Page:The American Cyclopædia (1879) Volume XIII.djvu/591

 PLANT 575 greatly, and is of course greatly modified by temperature and other influences ; some seeds sprout in 12 hours, those of most of the agri- cultural grains in four or five days, others require as many weeks, and there are several kinds which will lie dormant one and even two years. The time required by some seeds depends upon their treatment ; if sown as soon as ripe they germinate the following spring, but if not sown until spring they remain dor- mant until the year following. The growth of the young plant, which the embryo may be considered as soon as germination takes place, may be followed in the bean. The embryo when liberated from the seed coats consists of two very thick and fleshy leaves, a very short stem to which they are attached, and between them some rudimentary leaves, or a bud. The two fleshy leaves, commonly known as the seed leaves, are the cotyledons, the short stem is the radicle, and the small bud the plumule ; the plumule is not at first visible in all germi- nating seeds, but it soon manifests itself. The first step in growth is the elongation of the radicle, the cells of which multiply and grow by a similar process to that by which it grew in forming the seed ; if in the soil, one end pushes downward and the other makes its way upward, carrying with it the cotyledons, which it lifts above the surface of the soil. This growth is nourished by the material within the fleshy cotyledons ; in the forming and ripening of the seed the cotyledons were stored to re- pletion with starch and other proximate prin- ciples; these are now rendered soluble and carried to the part where growth is going on, and are there assimilated by the growing cells, where some go to build up cell walls, and others to form cell contents. These changes take place through the influence of diastase and other albuminoids, which act the part of ferments, converting starch from its fixed and insoluble condition into dextrine and sugar, forms which being soluble can be transferred. These changes are accompanied by the absorp- tion of oxygen, the oxidation of some of the elements of the seed, and the escape of carbonic acid, and both hydrogen and nitrogen are also given off ; and these changes are attended by the development of heat, which when germi- nation takes place in a large quantity of seed, as in malting (see BREWING), becomes striking- ly manifest. Before the store of nutriment in the cotyledons is exhausted preparation's made for drawing subsistence from the soil ; roots, organs especially designed for absorbing liquids, make their appearance on the radicle, and through them water, holding various sub- stances in solution, is conveyed to the inte- rior of the plant, where this crude sap, as it is called, is converted into compounds capable of nourishing the plant. Soon after the coty- ledons of the bean appear above the soil the plumule increases in size ; it is lifted up upon a stem, its rudimentary leaves enlarge and ex- pand and disclose another bud, which in turn is raised by the growth of a stem and develops its leaves, and so the growth is continued. In the bean the early leaves are in pairs, but soon only one appears at a place on the stem, and the plant elongates by a successive addition of leaves, each separated from the one below it by a portion of stem ; the cotyledons, though they become green like the other leaves, and no doubt to some extent perform the functions of foliage, after a while fall away. In exam- ining other seeds and watching their germina- tion, while the same general plan will be found to run through all, some marked deviations from the structure as seen in the bean will be noted. In the bean the seed leaves, having been diverted from their ordinary use and made the receptacles of nourishment for the young plant, are much distorted ; a condition carried still further in the pea, in which they do not appear above ground ; and in the acorn, horse chestnut, and others, they are so gorged that but little resemblance is to be traced be- tween them and ordinary leaves ; on the oth- er hand, the cotyledons will often be found thin, and though differing in shape from the leaves produced later, they remain long after germination and are equally useful with the others. In many seeds the nourishment for the growth of the young plant is placed out- side of the embryo, and not within it, as in the bean ; this material is termed albumen, and may be copious or scanty in proportion to the size of the embryo. The embryo is placed in the centre of the albumen, or at one end,' or may be coiled in a more or less complete ring around the outside of the albumen. The embryo differs greatly in degree of develop- ment; in some cases the cotyledons are large and the plumule is distinctly visible, while in other seeds it is very rudimentary, the little stem or radicle having merely a notch at one end, the rude lobes thus formed foreshadow- ing the cotyledons. The name albumen is used to designate the nutritious matter placed ^for its nourishment outside of the embryo, with- out reference to its chemical composition ; it was so called by the early botanists, who saw some analogy between its office and that of the white of an egg, and though endosperm, perisperm, and other distinctive names have been proposed, albumen is still used by the highest authorities. The character of the albu- men varies greatly ; it may be mucilaginous, fleshy, floury, oily, horny, &c. The presence or absence of albumen in seeds (albuminous and exalbuminous) is an important character in classifying plants. A still more important one is found in the number of cotyledons, which in our example, the bean, is two, that number being found in the majority of the plants in northern climates, which are dicotyle- donous. Plants in which but one cotyledon is present (monocotyledonous) are throughout their whole structure very different from the others, and this difference in the embryo, ac- companied by other characters, serves for the