_Laboratory exercise._--The structure (cross section) of a
woody stem.
_Demonstration._--To show that food passes downward in the
bark.
_Demonstration._--To show the condition of food passing
through the stem.
_Demonstration._--Plants with special digestive organs.
The Circulation and Final Uses of Foods in Green Plants.--We have seen that
cells of green plants make food and that such cells are mostly in the
leaves. But _all_ parts of the bodies of plants grow. Roots, stems, leaves,
flowers, and fruits grow. Seeds are storehouses of food. We must now
examine the stem of some plant in order to see how food is distributed,
stored, and finally used in the various parts of the plant.
The Structure of a Woody Stem.--If we cut a cross section through a young
willow or apple stem, we find it shows three distinct regions. The center
is occupied by the spongy, soft _pith_; surrounding this is found the
rather tough _wood_, while the outermost area is _bark_. More careful study
of the bark reveals the presence of three layers--an outer layer, a middle
green layer, and an inner fibrous layer, the latter usually brown in color.
This layer is made up largely of tough fiberlike cells known as _bast_
fibers. The most important parts of this inner bark, so far as the plant is
concerned, are many tubelike structures known as _sieve tubes_. These are
long rows of living cells, having perforated sievelike ends. Through these
cells food materials pass downward from the upper part of the plant, where
they are manufactured.
[Illustration: Section of a twig of box elder three years old, showing
three annual growth rings. The radiating lines (_m_) which cross the wood
(_w_) represent the pith rays, the principal ones extending from the pith
in the center to the cortex or bark. (From Coulter's _Plant Relations_.)]
In the wood will be noticed (see Figure) a number of lines radiating
outward from the pith toward the bark. These are thin plates of pith which
separate the wood into a number of wedge-shaped masses. These masses of
wood are composed of many elongated cells, which, placed end to end, form
thousands of little tubes connecting the leaves with the roots. In addition
to these are many thick-walled cells, which give strength to the mass of
wood. The bundles of tubes with their surrounding hard walled cells are the
continuation of the bundles of tubes which are found in the root. In
sections of wood which have taken several years to grow, we find so-called
_annual rings_. The distance between one ring and the next (see Figure)
usually represents the amount of growth in one year. Growth takes place
from an actively dividing layer of cells, known as the _cambium layer_.
This layer forms wood cells from its inner surface and bark from its outer
surface. Thus new wood is formed as a distinct ring around the old wood.
Use of the Outer Bark.--The outer bark of a tree is protective. The cells
are dead, the heavy woody skeletons serving to keep out cold and dryness,
as well as prevent the evaporation of fluids from within. The bark also
protects the tree from attack of other plants or animals which might harm
it. Most trees are provided with a layer of corky cells. This layer in the
cork oak is thick enough to be of commercial importance. The function of
the corky layer in preventing evaporation is well seen in the case of the
potato, which is a true stem, though found underground. If two potatoes of
equal weight are balanced on the scales, the skin having been peeled from
one, the peeled potato will be found to lose weight rapidly. This is due to
loss of water, which is held in by the skin of the unpeeled potato (see
right hand figure below).
There are also small breathing holes known as _lenticels_ scattered through
the surface of the bark. These can easily be seen in a young woody stem of
apple, beech, or horse-chestnut.
[Illustration: Experiment to show that the skin of the potato (a stem)
retards evaporation.]
Proof that Food passes down the Stem.--If freshly cut willow twigs are
placed in water, roots soon begin to develop from that part of the stem
which is under water. If now the stem is girdled by removing the bark in a
ring just above where the roots are growing, the latter will eventually
die, and new roots will appear above the girdled area. The food material
necessary for the outgrowth of roots evidently comes from above, and the
passage of food materials takes place in a downward direction just outside
the wood in the layer of bark which contains the bast fibers and sieve
tubes. This experiment with the willow explains why it is that trees die
when girdled so as to cut the sieve tubes of the inner bark. The food
supply is cut off from the protoplasm of the cells in the part of the tree
below the cut area. Many of the canoe birches of our Adirondack forest are
thus killed, girdled by thoughtless visitors. In the same manner mice and
other gnawing animals kill fruit trees. Food substances are also conducted
to a much less extent in the wood itself, and food passes from the inner
bark to the center of the tree by way of the pith plates. This can be
proved by testing for starch in the pith plates of young stems. It is found
that much starch is stored in this part of the tree trunk.
[Illustration: Experiment to show that food material passes down in the
inner bark.]
In what Form does Food pass through the Stem?--We have already seen that
materials in solution (those substances which will dissolve in the water)
will pass from cell to cell by the process of osmosis. This is shown in the
experiment illustrated in the figure. Two thistle tubes are partly filled,
one with starch and water, the other with sugar and water, and a piece of
parchment paper is tied over the end of each. The lower ends of both tubes
are placed in a glass dish under water. After twenty-four hours, the water
in the dish is tested for starch, and then for sugar. We find that only the
sugar, which has been dissolved by the water, can pass through the
membrane.
[Illustration: Experiment to show osmosis of sugar (right hand tube) and
non-osmosis of starch (left hand tube).]
Digestion.--Much of the food made in the leaves is stored in the form of
starch. But starch, being insoluble, cannot be passed from cell to cell in
a plant. It must be changed to a soluble form, for otherwise it could not
pass through the delicate cell membranes. This is accomplished by the
process of _digestion_. We have already seen that starch is changed to
grape sugar in the corn by the action of a substance (an enzyme) called
_diastase_. This process of digestion seemingly may take place in all
living parts of the plant, although most of it is done in the leaves. In
the bodies of all animals, including man, starchy foods are changed in a
similar manner, but by other enzymes, into soluble grape sugar.
The food material may be passed in a soluble form until it comes to a place
where food storage is to take place, then it can be transformed to an
insoluble form (starch, for example); later, when needed by the plant in
growth, it may again be transformed and sent in a soluble form through the
stem to the place where it will be used.
In a similar manner, protein seems to be changed and transferred to various
parts of the plant. Some forms of protein substance are _soluble_ and
others _insoluble_ in water. White of egg, for example, is slightly
soluble, but can be rendered insoluble by heating it so that it coagulates.
Insoluble proteins are digested within the plant; how and where is but
slightly understood. In a plant, soluble proteins pass down the sieve tubes
in the bast and then may be stored in the bast or medullary rays of the
wood in an insoluble form, or they may pass into the fruit or seeds of a
plant, and be stored there.
[Illustration: Diagram to show the areas in a plant through which the raw
food materials pass up the stem and food materials pass down.]
What forces Water up the Stem.--We have seen that the process of osmosis is
responsible for taking in soil water, and that the enormous absorbing
surface exposed by the root hairs makes possible the absorption of a large
amount of water. Frequently this is more than the weight of the plant in
every twenty-four hours.
Experiments have been made which show that at certain times in the year
this water is in some way forced up the tiny tubes of the stem. During the
spring season, in young and rapidly growing trees, water has been proved to
rise to a height of nearly ninety feet. The force that causes this rise of
water in stems is known as _root pressure_.
The greatest factor, however, is transpiration of water from leaves. This
evaporation of water in the form of vapor seems to result in a kind of
suction on the column of water in the stem. In the fall, after the leaves
have gone, much less water is taken in by roots, showing that an intimate
relation exists between the leaves and the root.
Summary of the Functions of Green Plants.--The processes which we have just
described (with the exception of food making) are those which occur in the
lives of any plant or animal. All plants and animals breathe, they oxidize
their foods to release energy, carbon dioxide being given off as the result
of the union of the carbon in the foods with the oxygen of the air. Both
plants and animals digest their food; plants may do this in the cells of
the root, stem, and leaf. Digestion must always occur so that food can be
moved in a soluble condition from cell to cell in the plant's body.
[Illustration: Leaf of sundew closing over a captured insect.]
[Illustration: The Venus fly trap, showing open and closed leaves.]
Plants with Special Digestive Organs.--Some plants have special organs of
digestion. One of these, the sundew, has leaves which are covered on one
side with tiny glandular hairs. These attract insects and later serve to
catch and digest the nitrogenous matter of these insects by means of
enzymes poured out by the same hairs. Another plant, the Venus fly trap,
catches insects in a sensitive leaf which folds up and holds the insect
fast until enzymes poured out by the leaf slowly digest it. Still others,
called pitcher plants, use as food the decayed bodies of insects which fall
into their cuplike leaves and die there. In this respect plants are like
those animals which have certain organs in the body set apart for the
digestion of food.
Assimilation.--The assimilation of foods, or making of foods into living
matter, is a process we know very little about. We know it takes place in
the living cells of plants and animals. But how foods are changed into
living matter is one of the mysteries of life which we have not yet solved.
Excretion.--The waste and repair of living matter seems to take place in
both plants and animals. When living plants breathe, they give off carbon
dioxide. In the process of starch-making, oxygen might be considered the
waste product. Water is evaporated from leaves and stems. The leaves fall
and carry away waste mineral substances which they contain.
[Illustration: The embryos of (_a_) the morning glory, (_b_) the barberry,
(_c_) the potato, (_d_) the four o'clock, showing the position of their
food supply. (After Gray.)]
Reproduction.--Finally, both plants and animals have organs of
reproduction. We have seen that the flower gives rise, after pollination,
to a fruit which holds the seeds. These seeds hold the _embryo_. Thus the
young plant is doubly protected for a time and is finally thrown off in the
seed with enough food to give it a start in life. In much the same way we
will find that animals reproduce, either by laying eggs which contain an
_embryo_ and food to start it in life or, as in the higher animals, by
holding and protecting the embryo within the body of the mother until it is
born, a helpless little creature, to be tenderly nourished by the mother
until able to care for itself.
The Life Cycle.--Ultimately both plants and animals grow old and die. Some
plants, for example the pea or bean, live but a season; others, such as the
big trees of California, live for hundreds of years. Some insects exist as
adults but a day, while the elephant is said to live almost two hundred
years. The span of life from the time the plant or animal begins to grow
until it dies is known as the _life cycle_.