168 EVOLUTION OF PLANTS 
cies gives them the strikingly symmetrical conical form 
so characteristic of most of the group. 
The leaves of the Conifers are usually slender 
“needles,” or are small and scale-like, as in the cypress 
and arbor-vite. Usually they remain attached to the 
stem for several years, but in a few cases, like the larch 
and bald cypress, they are shed annually. Like the 
Cycads, the Conifers generally have a main tap-root, 
which, like the stem, shows a continuous secondary 
growth in thickness. This in the stem results in the 
formation of the well-known annual growth-rings. This 
secondary growth is much like that found in the stems 
of normal Dicotyledons, and on the strength of this the 
older botanists united these with the Gymnosperms 
under the name “Exogens”; but the great differences . 
in the structure of the flower, and especially in the 
gametophyte, forbid the idea of such a union, and 
botanists are now agreed that no near relationship exists 
between the two. 
The flowers of the Coniferze are very simple in struct- 
ure. In the lowest types, like the yew (Taxus) (Fig. 
41, A~G), the macrosporangium is borne directly at the 
end of a shoot, and is in fact its transformed apex. It 
becomes invested with an integument like that found 
in Cycas, and is protected while young by several over- 
lapping scale-leaves. Within is produced a group of 
sporogenous cells, from one of which is developed the 
single macrospore which gives rise to a gametophyte 
of considerable size with several archegonia. The 
microsporangia are formed, several together, upon um- 
brella-shaped leaves, which are arranged in a cone which 
suggests that of Equisetum (F,G). The germinating 
