LEAVES
FACTS ABOUT LEAVES
We found in an earlier lesson that all of our farm plants have roots,
stems, leaves, flowers, fruit and seeds. We studied the root first as
being the most important part of the plant to the farmer. The seed was
the next part studied, for that was considered the next most
important, because the seed is the main reliance for new plants. The
part next in importance is the leaf and that we will now study.
If you will go into the field and observe the leaves on a number of
plants, you will find that the following facts are true:
They are all green.
They are flat and thin.
Many of them are very broad.
Some of the leaves on a single branch are larger than others on the
same branch, and some have longer stems than others.
Most of them have a rather dark glossy upper surface and a lighter
rougher under surface.
[Illustration: FIG. 59.
To show transpiration. Plant _A_ was set in the sunlight, plant _B_
was left in the darker part of the room. _A_ has transpired much more
than _B_, showing that sunlight is necessary for this work.]
[Illustration: FIG. 60.--AMOUNT OF TRANSPIRATION
This plant transpired within 48 hours an amount of water equal to the
colored liquid in the bottle standing on the jar, more than 6 ounces.]
The leaves on the lower branches of the trees are spread out in a more
or less flat layer and have their glossy surfaces all turned up,
while those on branches in the tops of trees or shrubs are arranged
all around the branch, the glossy surface being turned up.
What are the reasons for these facts?
A study of the work of the leaves and the conditions necessary for
them to perform their work will help us to answer this question.
THE USES OF LEAVES TO PLANTS
=Experiment.=--(See Fig. 59). Take a pot or tumbler in which a young
plant is growing, also a piece of pasteboard large enough to cover the
top of the pot; cut a slit from the edge to the centre of the
pasteboard, then place it on the top of the pot, letting the plant
enter the slit. Now close the slit with wax or tallow, making it
perfectly tight about the stem. If the plant is not too large, invert
a tumbler over it, letting the edge of the tumbler rest on the
pasteboard; if a tumbler is not large enough use a glass jar. If a
potted plant is not convenient a slip or a seedling bean or pea placed
in a tumbler of water will serve the purpose. Prepare several and
place some in a sunny window and leave others in the room where it is
darker, and observe them from time to time. In the case of those
plants that were set in the sunny window moisture will be seen
collecting on the inner surface of the tumbler. Where does this come
from? It is absorbed from the soil by the roots and is sent with its
load of dissolved plant food up through the stems to the leaves.
There most of the water is passed from the leaves to the air and is
condensed on the sides of the glass. A work of leaves then is to throw
off or to transpire moisture and thus make room for a new supply of
food-laden moisture. This water is thrown off through little pores or
mouths or stomata which are very small and very numerous on the under
side of the leaf. It will be noticed that the plant not placed in the
sunlight transpires very little moisture, showing that sunlight helps
the leaves in this work of transpiration.
How much water does a plant transpire or throw off from its leaves?
=Experiment.=--(See Fig. 60). Fill a common quart fruit jar or can
with soil and plant in it a kernel of corn, a bean, a cotton seed or
seed of some other plant. After the plant has grown to be twelve or
fifteen inches high, cut a piece of pasteboard a little larger than
the top of the jar, cut a hole in the centre as large as the stem of
the plant and make a slit from edge to centre. Soak the pasteboard in
melted wax or paraffine candle. Cool it and then place it over the
jar, slipping it around the plant stem. Now solder the pasteboard to
the jar with melted candle making the joints tight all the way around.
Then close up the slit and the hole about the stem. The jar is now
completely sealed and there is no way for water to escape except
through the plant. The plant should be well watered before the jar is
closed. Now weigh the jar and set in the sunlight. Weigh again the
next day. The difference in the two weights will represent the amount
of water transpired by the plant. The weighings may be repeated until
moisture gives out. If it is desired to continue this experiment some
time, a small hole should be cut in the pasteboard before it is
fastened to the jar. This hole is for adding water to the jar from
time to time. The hole should be kept closed with a cork. The amount
of water added should always be weighed and account taken of it in the
following weighings. While this plant is growing it will be well to
wrap the jar with paper to protect the roots from the light.
It has been found that the amount of water necessary to grow a plant
to maturity is equal to from 300 to 500 times the weight of the plant
when dry.
This gives us an idea of the very great importance of water to plants.
=Experiment.=--Take a few leaves from a plant of cotton, bean, clover
or other plant that has been growing in the sunlight; boil them for a
few minutes to soften the tissues, then place them in alcohol for a
day or until the green coloring matter is extracted by the alcohol.
Wash the leaves by taking them from the alcohol and putting them in a
tumbler of water. Then put them in saucers in a weak solution of
iodine. The leaf will be seen to gradually darken; this will continue
until it becomes dark purple or almost black (Fig. 61). We have
already learned that iodine turns starch this color, so we conclude
that leaves must contain starch. (Five or ten cents worth of tincture
of iodine from a drug store diluted to about the color of weak tea
will be sufficient for these leaf experiments.)
=Experiment.=--If a potted plant was used for the last experiment, set
it away in a dark closet after taking the leaves for the experiment. A
day or two after, take leaves from it before removing it from the
closet. Boil these leaves and treat them with alcohol as in the
previous experiment. Then wash them and test them with iodine as
before. No starch will be found in the leaves (Fig. 62). The starch
that was in them when placed in the closet has disappeared. Now paste
some thick paper labels on some of the leaves of a plant exposed to
the sunlight. After a few hours remove the leaves that have the labels
on them, boil, treat with alcohol and test with the iodine. In this
case starch will be found in all parts of the leaf except the part
over which the label was pasted (Fig. 63). If the sunlight is intense
and the label thin, some starch will appear under it.
According to these last experiments, leaves contain starch at certain
times, and this starch seems to appear when the leaf is in the
sunlight and to disappear when the light is cut off. The fact is that
the leaves manufacture starch for the plant and sunlight is necessary
for this work. The starch is then changed to sugar which is carried by
the sap to other parts of the plant where it is again changed to
starch to be built into the plant structure or stored for future use.
=Experiment.=--Take leaves from a plant of silver-leaf geranium
growing in the sunlight. If this plant cannot be had, the leaves from
some other variegated white and green leaved plant will do. Boil these
leaves, treat with alcohol, wash and test with iodine (Fig. 64).
Starch will be found in the leaf wherever there was green coloring
matter in it, while the parts that were white will show no starch. The
green coloring matter seems to have something to do with the starch
making, in fact starch is manufactured only where it is present. This
coloring matter is called chlorophyl or leaf green.
We are told by the chemists that this starch is made from carbon and
water. There exists in the air a gas called carbonic acid gas; this
gas is composed of carbon and oxygen. It is breathed out of the lungs
of animals and is produced by the burning and decay of organic matter.
The under side of the leaf contains hundreds of little pores or mouths
called stomata. This gas mixed with air enters these mouths. The green
part of the leaf aided by the sun takes hold of the gas and separates
the carbon from the oxygen. The oxygen is allowed to go free, but the
carbon is made to unite with water and form starch.
=Experiment.=--The escape of this oxygen gas may be seen by taking
some water weed from either fresh or salt water and placing it in a
glass jar of the kind of water from which it came, then set the jar in
the sunlight. After a time bubbles of gas will be seen collecting and
rising to the surface. If a mass of weed like the green scum of fresh
water ponds or green sea lettuce be used, the bubbles of gas will
become entangled in the mass and will cause it to rise to the surface
of the water. At the same time prepare another jar of the weed and
place it somewhere out of the sun; very few bubbles will be seen to
rise and the weed will settle to the bottom of the jar (Fig. 65).
All of the food of the plant, whether taken from the air or from the
soil is digested in the leaves, and sunlight and air are necessary for
this work.
Another function of leaves then is to digest food for the plant.
Important functions of leaves then are:
To transpire moisture sent up by the roots.
To manufacture starch by combining some of the water sent up by the
roots with carbon taken from the air.
To digest the starch and food sent up by the roots.
To do these things well leaves must be connected with a strong,
healthy root system and must have plenty of light and air.
We are now ready to give reasons for the facts about leaves mentioned
in the first part of the chapter (see page 109).
Leaves are green because the green coloring matter is necessary for
the leaf to do its work.
Leaves are flat and thin and broad in order that they may present a
large surface to the air and sunlight.
[Illustration: FIG. 61.
To show that growing leaves contain starch. 1. Represents a green
cotton leaf as picked from the plant. 2. Is the same leaf after taking
out the green coloring matter; the leaf is white. 3. The same leaf
after treatment with weak iodine turned to a dark purple, showing the
presence of starch. (Drawings by M.E. Feltham.)]
[Illustration: FIG. 62.
To show that starch disappears from the leaf when the plant is placed
in the dark. The plant from which was taken the leaf represented in
Fig 61, was immediately placed in a dark closet for 24 hours. Then
leaf 4 was taken from it; 5 represents this leaf after the chlorophyl
was taken from it: it is white; 6 is the same after treatment with
iodine. The leaf remains white, showing no starch. (Drawings by M.E.
Feltham.)]
[Illustration: FIG. 63.
To show that sunlight is necessary for starch-making by leaves. Leaf 7
had a paper label stuck to its upper surface a couple of hours while
the plant was exposed to sunlight; 8 is the same leaf after the
chlorophyl was taken out, and 9 represents it after treatment with
iodine. The leaf turned purple in all parts except the part that was
shaded by the label. Starch was removed from the portion under the
label, but was not renewed because the label kept out the necessary
sunlight. (Drawings by M.E. Feltham.)]
[Illustration: FIG. 64.
To show that chlorophyl is necessary for starch formation in the leaf.
10 is a variegated leaf from a silver-leaved geranium; the center is
an irregular patch of green, with an irregular border of white. 11,
after taking out the green. 12, after iodine treatment, the leaf turns
purple only where it was originally green, showing that no starch
forms in the white border. (Drawings by M.E. Feltham.)]
Some leaves on the branch are larger than others because in the
struggle for light and air they have had a better chance than the
others or they have had more of the food which has come up from the
root.
Some of the leaves have developed longer stems than others in their
effort to reach out after light and air.
Most leaves have the little mouths through which air is taken in and
water and oxygen given out on the rough side, and that side is turned
down toward the earth probably so that rain and dust will not choke
the little pores.
The leaves of the lower branches tend to spread out in a broad, flat
plane because in the effort to get light no leaf will grow directly
under and in the shadow of another, while on those branches which grow
straight up from the top of the tree the leaves can get light from all
sides and so arrange themselves around the stem.
Is it of any value to the plant grower to know these facts about
leaves? It is, for knowing these things he can better understand the
necessity of caring for the leaves of his growing plants to see that
their work is not interfered with.
HOW THE WORK OF SOME LEAVES IS INTERFERED WITH
Many people who grow house plants have trouble in keeping them well
clothed with leaves, for instance, the geranium and the rubber plant.
The leaves are constantly turning yellow and dropping off or drying
up. This sometimes occurs from over-watering or not sufficiently
watering the soil in the pot or box. If the watering is all right the
trouble may occur in this way: The air of the house is quite dry,
especially in winter. As a result transpiration from the leaf may be
excessive. More water is transpired than is necessary, consequently
more is pumped by the roots and with it more food is sent to the leaf
than it can take care of. As the excess of water is transpired the
excess of food is left in the leaf. The tendency is to clog its pores
and therefore interfere with its work, and gradually weaken and
finally kill it. The remedy for this is to spray the leaves frequently
so as to keep the air about them moist and so check transpiration.
Keeping a vessel of water near them helps also as this tends to keep
the air moist. Dust sometimes chokes the leaves. Washing or spraying
remedies this.
Sometimes house plants, and out-door plants as well, become covered
with a small, green insect called the plant louse or aphis. This
insect has a sharp beak like a mosquito and it sucks the juices from
the leaf and causes it to curl up, interfering with its work and
finally killing it. Frequent spraying with water will tend to keep
these away. A surer remedy against them is to spray the plants with
weak tobacco water made by soaking tobacco or snuff in water, or to
fumigate them with tobacco smoke. Sometimes the under side of the
leaf becomes infested with a very small mite called red spider
because it spins a web. These mites injure the leaf by sucking sap
from it. They can be kept in check by frequent spraying for they do
not like water. If, then, we are careful to frequently spray the
leaves of our house plants we will have very little trouble from
aphis, red spider or over transpiration. The aphis, or plant louse, is
often very numerous on out-door plants, for instance, the rose,
chrysanthemum, cabbage, and fruit trees. They vary in color from green
to dark brown or black. They are treated in the same way as those on
the house plants. Some familiar out-door insects which interfere with
leaf work are the common potato bug, the green cabbage worm, the rose
slug, the elm tree leaf beetle, the canker worm, the tomato worm.
These insects and many others eat the leaves (Fig. 67). They chew and
swallow their food and are called chewing insects. All insects which
chew the leaves of plants can be destroyed by putting poison on their
food. The common poisons used for this purpose are Paris green and
London purple, which contain arsenic, and are used at the rate of one
teaspoonful to a pail of water or one-fourth pound to a barrel of
water. This is sprinkled or sprayed on the leaves of the plants.
Another poison used is white hellebore. This loses its poisoning
qualities when exposed to the air for a time. Therefore it is safer to
use about the flower garden and on plants which are soon to be used as
food or whose fruit is to be used soon, like cabbages and current
bushes. This hellebore is sifted on the plant full strength, or it may
be diluted by mixing one part of hellebore with one or two parts of
flour, plaster, or lime. It is also used in water, putting one ounce
of hellebore in three gallons of water and then spraying it on the
plants. Plants may be sprayed by using a watering pot with a fine rose
or sprinkler, or an old hair-brush or clothes-brush. For large plants
or large numbers of smaller plants spray pumps of various sizes are
used. Sometimes chewing insects on food plants and sucking insects on
all plants are treated by spraying them with soapy solutions or oily
solutions which injure their bodies.
The work of the leaf is also interfered with by diseases which attack
the leaves and cause parts or the whole leaf to turn yellow or brown
or become blistered or filled with holes. The common remedy for most
of these diseases is called the "Bordeaux Mixture." It is prepared as
follows: Dissolve four pounds of blue vitriol (blue stone, or copper
sulphate) in several gallons of water. Then slake four pounds of lime.
Mix the two and add enough water to make a barrelful. The mixture is
then sprayed on the plants.
For more detailed directions for spraying plants and combating insects
and diseases write to your State Experiment Station and to the United
States Department of Agriculture at Washington, D.C.
[Illustration: FIG. 65.
To show the giving off of gas by leaves, and that sunlight is
necessary for it. The jars contain seaweed. _A_ was set in the sun and
developed enough gas to float part of the plant. _B_ was left in the
darker part of the room and developed very little gas.]
[Illustration: FIG. 66.
Seedling radishes reaching for light.]
[Illustration: FIG. 67.
Elm leaves injured by the "imported elm-tree leaf beetle," a chewing
insect.]
The work of the leaves of house plants is often interfered with by not
giving them sufficient sunlight. Garden and field plants are
sometimes planted so thick that they crowd each other and shut the
light and air from each other, or weeds are allowed to grow and do the
same thing, the result being that the leaves cannot do good work and
the plant becomes weak and sickly. Weeds are destroyed by pulling them
up and exposing their roots to the sun. This should be done before the
weeds blossom, to prevent them from producing fresh seeds for a new
crop of weeds. Some weeds have fleshy roots--for example, dock,
thistle--in which food is stored; these roots go deep in the ground,
and when the upper part of the plant is cut or broken off the root
sends up new shoots to take the place of the old. Some have
underground stems in which food is stored for the same purpose. The
surest way to get rid of such weeds, in fact, of all weeds, is to
prevent their leaves from growing and making starch and digesting food
for them. This is accomplished by constantly cutting off the young
shoots as soon as they appear above the soil, or by growing some crop
that will smother them. The constant effort to make new growth will
soon exhaust the supply of stored food and the weed will die.