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-vitse. 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 diflFerences 

 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 Coniferse are very simple in struc1> 

 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 



