an example). They are the
simplest of these very complex chemical compounds we call organic
nutrients. Starch and sugar are common examples of carbohydrates.
_Fats and Oils._--These foods are also composed of carbon, hydrogen, and
oxygen in a proportion which enables them to unite readily with oxygen.
_Proteins._--A third group of organic foods, proteins, are the most complex
of all in their composition, and have, besides carbon, oxygen, and
hydrogen, the element nitrogen and minute quantities of other elements.
[Illustration: Starch grains in the cells of a potato tuber.]
Test for Starch.--If we boil water with a piece of laundry starch in a test
tube, then cool it and add to the mixture two or three drops of iodine
solution,[3] we find that the mixture in the test tube turns purple or deep
blue. It has been discovered by experiment that starch, and no _other known
substance_, will be turned purple or dark blue by iodine. Therefore, iodine
solution has come to be used as a test for the presence of starch.
Footnote 3: Iodine solution is made by simply adding a few
crystals of the element iodine to 95 per cent alcohol; or,
better, take by weight 1 gram of iodine crystals, 2/3 gram
of iodide of potassium, and dilute to a dark brown color in
weak alcohol (35 per cent) or distilled water.
[Illustration: Test for Starch.]
Starch in the Bean.--If we mash up a little piece of a bean cotyledon which
has been previously soaked in water, and test for starch with iodine
solution, the characteristic blue-black color appears, showing the presence
of the starch. If a little of the stained material is mounted in water on a
glass slide under the compound microscope, you will find that the starch is
in the form of little ovoid bodies called _starch grains_. The starch
grains and other food products are made use of by the growing plant.
Test for Oils.--If the substance believed to contain oil is rubbed on brown
paper or is placed on paper and then heated in an oven, the presence of oil
will be known by a translucent spot on the paper.
[Illustration: Test for protein.]
Protein in the Bean.--Another nutrient present in the bean cotyledon is
_protein_. Several tests are used to detect the presence of this nutrient.
The following is one of the best known:--
Place in a test tube the substance to be tested; for example, a bit of
hard-boiled egg. Pour over it a little strong (60 per cent) nitric acid and
heat gently. Note the color that appears--a lemon yellow. If the egg is
washed in water and a little ammonium hydrate added, the color changes to a
deep orange, showing that a protein is present.
If the protein is in a liquid state, its presence may be proved by heating,
for when it coagulates or thickens, as does the white of an egg when
boiled, protein in the form of an _albumin_ is present.
Another characteristic protein test easily made at home is burning the
substance. If it burns with the odor of burning feathers or leather, then
protein forms part of its composition.[4]
Footnote 4: Other tests somewhat more reliable, but much
more delicate, are the biuret test and test with Millon's
reagent.
A test of the cotyledon of a bean for protein food with nitric acid and
ammonium hydrate shows us the presence of this food. Beans are found by
actual test to contain about 23 per cent of protein, 59 per cent of
carbohydrates, and about 2 per cent oils. The young plant within a pea or
bean is thus shown to be well supplied with nourishment until it is able to
take care of itself. In this respect it is somewhat like a young animal
within the egg, a bird or fish, for example.
Beans and Peas as Food for Man.--So much food is stored in legumes (as
beans and peas) that man has come to consider them a very valuable and
cheap source of food. Study carefully the following table:--
NUTRIENTS FURNISHED FOR TEN CENTS IN BEANS AND PEAS AT
CERTAIN PRICES PER POUND
=========================================================================
| | TEN CENTS WILL PAY FOR
| PRICES |------------------------------------------
FOOD MATERIALS | PER | TOTAL | | |
AS PURCHASED | POUND | FOOD | PROTEIN | FAT |CARBOHYDRATES
| |MATERIAL | | |
---------------------|--------|---------|---------|--------|-------------
| _Cents_| _Pounds_| _Pounds_|_Pounds_| _Pounds_
Kidney beans, dried | 5 | 2.00 | 0.45 | 0.04 | 1.19
Lima beans, fresh, | | | | |
shelled | 8 | 1.25 | .04 | -- | .12
Lima beans, dried | 6 | 1.67 | .30 | .03 | 1.10
String beans, fresh, | | | | |
30 cents per peck | 3 | 3.33 | .07 | .01 | .23
Beans, baked, canned | 5 | 2.00 | .14 | .05 | .39
Lentils, dried | 10 | 1.00 | .26 | .01 | .59
Peas, green, in pod, | | | | |
30 cents per peck | 3 | 3.33 | .12 | .01 | .33
Peas, dried | 4 | 2.50 | .62 | .03 | 1.55
=========================================================================
[Illustration: A series of early stages in the germination of the kidney
bean.]
Germination of the Bean.--If dry seeds are planted in sawdust or earth,
they will not grow. A moderate supply of water must be given to them. If
seeds were to be kept in a freezing temperature or at a very high
temperature, no growth would take place. A moderate temperature and a
moderate water supply are most favorable for their development.
[Illustration: Bean seedlings. The older seedlings at the left have used up
all of the food supply in the cotyledons.]
If some beans were planted so that we might make a record of their growth,
we would find the first signs of germination to be the breaking of the
testa and the pushing outward of the hypocotyl to form the first root. A
little later the hypocotyl begins to curve downward. A later stage shows
the hypocotyl lifting the cotyledon upward. In consequence the hypocotyl
forms an arch, dragging after it the bulky cotyledons. The stem, as soon as
it is released from the ground, straightens out. From between the
cotyledons the budlike plumule or epicotyl grows upward, forming the first
true leaves and all of the stem above the cotyledons. As growth continues,
we notice that the cotyledons become smaller and smaller, until their food
contents are completely absorbed into the young plant. The young plant is
now able to care for itself and may be said to have passed through the
stages of germination.
What makes an Engine Go.--If we examine the sawdust or soil in which the
seeds are growing, we find it forced up by the growing seed. Evidently work
was done; in other words, _energy_ was released by the seeds. A familiar
example of release of energy is seen in an engine. Coal is placed in the
firebox and lighted, the lower door of the furnace is then opened so as to
make a draft of air which will reach the coal. You know the result. The
coal burns, heat is given off, causing the water in the boiler to make
steam, the engine wheels to turn, and work to be done. Let us see what
happens from the chemical standpoint.
Coal, Organic Matter.--Coal is made largely from dead plants, long since
pressed into its present hard form. It contains a large amount of a
chemical element called carbon, the presence of which is characteristic of
all organic material.
[Illustration: The limewater test. The tube at the right shows the effect
of the carbon dioxide.]
Oxidation, its Results.--When things containing carbon are lighted, they
burn. If we place a lighted candle which contains carbon in a closed glass
jar, the candle soon goes out. If we then carefully test the air in the jar
with a substance known as _limewater_,[5] the latter, when shaken up with
the air in the jar, turns milky. This test proves the presence in the jar
of a gas, known as _carbon dioxide_. This gas is formed by the carbon of
the candle uniting with the oxygen in the air. When the oxygen of the air
in the jar was used up, the flame went out, showing that oxygen is
necessary to make a thing burn. This uniting of oxygen with some other
substance is called _oxidation_.
Footnote 5: Limewater can be made by shaking up a piece of
quicklime the size of your fist in about two quarts of
water. Filter or strain the limewater into bottles and it is
ready for use.
[Illustration: Diagram to show that when a piece of wood is burned it forms
water and carbon dioxide.]
Oxidation possible without a Flame.--But a flame is not necessary for
oxidation. Iron, if left in a damp place, becomes rusty. A union between
the oxygen in the water or air and the iron makes what is known as iron
oxide or rust. This is an example of _slow oxidation_.
Oxidation in our Bodies.--If we expel the air from our lungs through a tube
into a bottle of limewater, we notice the limewater becomes milky.
Evidently carbon dioxide is formed in our own bodies and oxidation takes
place there. Is it fair to believe that the heat of our body (for example,
98.6 deg. Fahrenheit under the tongue) is due to oxidation within the body, and
that the work we do results from this chemical process. If so, what is
oxidized?
Energy comes from Foods.--From the foregoing experiment it is evident that
food is oxidized within the human body to release energy for our daily
work. Is it not logical to suppose that all living things, both plant and
animal, release energy as the result of oxidation of foods within their
cells? Let us see if this is true in the case of the pea.
Food oxidized in Germinating Seeds.--If we take equal numbers of soaked
peas, placed in two bottles, one tightly stoppered, the other having no
stopper, both bottles being exposed to identical conditions of light,
temperature, and moisture, we find that the seeds in both bottles start to
germinate, but that those in the closed bottle soon stop, while those in
the open jar continue to grow almost as well as similar seeds placed in an
open dish would.
[Illustration: Experiment that shows the necessity for air in germination.]
Why did not the seeds in the covered jar germinate? To answer this
question, let us carefully remove the stopper from the stoppered jar and
insert a lighted candle. The candle goes out at once. The surer test of
limewater shows the presence of carbon dioxide in the jar. The carbon of
the foodstuffs of the pea united with the oxygen of the air, forming carbon
dioxide. Growth stopped as soon as the oxygen was exhausted. The presence
of carbon dioxide in the jar is an indication that a very important process
which we associate with animals rather than plants, that of _respiration_,
is taking place. The seed, in order to release the energy locked up in its
food supply, must have oxygen, so that the oxidation of the food may take
place. _Hence a constant supply of fresh air is an important factor in
germination._ It is important that air should penetrate between the grains
of soil around a seed. The frequent stirring of the soil enables the air to
reach the seed. Air also acts upon some materials in the soil and puts them
in a form that the germinating seed can use. This necessity for oxygen
shows us at least one reason why the farmer plows and harrows a field and
one important use of the earthworm. Explain.
[Illustration: A grain of corn cut lengthwise. _C_, cotyledon; _E_,
endosperm; _H_, hypocotyl; _P_, plumule.]
Structure of a Grain of Corn.--Examination of a well-soaked grain of corn
discloses a difference in the two flat sides of the grain. A light-colored
area found on one surface marks the position of the embryo; the rest of the
grain contains the food supply. The interesting thing to remember here is
that the food supply is _outside_ of the embryo.
A grain cut lengthwise perpendicular to the flat side and then dipped in
weak iodine shows two distinct parts, an area containing considerable
starch, the _endosperm_, and the embryo or young plant. Careful inspection
shows the hypocotyl and plumule (the latter pointing toward the free end of
the grain) and a part surrounding them, the _single_ cotyledon (see
Figure). Here again we have an example of a fitting for future needs, for
in this fruit the one seed has at hand all the food material necessary for
rapid growth, although the food is here outside the embryo.
[Illustration: Longitudinal section of young ear of corn. _O_, the fruits;
_S_, the stigmas; _SH_, the sheath-like leaves; _ST_, the flower stalk.
(After Sargent.)]
Endosperm the Food Supply of Corn.--We find that the one cotyledon of the
corn grain does not serve the same purpose to the young plant as do the two
cotyledons of the bean. Although we find a little starch in the corn
cotyledon, still it is evident from our tests that the endosperm is the
chief source of food supply. The study of a thin section of the corn grain
under the compound microscope shows us that the starch grains in the
endosperm are large and regular in size. When the grain has begun to grow,
examination shows that the starch grains near the edge of the cotyledon are
much smaller and quite irregular, having large holes in them. We know that
the germinating grain has a much sweeter taste than that which is not
growing. This is noticed in sprouting barley or malt. We shall later find
that, in order to make use of starchy food, a plant or animal must in some
manner change it over to sugar. This change is necessary, because starch
will not dissolve in water, while sugar will; in this form substances can
pass from cell to cell in the plant and thus distribute the food where it
is needed.
[Illustration: Test for grape sugar.]
A Test for Grape Sugar.--Place in a test tube the substance to be tested
and heat it in a little water so as to dissolve the sugar. Add to the fluid
twice its bulk of Fehling's solution,[6] which has been previously
prepared. Heat the mixture, which should now have a blue color, in the test
tube. If grape sugar is present in considerable quantity, the contents of
the tube will turn first a greenish, then yellow, and finally a brick-red
color. Smaller amounts will show less decided red. No other substance than
sugar will give this reaction. If Benedict's test[7] is used, a colored
precipitate will appear in the test tube after boiling.
Footnotes 6 and 7: Directions for making these solutions
will be found in Hunter's _Laboratory Problems in Civic
Biology_.
Starch changed to Grape Sugar in the Corn.--That starch is being changed to
grape sugar in the germinating corn grain can easily be shown if we cut
lengthwise through the embryos of half a dozen grains of corn that have
just begun to germinate, place them in a test tube with some Fehling's
solution, and heat almost to the boiling point. They will be found to give
a reaction showing the presence of sugar along the edge of the cotyledon
and between it and the endosperm.
Digestion.--This change of starch to grape sugar in the corn is a process
of _digestion_. If you chew a bit of unsweetened cracker in the mouth for a
little time, it will begin to taste sweet, and if the chewed cracker, which
we know contains starch, is tested with Fehling's solution, some of the
starch will be found to have changed to grape sugar. Here, again, a process
of digestion has taken place. In both the corn and in the mouth, the change
is brought about by the action of peculiar substances known as digestive
ferments, or _enzymes_. Such substances have the power under certain
conditions to change insoluble foods--solids--into soluble
substances--liquids. The result is that substances which before digestion
would not dissolve in water now will dissolve.
[Illustration: A germinating corn grain. _C_, cotyledon; _H_, growing root
(_hypocotyl_); _P_, growing stem (_plumule_); _S_, endosperm; _d.s._,
digested starch; _p.r._, primary root; _s.r._, secondary root; _r.h._, root
hairs.]
The Action of Diastase on Starch.--The enzyme found in the cotyledon of the
corn, which changes starch to grape sugar, is called _diastase_. It may be
separated from the cotyledon and used in the form of a powder.
To a little starch in half a cup of water we add a very little (1 gram) of
diastase and put the vessel containing the mixture in a warm place, where
the temperature will remain nearly constant at about 98 deg. Fahrenheit. On
testing part of the contents at the end of half an hour, and the remainder
the next morning, for starch and for grape sugar, we find from the morning
test that the starch has been almost completely changed to grape sugar.
Starch and warm water alone under similar conditions will not react to the
test for grape sugar.
Digestion has the Same Purpose in Plants and Animals.--In our own bodies we
know that solid foods taken into the mouth are broken up by the teeth and
moistened by saliva. If we could follow that food, we would find that
eventually it became part of the blood. It was made soluble by digestion,
and in a liquid form was able to reach the blood. Once a part of the body,
the food is used either to release energy or to build up the body.
Summary.--We have seen:
1. That seeds, in order to grow, must possess a food supply either in or
around their bodies.