A Civic Biology Part 5

_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 pa.s.sed out. Plants and animals alike pa.s.s 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.

[Ill.u.s.tration: 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 ma.s.s 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.

[Ill.u.s.tration: A pollen grain greatly magnified. Two nuclei are found (_n_, _n'_) at this stage of its growth.]

[Ill.u.s.tration: 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 ma.s.s 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_.

[Ill.u.s.tration: 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_, s.p.a.ce 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 s.p.a.ce 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 s.p.a.ce, 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._

[Ill.u.s.tration: 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 s.p.a.ce 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 ill.u.s.trates 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.

[Ill.u.s.tration: 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 pa.s.sed 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 gra.s.splots and gardens.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_. American Book Company.

Andrews, _A Practical Course in Botany_, pages 250-270.

American Book Company.

Atkinson, _First Studies of Plant Life_, Chaps. XXV-XXVI.

Ginn and Company.

Bailey, _Lessons with Plants_, Part III, pages 131-250. The Macmillan Company.

Coulter, _Plant Life and Plant Uses_. American Book Company.

Dana, _Plants and their Children_, pages 187-255. American Book Company.

Lubbock, _Flowers, Fruit, and Leaves_, Part I. The Macmillan Company.

Newell, _A Reader in Botany_, Part II, pages 1-96. Ginn and Company.

ADVANCED

Bailey, _Plant Breeding_. The Macmillan Company.

Campbell, _Lectures on the Evolution of Plants_. The Macmillan Company.

Coulter, Barnes, and Cowles, _A Textbook of Botany_, Part II. American Book Company.

Darwin, _Different Forms of Flowers on Plants of the Same Species_. Appleton.

Darwin, _Fertilization in the Vegetable Kingdom_, Chaps. I and II. Appleton.

Darwin, _Orchids Fertilized by Insects_. D. Appleton and Company.

Muller, _The Fertilization of Flowers_. The Macmillan Company.

V. PLANT GROWTH AND NUTRITION. CAUSES OF GROWTH

_Problem.--What causes a young plant to grow?_ _(a) The relation of the young plant to its food supply._ _(b) The outside conditions necessary for germination._ _(c) What the young plant does with its food supply._ _(d) How a plant or animal is able to use its food supply._ _(e) How a plant or animal prepares food to use in various parts of the body._

LABORATORY SUGGESTIONS

_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 cla.s.s.

_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.

[Ill.u.s.tration: 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.