592 PLANT their external appearance. It may be soft and membranous or hard and bony; its surface is often marked by furrows, pits, and other irregularities; it sometimes expands into a broad wing (<?. <?., catalpa), or produces a tuft of silky hairs (e. g., milkweed). The append- ages to the seed coat of one plant are of vast importance to the world ; the fibre, cotton, is only elongated simple cells produced from the spongy testa of the seed coat of gossypium. These and other appendages aid in the disper- sion of the seeds ; the one-seeded akenes of the composite family, often mistaken for seeds, also present a wonderful variety of contrivances for dissemination. The stalk of the ovary (funiculus) retains the same name in the seed. In falling away from the stalk when ripe the seed shows a scar upon its testa, the liilum, sometimes a mere point and obscure, often quite noticeable, as in the bean, and in the horse chestnut very broad and conspicuous. Some appendages of the seed are due to an expansion of the funiculus, while others are regarded as a growth from the micropyle, but the distinc- tion between the two is not always manifest. The seed of the white water lily (nympJicea) is surrounded by a delicate transparent sac (arillus) of this nature ; the pulpy covering of the seeds of the burning-bush (euonymus) and waxwork (celastrus) is an aril, and in the nut- meg the same organ becomes an important product, mace. The content of the seed, whe- ther it consists of the embryo only, or both that and the albumen, is the nucleus, the struc- ture of which has been sufficiently described in speaking of germination. In this sketch of the plant but little reference has been made to the great series of cryptogamous or flower- less plants, which are subdivided into several classes, each having its peculiar structure and method of reproduction ; accounts of these may be found in the articles ALG.E, FERNS, FUNGI, LIVERWORTS, LTCOPODIUM, and MOSSES. Classification of Plants. While plants ev- erywhere are engaged in essentially the same work, that of vegetation or the growth of the individual, and reproduction or providing for the continuation of the kind by seeds, the details of these operations, as has been shown, are wonderfully varied ; to study such a mul- titude of plants understandingly, they must be grouped and arranged into a system. Some account of the different methods that have been from time to time employed is given in the article BOTANY. The natural system, at- tributed to Jussieu, who more distinctly pre- sented it than any of his predecessors, has been greatly modified and improved by the labors of subsequent botanists ; the most recent pres- entation of this is in the " Synopsis " appended by Dr. J. D. Hooker to the English edition of Le Maout and Decaisne's " General System of Botany," translated by Mrs. Hooker; in this the arrangement to be adopted in Hooker and Bentham's Genera Plantarum is indicated. It has already been shown that flowering plants are divided into two great classes founded upon the structure of the seed, monocotyledons and dicotyledons, which are accompanied by differ- ences in the structure of the stem, called endo- genous and exogenous growth. These are subdi- vided into subclasses, divisions, and other groups founded upon the structure of the flower. Classification by whatever system commences with grouping individuals into species ; it has been suggested that it is difficult to define the limits of the individual in plants, as they are made up of a multitude of parts, each of which with proper care may be separated and con- tinue an independent existence. If the question of individuality has given rise to much discus- sion, not less so has that of the species. It is not easy to give a brief and satisfactory answer to the question, What constitutes a species? One of the best definitions is, that it is a col- lection of individuals so nearly alike that they may have been the progeny of one parent. Some botanists believe that a species may vary widely, and others rank what the former would consider mere varieties as distinct spe- cies ; while Bentham considers that the various forms of the European blackberry and dew- berry are reducible to two species, some other botanists enumerate 36 ; the rich collection of facts brought together by Mr. Darwin in his " Origin of Species" and "Variation of Plants and Animals under Domestication " shows the wide variations of which species are capable, and has led naturalists to take very liberal views of their limits. This liability to vary, and the fact that peculiarities in species may be rendered permanent, are of the greatest importance economically ; while many desira- ble departures from the normal form can only be continued by subdivision, others, as in an- nual and biennial plants, may be propagated by the seed ; by careful selection of plants as seed-bearers, and continuous sowing of seed for several years in succession, a peculiarity becomes fixed and a race is produced. Species are grouped into genera founded upon simi- larities of structure, and where the genus is a large one it is divided into subgenera, and these again into sections. We find genera large or small, according to the importance attached to points of structure by different botanists ; while most regard the pine, spruce, larch, and oth- ers as sufficiently unlike to rank botanically, as they do popularly, as distinct genera, some class them all in the pine genus. Kindred genera are grouped as families or orders ; these when large are often subdivided, and we have suborders, tribes, and subtribes. The peach, strawberry, blackberry, rose, and apple have certain characters in common, which bring them all into the rose family ; but they differ so much in other respects that they fall into three subfamilies, the peach in one, the apple in another, and the others in a third; still further, the strawberry, blackberry, and rose, though in one subfamily, are sufficiently unlike to be placed in separate tribes. The next
