IV. FLOWERS AND THEIR WORK

Problem VI. The structure and work of the parts of a flower. (Laboratory Manual, Prob. VI.)

Structure of a Simple Flower. Flowers of different kinds of plants vary greatly in size, shape, and color. In our study of the flower our problem will be primarily to find out the use of the flower to the plant which produces it. To solve this problem we must first learn something of the structure and uses of the parts

A flower of the sedum, from the side, considerably enlarged; A, anther of stamen; C, carpel; F, filament; P, petal; S, sepal.

of a very simple flower. Examples of such flowers are the evening primrose and the sedum (live-forever), both of which are plentiful in the fall.

The Floral Envelope. In such a flower the expanded portion of the flower stalk, which holds the parts of the flower, is called the receptacle. The five green leaflike parts covering the unopened flower are called the sepals. Sometimes the sepals are all joined or united in one piece. Taken together, they are called the calyx. The sepals come out in a circle or whorl on the flower stalk.

The more brightly colored structures are the petals. They form the corolla. The corolla is of importance, as we shall see later, in making the flower conspicuous.

The Essential Organs. A flower, however, could live without sepals or petals and still do the work for which it exists. The essential organs of the flower are within the so-called floral envelope. They consist of the stamens and carpels (or pistil), the

latter being in the center of the flower. The structures with the knobbed ends are called stamens. In a single stamen the boxlike part at the end is the anther; the stalk is called the filament. The anther is in reality a hollow box which produces a large number of little grains called pollen. It is neces

sary for the reproduction of new plants that the pollen get out of the anther. Each carpel or pistil is composed of a rather stout base called the ovary, and a more or less lengthened portion rising from the ovary called the style. The upper end of the style, which in some cases is somewhat broadened, is called the stigma. The stigmatic surface usually secretes a sweet fluid in which grains of pollen from flowers of the same kind can grow.

A flower of the sedum from above; A, anther; C, carpel; F, lament; P, petal; S, sepal. Notice how the parts come out in circles or whorls.

scope, differ greatly in form and appearance.

Some are rela

tively large, some small, some rough, others smooth, some spherical, and others angular. They all agree, however, in having a thick

n

A pollen grain highly magnified.
It contains two nuclei (n, n') at

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. When we see it, however, we can distinguish two nuclei in the protoplasm. Hence we know that at least two cells exist there.

Growth of Pollen Grains. - Under certain conditions a pollen grain will burst open and grow. This growth the stage here represented. 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, one of the nuclei enters the threadlike growth (this growth is called the pollen tube; see Figure). The cell which grows into the pollen tube is known as the sperm cell.

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

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 cell of the pollen tube grows toward; ultimately the sperm cell reaches the egg cell and unites with it. The two cells,

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. The first beginning of the growth of the seed takes place at the moment of fertilization. From that time on there is a growth within the ovule of a little structure called the embryo. The embryo will give rise to the future plant. After fertilization the ovule grows into a seed.

Problem VII. A study of cross-pollination and some means of bringing it about. (Laboratory Manual, Prob. VII.)

(a) Adaptations in the flower.

(b) Adaptations in an insect agent. (c) Other agents.

History of the Discoveries regarding Pol

- Although the ancient

lination of Flowers.
Greek and Roman naturalists had some
vague ideas on the subject of fertilization,
it was not until the latter part of the eight-
eenth century that it was demonstrated that
pollen was necessary for the growth of the
embryo within a seed. In the latter part
of the eighteenth century 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

Fertilization of the ovule. A pistil cut down lengthwise (only one side shown). The pollen tube is seen entering the cavity (locule) of the ovary.

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, worked out the true relation of insects to flowers by his investigations upon the cross-pollination of flowers. 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 of the same kind. 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 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 many years on the pollination of many