_Laboratory study of a living plant._--Any whole plant may
be used; a weed is preferable.
_Laboratory demonstration or home study._--The functions of
a living animal.
_Demonstration._--The growth of pollen tubes.
_Laboratory exercise._--The growth of the mature ovary into
the fruit, _e.g._ bean or pea pod.
A Living Plant and a Living Animal Compared.--A walk into the fields or any
vacant lot on a day in the early fall will give us first-hand acquaintance
with many common plants which, because of their ability to grow under
somewhat unfavorable conditions, are called _weeds_. Such plants--the
dandelion, butter and eggs, the shepherd's purse--are particularly well
fitted by nature to produce many of their kind, and by this means drive out
other plants which cannot do this so well. On these or other plants we find
feeding several kinds of animals, usually insects.
If we attempt to compare, for example, a grasshopper with the plant on
which it feeds, we see several points of likeness and difference at once.
Both plant and insect are made up of parts, each of which, as the stem of
the plant or the leg of the insect, appears to be distinct, but which is a
part of the whole living plant or animal. Each part of the living plant or
animal which has a separate work to do is called an _organ_. Thus plants
and animals are spoken of as living _organisms_.
[Illustration: A weed--notice the unfavorable environment.]
Functions of the Parts of a Plant.--We are all familiar with the parts of a
plant,--the root, stem, leaves, flowers, and fruit. But we may not know so
much about their uses to the plant. Each of these structures differs from
every other part, and each has a separate work or function to perform for
the plant. _The root holds the plant firmly in the ground and takes in
water and mineral matter from the soil; the stem holds the leaves up to the
light and acts as a pathway for fluids between the root and leaves; the
leaves, under certain conditions, manufacture food for the plant and
breathe; the flowers form the fruits; the fruits hold the seeds, which in
turn hold young plants which are capable of reproducing adult plants of the
same kind._
The Functions of an Animal.--As we have already seen, the grasshopper has a
head, a jointed body composed of a middle and a hind part, three pairs of
jointed legs, and two pairs of wings. Obviously, the wings and legs are
used for movement; a careful watching of the hind part of the animal shows
us that breathing movements are taking place; a bit of grass placed before
it may be eaten, the tiny black jaws biting little pieces out of the grass.
If disturbed, the insect hops away, and if we try to get it, it jumps or
flies away, evidently seeing us before we can grasp it. Hundreds of little
grasshoppers on the grass indicate that the grasshopper can reproduce its
own kind, but in other respects the animal seems quite unlike the plant.
The animal moves, breathes, feeds, and has sensation, while _apparently_
the plant does none of these. It will be the purpose of later chapters to
prove that the functions of plants and animals are in many respects similar
and that _both plants_ and _animals breathe_, _feed_, and _reproduce_.
[Illustration: Section through the blade of a leaf. _e_, cells of the upper
surface; _d_, cells of the lower surface; _i_, air spaces in the leaf; _v_,
vein in cross sections; _p_, green cells.]
Organs.--If we look carefully at the organ of a plant called a leaf, we
find that the materials of which it is composed do not appear to be
everywhere the same. The leaf is much thinner and more delicate in some
parts than in others. Holding the flat, expanded blade away from the branch
is a little stalk, which extends into the blade of the leaf. Here it splits
up into a network of tiny "veins" which evidently form a framework for the
flat blade somewhat as the sticks of a kite hold the paper in place. If we
examine under the compound microscope a thin section cut across the leaf,
we shall find that the veins as well as the other parts are made up of many
tiny boxlike units of various sizes and shapes. These smallest units of
building material of the plant or animal disclosed by the compound
microscope are called _cells_. The organs of a plant or animal are built of
these tiny structures.
[Illustration: Several cells of _Elodea_, a water plant. _chl._,
chlorophyll bodies; _c.s._, cell sap; _c.w._, cell wall; _n._, nucleus;
_p._, protoplasm. The arrows show the direction of the protoplasmic
movement.]
Tissues.[2]--The cells which form certain parts of the veins, the flat
blade, or other portions of the plant, are often found in groups or
collections, the cells of which are more or less alike in size and shape.
Such a collection of cells is called a _tissue_. Examples of tissues are
the cells covering the outside of the human body, the muscle cells, which
collectively allow of movement, bony tissues which form the framework to
which the muscles are attached, and many others.
Footnote 2: _To the Teacher._--Any simple plant or animal
tissue can be used to demonstrate the cell. Epidermal cells
may be stripped from the body of the frog or obtained by
scraping the inside of one's mouth. The thin skin from an
onion stained with tincture of iodine shows well, as do thin
sections of a young stem, as the bean or pea. One of the
best places to study a tissue and the cells of which it is
composed is in the leaf of a green water plant, _Elodea_. In
this plant the cells are large, and not only their outline,
but the movement of the living matter within the cells, may
easily be seen, and the parts described in the next
paragraph can be demonstrated.
[Illustration: A cell. _ch._, chromosomes; _c.w._, cell wall; _n._,
nucleus; _p._, protoplasm.]
Cells.--_A cell may be defined as a tiny mass of living matter containing a
nucleus, either living alone or forming a unit of the building material of
a living thing._ The living matter of which all cells are formed is known
as _protoplasm_ (formed from two Greek words meaning _first form_). If we
examine under a compound microscope a small bit of the water plant
_Elodea_, we see a number of structures resembling bricks in a wall. Each
"brick," however, is really a plant cell bounded by a thin wall. If we look
carefully, we can see that the material inside of this wall is slowly
moving and is carrying around in its substance a number of little green
bodies. This moving substance is living matter, the protoplasm of the cell.
The green bodies (the _chlorophyll_ bodies) we shall learn more about
later; they are found only in plant cells. All plant and animal cells
appear to be alike in the fact that every living cell possesses a structure
known as the _nucleus_ (pl. _nuclei_), which is found within the body of
the cell. This nucleus is not easy to find in the cells of _Elodea_. Within
the nucleus of all cells are found certain bodies called _chromosomes_.
These chromosomes in a given plant or animal are always constant in number.
These chromosomes are supposed to be the bearers of the qualities which we
believe can be handed down from plant to plant and from animal to animal,
in other words, the inheritable qualities which make the offspring like its
parents.
How Cells form Others.--Cells grow to a certain size and then split into
two new cells. In this process, which is of very great importance in the
growth of both plants and animals, the nucleus divides first. The
chromosomes also divide, each splitting lengthwise and the parts going in
equal numbers to each of the two cells formed from the old cell. In this
way the matter in the chromosomes is divided equally between the two new
cells. Then the rest of the protoplasm separates, and two new cells are
formed. This process is known as _fission_. It is the usual method of
growth found in the tissues of plants and animals.
[Illustration: Stages in the division of one cell to form two. Which part
of the cell divides first? What seems to become of the chromosomes?]
Cells of Various Sizes and Shapes.--Plant cells and animal cells are of
very diverse shapes and sizes. There are cells so large that they can
easily be seen with the unaided eye; for example, the root hairs of plants
and eggs of some animals. On the other hand, cells may be so minute, as in
the case of the plant cells named bacteria, that several million might be
present in a few drops of milk. The forms of cells may be extremely varied
in different tissues; they may assume the form of cubes, columns, spheres,
flat plates, or may be extremely irregular in shape. One kind of tissue
cell, found in man, has a body so small as to be quite invisible to the
naked eye, although it has a prolongation several feet in length. Such are
some of the cells of the nervous system of man and other large animals, as
the ox, elephant, and whale.
Varying Sizes of Living Things.--Plant cells and animal cells may live
alone, or they may form collections of cells. Some plants are so simple in
structure as to be formed of only one kind of cells. Usually living
organisms are composed of several groups of different kinds of cells. It is
only necessary to call attention to the fact that such collections of cells
may form organisms so tiny as to be barely visible to the eye; as, for
instance, some of the small flowerless plants or many of the tiny animals
living in fresh water or salt water. On the other hand, among animals, the
bulk of the elephant and whale, and among plants the big trees of
California, stand out as notable examples. The large plants and animals are
made up of _more_, not necessarily larger, cells.
What Protoplasm can Do.--It responds to influences or stimulation from
without its own substance. Both plants and animals are sensitive to touch
or stimulation by light, heat or cold, certain chemical substances,
gravity, and electricity. Green plants turn toward the source of light.
Some animals are attracted to light and others repelled by it; the
earthworm is an example of the latter. _Protoplasm is thus said to be
irritable._
_Protoplasm has the power to contract and to move._ Muscular movement is a
familiar instance of this power. Movement may also take place in plants.
Some plants fold up their leaves at night; others, like the sensitive
plant, fold their leaflets when touched.
_Protoplasm can form new living matter out of food._ To do this, food
materials must be absorbed into the cells of the living organism. To make
protoplasm, it is evident that the same chemical elements must enter into
the composition of the food substances as are found in living matter. The
simplest plants and animals have this wonderful power as certainly
developed as the most complex forms of life.
_Protoplasm, be it in plant or animal, breathes and throws off waste
materials._ When a living thing does work oxygen unites with food in the
body; the food is burned or _oxidized_ and work is done by means of the
energy released from the food. The waste materials are _excreted_ or passed
out. Plants and animals alike pass off the carbon dioxide which results
from the oxidation of food and of parts of their own bodies. Animals
eliminate wastes containing nitrogen through the skin and the kidneys.
_Protoplasm can reproduce, that is, form other matter like itself._ New
plants are constantly appearing to take the places of those that die. The
supply of living things upon the earth is not decreasing; reproduction is
constantly taking place. In a general way it is possible to say that plants
and animals reproduce in a very similar manner.
The Importance of Reproduction.--Reproduction is the final process that
plants and animals are called upon to perform. Without the formation of
_new_ living things no progress would be possible on the earth. We have
found that insects help flowering plants in this process. Let us now see
exactly what happens when pollen is placed by the bee on the stigma of
another flower of the same kind. To understand this process of reproduction
in flowers, we must first study carefully pollen grains from the anther of
some growing flower.
[Illustration: Pollen grains of different shapes and sizes.]
Pollen.--Pollen grains of various flowers, when seen under the microscope,
differ greatly in form and appearance. Some are relatively large, some
small, some rough, others smooth, some spherical, and others angular. They
all agree, however, in having a thick wall, with a thin membrane under it,
the whole inclosing a mass of protoplasm. At an early stage the pollen
grain contains but a single cell. A little later, however, two nuclei may
be found in the protoplasm. Hence we know that at least two cells exist
there, one of which is called the sperm cell; its nucleus is the sperm
nucleus.
[Illustration: A pollen grain greatly magnified. Two nuclei are found (_n_,
_n'_) at this stage of its growth.]
[Illustration: Three stages in the germination of the pollen grain. The
nuclei in the tube in (3) are the sperm nuclei. Drawn under the compound
microscope.]
Growth of Pollen Grains.--Under certain conditions a pollen grain will grow
or germinate. This growth can be artificially produced in the laboratory by
sprinkling pollen from well-opened flowers of sweet pea or nasturtium on a
solution of 15 parts of sugar to 100 of water. Left for a few hours in a
warm and moist place and then examined under the microscope, the grains of
pollen will be found to have germinated, a long, threadlike mass of
protoplasm growing from it into the sugar solution. The presence of this
sugar solution was sufficient to induce growth. When the pollen grain
germinates, the nuclei enter the threadlike growth (this growth is called
the pollen tube; see Figure). One of the nuclei which grows into the pollen
tube is known as the _sperm nucleus_.
[Illustration: Fertilization of the ovule. A flower cut down lengthwise
(only one side shown). The pollen tube is seen entering the ovule. _a_,
anther; _f_, filament; _pg_, pollen grain; _s_, stigmatic surface; _pt_,
pollen tube; _st_, style; _o_, ovary; _m_, micropyle; _sp_, space within
ovary; _e_, egg cell; _P_, petal; _S_, sepal.]
Fertilization of the Flower.--If we cut the pistil of a large flower (as a
lily) lengthwise, we notice that the style appears to be composed of rather
spongy material in the interior; the ovary is hollow and is seen to contain
a number of rounded structures which appear to grow out from the wall of
the ovary. These are the _ovules_. The ovules, under certain conditions,
will become _seeds_. An explanation of these conditions may be had if we
examine, under the microscope, a very thin section of a pistil, on which
pollen has begun to germinate. The central part of the style is found to be
either hollow or composed of a soft tissue through which the pollen tube
can easily grow. Upon germination, the pollen tube grows downward through
the spongy center of the style, follows the path of least resistance to the
space within the ovary, and there enters the ovule. It is believed that
some chemical influence thus attracts the pollen tube. When it reaches the
ovary, the sperm cell penetrates an ovule by making its way through a
little hole called the _micropyle_. It then grows toward a clear bit of
protoplasm known as the _embryo sac_. The embryo sac is an ovoid space,
microscopic in size, filled with semifluid protoplasm containing several
nuclei. (See Figure.) _One of the nuclei, with the protoplasm immediately
surrounding it, is called the egg cell._ It is this cell that the sperm
nucleus of the pollen tube grows toward; ultimately the sperm nucleus
reaches the egg nucleus and unites with it. _The two nuclei, after coming
together, unite to form a single cell. This process is known as
fertilization._ This single cell formed by the union of the pollen tube
cell or sperm and the egg cell is now called a _fertilized egg_.
Development of Ovule into Seed.--_The primary reason for the existence of a
flower is that it may produce seeds from which future plants will grow.
After fertilization the ovule grows into a seed._ The first beginning of
the growth of the seed takes place at the moment of fertilization. From
that time on there is a growth of the fertilized egg within the ovule which
makes a baby plant called the _embryo_. _The embryo will give rise to the
adult plant._
[Illustration: The fruit of the locust, a bean-like fruit. _p_, the
attachment to the placenta; _s_, the stigma.]
A Typical Fruit,--the Pea or Bean Pod.--If a withered flower of any one of
the pea or bean family is examined carefully, it will be found that the
pistil of the flower continues to grow after the rest of the flower
withers. If we remove the pistil from such a flower and examine it
carefully, we find that it is the ovary that has enlarged. The space within
the ovary has become nearly filled with a number of nearly ovoid bodies,
attached along one edge of the inner wall. These we recognize as the young
seeds.
The pod of a bean, pea, or locust illustrates well the growth from the
flower. The pod, which is in reality a ripened ovary with other parts of
the pistil attached to it, is considered as a _fruit_. By definition, _a
fruit is a ripened ovary and its contents together with any parts of the
flower that may be attached to it_. The chief use of the fruit to the
flower is to hold and to protect the seeds; it may ultimately distribute
them where they can reproduce young plants.
[Illustration: The development of an apple. Notice that in this fruit
additional parts besides the ovary (_o_) become part of the fruit. Certain
outer parts of the flower, the sepals (_s_) and receptacle, become the
fleshy part of the fruit, while the ovary becomes the core. Stages numbered
1 to 7 are in the order of development.]
The Necessity of Fruit and Seed Dispersal to a Plant.--We have seen that
the chief reason for flowers, from the plant's standpoint, is to produce
fruits which contain seeds. Reproduction and the ultimate scattering of
fruits and seeds are absolutely necessary in order that colonies of plants
may reach new localities. It is evident that plants best fitted to scatter
their seeds, or place fruits containing the seeds some little distance from
the parent plants, are the ones which will spread most rapidly. A plant, if
it is to advance into new territory, must get its seeds there first. Plants
which are best fitted to do this are the most widely distributed on the
earth.
How Seeds and Fruits are Scattered.--Seed dispersal is accomplished in many
different ways. Some plants produce enormous numbers of seeds which may or
may not have special devices to aid in their scattering. Most weeds are
thus started "in pastures new." Some prolific plants, like the milkweed,
have _seeds_ with a little tuft of hairlike down which allows them to be
carried by the wind. Others, as the omnipresent dandelion, have their
_fruits_ provided with a similar structure, the pappus. Some plants, as the
burdock and clotbur, have fruits provided with tiny hooks which stick to
the hair of animals, thus proving a means of transportation. Most fleshy
fruits contain indigestible seeds, so that when the fruits are eaten by
animals the seeds are passed off from the body unharmed and may, if
favorably placed, grow. Nuts of various kinds are often carried off by
animals, buried, and forgotten, to grow later. Such are a few of the ways
in which seeds are scattered. All other things being equal, the plants best
equipped to scatter seeds or fruits are those which will drive out other
plants in a given locality. Because of their adaptations they are likely to
be very numerous, and when unfavorable conditions come, for that reason, if
for no other, are likely to survive. Such plants are best exemplified in
the weeds of the grassplots and gardens.