Chapter II: Part 2

The Sight of the Bumblebee.--The large eyes located on the sides of the head are made up of a large number of little units, each of which is considered to be a very simple eye. The large eyes are therefore called the _compound eyes_. All insects are provided with compound eyes, with simple eyes, or in most cases with both. The simple eyes of the bee may be found by a careful observer between and above the compound eyes.

Insects can, as we have already learned, distinguish differences in color at some distance; they can see _moving_ objects, but they do not seem to be able to make out form well. To make up for this, they appear to have an extremely well-developed sense of smell. Insects can distinguish at a great distance odors which to the human nose are indistinguishable. Night-flying insects, especially, find the flowers by the odor rather than by color.

Mouth Parts of the Bee.--The mouth of the bee is adapted to take in the foods we have mentioned, and is used for the purposes for which man would use the hands and fingers. The honeybee laps or sucks nectar from flowers, it chews the pollen, and it uses part of the mouth as a trowel in making the honeycomb. The uses of the mouth parts may be made out by watching a bee on a well-opened flower.

Suggestions for Field Work.--In any locality where flowers are abundant, try to answer the following questions: How many bees visit the locality in ten minutes? How many other insects alight on the flowers? Do bees visit flowers of the same kinds in succession, or fly from one flower on a given plant to another on a plant of a different kind? If the bee lights on a flower cluster, does it visit more than one flower in the same cluster? How does a bee alight? Exactly what does the bee do when it alights?

Butter and Eggs (_Linaria vulgaris_).--From July to October this very abundant weed may be found especially along roadsides and in sunny fields. The flower cluster forms a tall and conspicuous cluster of orange and yellow flowers.

The corolla projects into a spur on the lower side; an upper two-parted lip shuts down upon a lower three-parted lip. The four stamens are in pairs, two long and two short.

Certain parts of the corolla are more brightly colored than the rest of the flower. This color is a guide to insects. Butter and eggs is visited most by bumblebees, which are guided by the orange lip to alight just where they can push their way into the flower. The bee, seeking the nectar secreted in the spur, brushes his head and shoulders against the stamens. He may then, as he pushes down after nectar, leave some pollen upon the pistil, thus assisting in _self-pollination_. Visiting another flower of the cluster, it would be an easy matter accidentally to transfer this pollen to the stigma of another flower. In this way pollen is carried by the insect to another flower of the same kind. This is known as _cross-pollination_. _By pollination we mean the transfer of pollen from an anther to the stigma of a flower. Self-pollination is the transfer of pollen from the anther to the stigma of the same flower; cross-pollination is the transfer of pollen from the anthers of one flower to the stigma of another flower on the same or another plant of the same kind._

History of the Discoveries regarding Pollination of Flowers.--Although the ancient Greek and Roman naturalists had some vague ideas on the subject of pollination, it was not until the first part of the nineteenth century that a book appeared in which a German named Conrad Sprengel worked out the facts that the structure of certain flowers seemed to be adapted to the visits of insects. Certain facilities were offered to an insect in the way of easy foothold, sweet odor, and especially food in the shape of pollen and nectar, the latter a sweet-tasting substance manufactured by certain parts of the flower known as the nectar glands. Sprengel further discovered the fact that pollen could be and was carried by the insect visitors from the anthers of the flower to its stigma. It was not until the middle of the nineteenth century, however, that an Englishman, Charles Darwin, applied Sprengel's discoveries on the relation of insects to flowers by his investigations upon cross-pollination. The growth of the pollen on the stigma of the flower results eventually in the production of seeds, and thus new plants. Many species of flowers are self-pollinated and do not do so well in seed production if cross-pollinated, but Charles Darwin found that some flowers which were self-pollinated did not produce so many seeds, and that the plants which grew from their seeds were smaller and weaker than plants from seeds produced by cross-pollinated flowers of the same kind. He also found that plants grown from cross-pollinated seeds tended to _vary_ more than those grown from self-pollinated seed. This has an important bearing, as we shall see later, in the production of new varieties of plants. Microscopic examination of the stigma at the time of pollination also shows that the pollen from another flower usually germinates before the pollen which has fallen from the anthers of the same flower. This latter fact alone in most cases renders it unlikely for a flower to produce seeds by its own pollen. Darwin worked for years on the pollination of many insect-visited flowers, and discovered in almost every case that showy, sweet-scented, or otherwise attractive flowers were adapted or fitted to be cross-pollinated by insects. He also found that, in the case of flowers that were inconspicuous in appearance, often a compensation appeared in the odor which rendered them attractive to certain insects. The so-called carrion flowers, pollinated by flies, are examples, the odor in this case being like decayed flesh. Other flowers open at night, are white, and provided with a powerful scent. Thus they attract night-flying moths and other insects.

Other Examples of Mutual Aid between Flowers and Insects.--Many other examples of adaptations to secure cross-pollination by means of the visits of insects might be given. The mountain laurel, which makes our hillsides so beautiful in late spring, shows a remarkable adaptation in having the anthers of the stamens caught in little pockets of the corolla. The weight of the visiting insect on the corolla releases the anther from the pocket in which it rests so that it springs up, dusting the body of the visitor with pollen.

In some flowers, as shown by the primroses or primula of our hothouses, the stamens and pistils are of different lengths in different flowers. Short styles and long or high-placed filaments are found in one flower, and long styles with short or low-placed filaments in the other. Pollination will be effected only when some of the pollen from a low-placed anther reaches the stigma of a short-styled flower, or when the pollen from a high anther is placed upon a long-styled pistil. There are, as in the case of the loosestrife, flowers having pistils and stamens of three lengths. Pollen only grows on pistils of the same length as the stamens from which it came.

The milkweed or butterfly weed already mentioned is another example of a flower adapted to insect pollination.[1]

Footnote 1: For an excellent account of cross-pollination of this
flower, the reader is referred to W. C. Stevens, _Introduction to
Botany_. Orchids are well known to botanists as showing some very
wonderful adaptations. A classic easily read is Darwin, _On the
Fertilization of Orchids_.

A very remarkable instance of insect help is found in the pollination of the yucca, a semitropical lily which lives in deserts (to be seen in most botanic gardens). In this flower the stigmatic surface is above the anther, and the pollen is sticky and cannot be transferred except by insect aid. This is accomplished in a remarkable manner. A little moth, called the _pronuba_, after gathering pollen from an anther, deposits an egg in the ovary of the pistil, and then rubs its load of pollen over the stigma of the flower. The young hatch out and feed on the young seeds which have grown because of the pollen placed on the stigma by the mother. The baby caterpillars eat some of the developing seeds and later bore out of the seed pod and escape to the ground, leaving the plant to develop the remaining seeds without further molestation.

The fig insect (_Blastophaga grossorum_) is another member of the insect tribe that is of considerable economic importance. It is only in recent years that the fruit growers of California have discovered that the fertilization of the female flowers is brought about by a gallfly which bores into the young fruit. By importing the gallflies it has been possible to grow figs where for many years it was believed that the climate prevented figs from ripening.

Other Flower Visitors.--Other insects besides those already mentioned are pollen carriers for flowers. Among the most useful are moths and butterflies. Projecting from each side of the head of a butterfly is a fluffy structure, the palp. This collects and carries a large amount of pollen, which is deposited upon the stigmas of other flowers when the butterfly pushes its head down into the flower tube after nectar. The scales and hairs on the wings, legs, and body also carry pollen.

Flies and some other insects are agents in cross-pollination. Humming birds are also active agents in some flowers. Snails are said in rare instances to carry pollen. Man and the domesticated animals undoubtedly frequently pollinate flowers by brushing past them through the fields.

Pollination by the Wind.--Not all flowers are dependent upon insects or other animals for cross-pollination. Many of the earliest of spring flowers appear almost before the insects do. Such flowers are dependent upon the wind for carrying pollen from the stamens of one flower to the pistil of another. Most of our common trees, oak, poplar, maple, and others, are cross-pollinated almost entirely by the wind.

Flowers pollinated by the wind are generally inconspicuous and often lack a corolla. The anthers are exposed to the wind and provided with much pollen, while the surface of the stigma may be long and feathery. Such flowers may also lack odor, nectar, and bright color. Can you tell why?

Imperfect Flowers.--Some flowers, the wind-pollinated ones in particular, are imperfect; that is, they lack either stamens or pistils. Again, in some cases, imperfect flowers having stamens only are alone found on one plant, while those flowers having pistils only are found on another plant of the same kind. In such flowers, cross-pollination must of necessity follow. Many of our common trees are examples.

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.
Mueller, _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_.

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_.

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.

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.

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.

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.

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.

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_.

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._

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.

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.

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.
Mueller, _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 class.
_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.

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.

Organic Nutrients.--Organic foods (those which come from living sources) are made up of two kinds of substances, the _nutrients_ or food substances and _wastes_ or _refuse_. An egg, for example, contains the white and the yolk, composed of nutrients, and the shell, which is waste. The organic nutrients are classed in three groups.

_Carbohydrates_, foods which contain carbon, hydrogen, and oxygen in a certain fixed proportion (C{6}H{10}O{5} is an example). They are the simplest of these very complex chemical compounds we call organic nutrients. Starch and sugar are common examples of carbohydrates.

_Fats and Oils._--These foods are also composed of carbon, hydrogen, and oxygen in a proportion which enables them to unite readily with oxygen.

_Proteins._--A third group of organic foods, proteins, are the most complex of all in their composition, and have, besides carbon, oxygen, and hydrogen, the element nitrogen and minute quantities of other elements.

Test for Starch.--If we boil water with a piece of laundry starch in a test tube, then cool it and add to the mixture two or three drops of iodine solution,[3] we find that the mixture in the test tube turns purple or deep blue. It has been discovered by experiment that starch, and no _other known substance_, will be turned purple or dark blue by iodine. Therefore, iodine solution has come to be used as a test for the presence of starch.

Footnote 3: Iodine solution is made by simply adding a few
crystals of the element iodine to 95 per cent alcohol; or,
better, take by weight 1 gram of iodine crystals, 2/3 gram
of iodide of potassium, and dilute to a dark brown color in
weak alcohol (35 per cent) or distilled water.

Starch in the Bean.--If we mash up a little piece of a bean cotyledon which has been previously soaked in water, and test for starch with iodine solution, the characteristic blue-black color appears, showing the presence of the starch. If a little of the stained material is mounted in water on a glass slide under the compound microscope, you will find that the starch is in the form of little ovoid bodies called _starch grains_. The starch grains and other food products are made use of by the growing plant.

Test for Oils.--If the substance believed to contain oil is rubbed on brown paper or is placed on paper and then heated in an oven, the presence of oil will be known by a translucent spot on the paper.

Protein in the Bean.--Another nutrient present in the bean cotyledon is _protein_. Several tests are used to detect the presence of this nutrient. The following is one of the best known:--

Place in a test tube the substance to be tested; for example, a bit of hard-boiled egg. Pour over it a little strong (60 per cent) nitric acid and heat gently. Note the color that appears--a lemon yellow. If the egg is washed in water and a little ammonium hydrate added, the color changes to a deep orange, showing that a protein is present.

If the protein is in a liquid state, its presence may be proved by heating, for when it coagulates or thickens, as does the white of an egg when boiled, protein in the form of an _albumin_ is present.

Another characteristic protein test easily made at home is burning the substance. If it burns with the odor of burning feathers or leather, then protein forms part of its composition.[4]

Footnote 4: Other tests somewhat more reliable, but much
more delicate, are the biuret test and test with Millon's
reagent.

A test of the cotyledon of a bean for protein food with nitric acid and ammonium hydrate shows us the presence of this food. Beans are found by actual test to contain about 23 per cent of protein, 59 per cent of carbohydrates, and about 2 per cent oils. The young plant within a pea or bean is thus shown to be well supplied with nourishment until it is able to take care of itself. In this respect it is somewhat like a young animal within the egg, a bird or fish, for example.

Beans and Peas as Food for Man.--So much food is stored in legumes (as beans and peas) that man has come to consider them a very valuable and cheap source of food. Study carefully the following table:--

NUTRIENTS FURNISHED FOR TEN CENTS IN BEANS AND PEAS AT
CERTAIN PRICES PER POUND

=========================================================================
| | TEN CENTS WILL PAY FOR
| PRICES |------------------------------------------
FOOD MATERIALS | PER | TOTAL | | |
AS PURCHASED | POUND | FOOD | PROTEIN | FAT |CARBOHYDRATES
| |MATERIAL | | |
---------------------|--------|---------|---------|--------|-------------
| _Cents_| _Pounds_| _Pounds_|_Pounds_| _Pounds_
Kidney beans, dried | 5 | 2.00 | 0.45 | 0.04 | 1.19
Lima beans, fresh, | | | | |
shelled | 8 | 1.25 | .04 | -- | .12
Lima beans, dried | 6 | 1.67 | .30 | .03 | 1.10
String beans, fresh, | | | | |
30 cents per peck | 3 | 3.33 | .07 | .01 | .23
Beans, baked, canned | 5 | 2.00 | .14 | .05 | .39
Lentils, dried | 10 | 1.00 | .26 | .01 | .59
Peas, green, in pod, | | | | |
30 cents per peck | 3 | 3.33 | .12 | .01 | .33
Peas, dried | 4 | 2.50 | .62 | .03 | 1.55
=========================================================================

Germination of the Bean.--If dry seeds are planted in sawdust or earth, they will not grow. A moderate supply of water must be given to them. If seeds were to be kept in a freezing temperature or at a very high temperature, no growth would take place. A moderate temperature and a moderate water supply are most favorable for their development.

If some beans were planted so that we might make a record of their growth, we would find the first signs of germination to be the breaking of the testa and the pushing outward of the hypocotyl to form the first root. A little later the hypocotyl begins to curve downward. A later stage shows the hypocotyl lifting the cotyledon upward. In consequence the hypocotyl forms an arch, dragging after it the bulky cotyledons. The stem, as soon as it is released from the ground, straightens out. From between the cotyledons the budlike plumule or epicotyl grows upward, forming the first true leaves and all of the stem above the cotyledons. As growth continues, we notice that the cotyledons become smaller and smaller, until their food contents are completely absorbed into the young plant. The young plant is now able to care for itself and may be said to have passed through the stages of germination.

What makes an Engine Go.--If we examine the sawdust or soil in which the seeds are growing, we find it forced up by the growing seed. Evidently work was done; in other words, _energy_ was released by the seeds. A familiar example of release of energy is seen in an engine. Coal is placed in the firebox and lighted, the lower door of the furnace is then opened so as to make a draft of air which will reach the coal. You know the result. The coal burns, heat is given off, causing the water in the boiler to make steam, the engine wheels to turn, and work to be done. Let us see what happens from the chemical standpoint.

Coal, Organic Matter.--Coal is made largely from dead plants, long since pressed into its present hard form. It contains a large amount of a chemical element called carbon, the presence of which is characteristic of all organic material.

Oxidation, its Results.--When things containing carbon are lighted, they burn. If we place a lighted candle which contains carbon in a closed glass jar, the candle soon goes out. If we then carefully test the air in the jar with a substance known as _limewater_,[5] the latter, when shaken up with the air in the jar, turns milky. This test proves the presence in the jar of a gas, known as _carbon dioxide_. This gas is formed by the carbon of the candle uniting with the oxygen in the air. When the oxygen of the air in the jar was used up, the flame went out, showing that oxygen is necessary to make a thing burn. This uniting of oxygen with some other substance is called _oxidation_.

Footnote 5: Limewater can be made by shaking up a piece of
quicklime the size of your fist in about two quarts of
water. Filter or strain the limewater into bottles and it is
ready for use.

Oxidation possible without a Flame.--But a flame is not necessary for oxidation. Iron, if left in a damp place, becomes rusty. A union between the oxygen in the water or air and the iron makes what is known as iron oxide or rust. This is an example of _slow oxidation_.

Oxidation in our Bodies.--If we expel the air from our lungs through a tube into a bottle of limewater, we notice the limewater becomes milky. Evidently carbon dioxide is formed in our own bodies and oxidation takes place there. Is it fair to believe that the heat of our body (for example, 98.6 deg. Fahrenheit under the tongue) is due to oxidation within the body, and that the work we do results from this chemical process. If so, what is oxidized?

Energy comes from Foods.--From the foregoing experiment it is evident that food is oxidized within the human body to release energy for our daily work. Is it not logical to suppose that all living things, both plant and animal, release energy as the result of oxidation of foods within their cells? Let us see if this is true in the case of the pea.

Food oxidized in Germinating Seeds.--If we take equal numbers of soaked peas, placed in two bottles, one tightly stoppered, the other having no stopper, both bottles being exposed to identical conditions of light, temperature, and moisture, we find that the seeds in both bottles start to germinate, but that those in the closed bottle soon stop, while those in the open jar continue to grow almost as well as similar seeds placed in an open dish would.

Why did not the seeds in the covered jar germinate? To answer this question, let us carefully remove the stopper from the stoppered jar and insert a lighted candle. The candle goes out at once. The surer test of limewater shows the presence of carbon dioxide in the jar. The carbon of the foodstuffs of the pea united with the oxygen of the air, forming carbon dioxide. Growth stopped as soon as the oxygen was exhausted. The presence of carbon dioxide in the jar is an indication that a very important process which we associate with animals rather than plants, that of _respiration_, is taking place. The seed, in order to release the energy locked up in its food supply, must have oxygen, so that the oxidation of the food may take place. _Hence a constant supply of fresh air is an important factor in germination._ It is important that air should penetrate between the grains of soil around a seed. The frequent stirring of the soil enables the air to reach the seed. Air also acts upon some materials in the soil and puts them in a form that the germinating seed can use. This necessity for oxygen shows us at least one reason why the farmer plows and harrows a field and one important use of the earthworm. Explain.

Structure of a Grain of Corn.--Examination of a well-soaked grain of corn discloses a difference in the two flat sides of the grain. A light-colored area found on one surface marks the position of the embryo; the rest of the grain contains the food supply. The interesting thing to remember here is that the food supply is _outside_ of the embryo.

A grain cut lengthwise perpendicular to the flat side and then dipped in weak iodine shows two distinct parts, an area containing considerable starch, the _endosperm_, and the embryo or young plant. Careful inspection shows the hypocotyl and plumule (the latter pointing toward the free end of the grain) and a part surrounding them, the _single_ cotyledon (see Figure). Here again we have an example of a fitting for future needs, for in this fruit the one seed has at hand all the food material necessary for rapid growth, although the food is here outside the embryo.

Endosperm the Food Supply of Corn.--We find that the one cotyledon of the corn grain does not serve the same purpose to the young plant as do the two cotyledons of the bean. Although we find a little starch in the corn cotyledon, still it is evident from our tests that the endosperm is the chief source of food supply. The study of a thin section of the corn grain under the compound microscope shows us that the starch grains in the endosperm are large and regular in size. When the grain has begun to grow, examination shows that the starch grains near the edge of the cotyledon are much smaller and quite irregular, having large holes in them. We know that the germinating grain has a much sweeter taste than that which is not growing. This is noticed in sprouting barley or malt. We shall later find that, in order to make use of starchy food, a plant or animal must in some manner change it over to sugar. This change is necessary, because starch will not dissolve in water, while sugar will; in this form substances can pass from cell to cell in the plant and thus distribute the food where it is needed.

A Test for Grape Sugar.--Place in a test tube the substance to be tested and heat it in a little water so as to dissolve the sugar. Add to the fluid twice its bulk of Fehling's solution,[6] which has been previously prepared. Heat the mixture, which should now have a blue color, in the test tube. If grape sugar is present in considerable quantity, the contents of the tube will turn first a greenish, then yellow, and finally a brick-red color. Smaller amounts will show less decided red. No other substance than sugar will give this reaction. If Benedict's test[7] is used, a colored precipitate will appear in the test tube after boiling.

Footnotes 6 and 7: Directions for making these solutions
will be found in Hunter's _Laboratory Problems in Civic
Biology_.

Starch changed to Grape Sugar in the Corn.--That starch is being changed to grape sugar in the germinating corn grain can easily be shown if we cut lengthwise through the embryos of half a dozen grains of corn that have just begun to germinate, place them in a test tube with some Fehling's solution, and heat almost to the boiling point. They will be found to give a reaction showing the presence of sugar along the edge of the cotyledon and between it and the endosperm.

Digestion.--This change of starch to grape sugar in the corn is a process of _digestion_. If you chew a bit of unsweetened cracker in the mouth for a little time, it will begin to taste sweet, and if the chewed cracker, which we know contains starch, is tested with Fehling's solution, some of the starch will be found to have changed to grape sugar. Here, again, a process of digestion has taken place. In both the corn and in the mouth, the change is brought about by the action of peculiar substances known as digestive ferments, or _enzymes_. Such substances have the power under certain conditions to change insoluble foods--solids--into soluble substances--liquids. The result is that substances which before digestion would not dissolve in water now will dissolve.

The Action of Diastase on Starch.--The enzyme found in the cotyledon of the corn, which changes starch to grape sugar, is called _diastase_. It may be separated from the cotyledon and used in the form of a powder.