The distribution of mechanical tissue in the stem presents a wonderful example of useful arrange ment to secure the highest degree of strength with the least expenditure of material. The prin ciple of the girder and of the hollow cylinder is everywhere employed (Fig. 39) in leaf and stem. It results that a wheat stalk is a model of light ness and strength, and at the same time it is elas tic enough to bend sufficiently in the wind. In the root (Fig. 29) the strengthening tissue forms strands in the center, known as cable construction, that enable it to resist pulling strains. Some stems economize material by climbing on walls, trees, or other supports. Some weave themselves in and out of the branches of other plants (black berry), others form tendrils by modifying a branch (squash, grape), or a leaf (pea), or a leaf-stalk (clematis). The coiling of the tendril is due to a stimulus such that the contact side grows less rapidly than the opposite side. The tendril, and the tip of the stem as well, usually has a sweeping circular movement that assists in finding a sup port. The tendrils of the Boston ivy fasten them selves to walls ; the roots of the English ivy answer the same purpose.
Plants which twine do so apparently under the influence of gravity, which causes one side to grow faster than another, so that the tip circles in the same direction as the hands of a watch, to the right, or with the sun (as in the hop), or in the opposite direction (as in the morning - glory). Such plants unwind and reverse their direction if placed upside down, and they will not twine on a horizontal or nearly horizontal support.
The the work of root, stem and leaf has stored a sufficient surplus of food, the plant proceeds to flower. The century plant spends many years in this process ; trees usually take four or five years or more ; biennials, as the beet and turnip, require two, while annuals complete their preparation in a few days or weeks. The food is stored in roots, tubers, root stocks, stems, or in modified leaves such as we find in bulbs. In the latter case, the fully formed minia ture flowers can often be seen on cutting open the bulb.
The flower is usu ally spoken of as a modified branch. In their early stages, flower-buds are so much like leaf-buds that they cannot be told apart. But the growing leaf-bud pro duces leaves that soon become separated by the elongation of the stem, while in the flower-bud they remain crowded to gether, and become modified into dissimilar struc tures.
The outermost set of these structures, the calyx (Fig. 14), consists of green, leaf-like sepals whose function is to protect the internal parts, much as the outer leaves of a bud protect the innermost.
Next comes the corolla, consisting of petals, which are leaf-like except in color. Instead of chloro phyll they possess a number of pigments that are either held in solution in the cell sap or appear in solid form. These, by their combination, produce an endless variety of coloration. The appearance of white petals is due (like that of snow) to the presence of air.
The next set of organs, the stamens, often have a leaf-like basal part, while the upper part pro duces an anther, i. e., a structure consisting usually of four cavities filled with pollen-grains or micro spores. At maturity these cavities open and dis charge their pollen in the form of yellow dust. The innermost set of organs, known as the carpels, are often leaflike, like, as, for example, in the pea-pod, whose texture, color and veining are essentially those of a leaf. It corresponds to a leaf folded lengthwise on the midrib, so as to bring the edges together. Along the united edges are borne the seeds. Such a carpel is called a simple pistil or ovary ; when several are united, as usually is the case, the resulting structure is called a compound pistil or ovary. The term ovary is applied to the part that contains the seeds or ovules, while the term pistil includes also the style and stigma. The stigma is borne on the summit of the ovary, often on a stalk called the style, and consists of a sticky or hairy surface designed to catch and retain the pollen. Inside the ovary are found the rudi mentary seeds, or ovules (Fig. 40). An ovule usually has two coats, inside of which is a mass of tissue called the nucellus, containing a cavity called the embryo-sac, or macrospore. Inside of this are found one or more eggs.
In order that the egg may develop, pollen must be brought to the stigma and there germinate (Fig. 41), sending out a long germ tube that makes its way down the style to the embryo-sac.
A nucleus (Fig. 42, pn) makes its way from the pollen-tube through an open ing that is formed at its end, and en ters the embryo-sac, where it unites with the nucleus of the egg (e). This constitutes the act of fertilization, and the characters of both parents are thereby united in the nucleus so formed. This nucleus is called the fertilized egg. Since each of the fusing nuclei has the same number of chromo somes, the fertilized egg has twice as many, and this double number is found in all the cells of the plant that develop from the fertilized egg, until, in the mother-cells, that give rise to the pollen-grains and embryo-sac, the number is suddenly reduced to one-half.