Other Cases.--The stamens and pistil ripen at different times in some flowers. The "Lady Washington" geranium, a common house plant, shows this condition. Here also cross-pollination must take place if seeds are to be formed.
Summary.--If we now collect our observations upon flowers with a view to making a summary of the different devices flowers have assumed to prevent self-pollination and to secure cross-pollination, we find that they are as follows:--
_(1) The stamens and pistils may be found in separate flowers, either on the same or on different plants._
_(2) The stamens may produce pollen before the pistil is ready to receive it, or vice versa._
_(3) The stamens and pistils may be so placed with reference to each other that pollination can be brought about only by outside assistance._
Artificial Cross-pollination and its Practical Benefits to Man.--Artificial cross-pollination is practiced by plant breeders and can easily be tried in the laboratory or at home. First the anthers must be carefully removed from the bud of the flower so as to eliminate all possibility of self-pollination. The flower must then be covered so as to prevent access of pollen from without; when the ovary is sufficiently developed, pollen from another flower, having the characters desired, is placed on the stigma and the flower again covered to prevent any other pollen reaching the flower. The seeds from this flower when planted _may_ give rise to plants with the best characters of each of the plants which contributed to the making of the seeds.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American Book Company.
Andrews, _A Practical Course in Botany_, pages 214-249.
American Book Company.
Atkinson, _First Studies of Plant Life_, Chaps. XXV-XXVI.
Ginn and Company.
Coulter, _Plant Life and Plant Uses_, pages 301-322.
American Book Company.
Dana, _Plants and their Children_, pages 187-255. American Book Company.
Lubbock, _Flowers, Fruits, and Leaves_, Part I. The Macmillan Company.
Needham, _General Biology_, pages 1-50. The Comstalk Publishing Company.
Newell, _A Reader in Botany_, Part II, pages 1-96. Ginn and Company.
Sharpe, _A Laboratory Manual in Biology_, pages 43-48.
American Book 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_, D. Appleton and Company.
Darwin, _Fertilization in the Vegetable Kingdom_, Chaps. I and II. D. Appleton and Company.
Darwin, _Orchids Fertilized by Insects_, D. Appleton and Company.
Lubbock, _British Wild Flowers_. The Macmillan Company.
Muller, _The Fertilization of Flowers_. The Macmillan Company.
IV. THE FUNCTIONS AND COMPOSITION OF LIVING THINGS
_Problems.--To discover the functions of living matter._ _(a) In a living plant._ _(b) In a living animal._
LABORATORY SUGGESTIONS
_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._