IOO EVOLUTION 7 
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 hav- 
ing 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 Mono- 
cotyledonous 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 inter- 
mingling 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 Lycopodiacee, 
Cycadze, and Coniferz, we have noticed they have important 
features in common: the reproductive apparatus is essen- 
tially the same; the form of the embryo in some genera 
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