ENDOGENOUS PLANTS, or ENDOGENS (Gr. en don, within, and genos, birth or origin), one of the great classes into which the vegetable kingdom is divided, the others receiv ing the corresponding designations of exogenous plants and acrogenous plants. The char acter from which this designation is derived is found in the structure of the stem, which does not increase in thickness by additional layers on the outside like the exogenous stem, familiarly illustrated in all the trees of the colder parts of the world, but receives its additions of woody matter in the interior; and in general does not continue to increase indefinitely in thickness like the exogenous stem, but is arrested when a certain thick ness has been attained, different in different species, and afterwards increases only in length. When a transverse section is made of an endogenous stem, numerous bundles of vessels are seen irregularly in cellular tissues, the younger and softer parts of the stem exhibiting the cellular tissue in greatest proportion, the older and lower parts chiefly abounding in vascular bundles, which are, however, somewhat scattered in the central part of the stem, and are densely aggregated towards the circumference, there, in the palms generally, forming very hard wood, in some of them wood so hard that it cannot be cut with a hatchet. The stems of E. P. in the far greater number of cases produce terminal buds only, and not lateral buds, and are therefore uubranched. From the bases of the leaves, definite bundles of vascular tissue converge towards the center; but these extending downwards extend also outwards, and thus an interlacing of fibers takes place, which contributes not a little to the strength and compactness of the wood in the lower part of the stem. As the fibers extend downwards, they also become attenuated, spiral and porous vessels disappearing, and nothing but the most ligneous substance remaining. It is the hardening of the outer part of the stem which arrests its increase in thickness. Endogenous sterns have not a distinct pith nor any medullary rays. When the central part is soft and pith-like, yet it is not distinctly separated from the surrounding wood, and has no medullary sheath. In many E. P., as in the greater number of grasses, the center of. the stem is hollow. This is not the case at first, when the stem begins to grow; and when any cause makes the growth of the stem unusually slow, so that it is much stunted, it remains solid; the fistular character of the stem is the result of its rapid growth, rupturing the cells of the central portion, which finally disappear. Endogenous stems have no cambium and no proper bark. There
is, indeed, a cellular epidermis; and there is also within it, and exterior to the hardest woody part of the stem, a comparatively soft layer of a corky substance, which is sometimes called bark, sometimes false bark, which does not separate from the wood below it without leaving myriads of little broken threads, the ends of the fibers which have extended into it from the hardest part of the stem. In those exogenous plants which produce lateral buds and branclies„,the fibers of the branches on descend ing to the stem extend on the outside of the proper stein, between its hardest portion and the false bark; and in this way a great thickness is sometimes attained, as in the dragon-tree. In the grasses, a plexus of fibers takes place at the nodes, the fibers cross ing from one side to the other. No British tree—and it may almost he said, no tree of temperate or colder climates—is endogenous. Almost all the endogenous trees are palms, although a few, as the dragon-tree, belong to other orders. E. P., however, are numerous in all parts of the world. Among E. P. are many of the plants most useful to mankind, particularly palms and grasses, all the true corn-plants being included among the latter. Nutritious substances are very extensively produced both in the fruit or seed, and in other parts; poisonous products are comparatively rare, although found in the aracece, liliacece, melanthacece, and other orders. Aromatic secretions are characteristic chiefly of one order, scitamin&e. Besides palms and grasses,, many of the E. P. are of great beauty, and matey produce most beautiful flowers. Lilies and orchids may be mentioned as instances.
E. P. are monocotyledonous; and the terms endogenous and monocotyledonous are there fore often employed indiscriminately to designate the class. But Lindley distinguishes a class of dictyogens (q.v.), which, although mon ocotyledonous, have stems approaching to the exogenous character. The leaves of E. P. generally exhibit parallel venation, which is indeed strictly confined to them, although a venation resembling it, or rather simulating it, may be seen in some exogenous plants. The seed also germinates in a peculiar manner, different from that of exogenous plants, and to which the name endor hizal has been given, the radicle being protruded from within the substance of the embryo, and surrounded by a cellular sheath formed from the integument which it breaks in its egress.