Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/140

100 the embryo-cell appearing in the Archegonia or Corpuscles, to which there is nothing to correspond in the higher plants, which necessarily want the Prothallus or Endosperm as well. After the contact of the germinal vesicle and the pollen, the life of the new Gymnosperm begins in the formation of the embryo, which consists of a stem or radical supporting two or more leaves (Figs. 139, 138, a), called cotyledons. The embryo of the Cypress in its two cotyledons recalls that of Selaginella (a Lycopod). Those plants whose embryos have only one leaf (Fig. 140, a) or cotyledon are called Monocotyledonous, while those having two are known as Dicotyledonous. The Dicotyledonous plants further offer in their stem the destruction of pith, wood, and bark, and increase the diameter of their stem by layer after layer (Fig. 141, 1, 2, 3) of wood being added, the new layer being deposited between the old layer and the bark, this new layer growing at the expense of the Cambium, a layer (Fig. 141, C) found always between the last and most external layer of wood and the bark. Such plants are called outside growths, or Exogens. The Monocotyledonous plants, however, do not present, in their stem, the difference of pith, wood, and bark so well defined as in Exogens; the new wood being added in bundles intermingling with the old, and deposited principally towards the centre of the plant. (Fig. 142.) Such forms are called inside growths, or Endogens. The wood of an Exogen is oldest and hardest in the centre, whereas the wood of an Endogen is newest and softest towards the centre. The increase in the diameter of the trunk of an Exogen, as in the Oak, is indefinite; the stem of the Endogen, as in the Palm, is limited as regards its diameter, the tendency being rather to grow upwards. In speaking of the Lycopodiaceae, Cycadae, and Coniferae, we have noticed they have important features in common: the reproductive apparatus is essentially the same; the form of the embryo in some genera