Jan., 1905.] 
Heterosporous Pteridophyte. 
257 
have been unable to adapt themselves to the new condition of 
things. The next higher subkingdom, the G}'mnosperms, be- 
came the dominant group of plants. 
In tracing out the life cycle, Selaginella kraussiana will be 
taken as a representative with incidental references to other 
tvpes. There is. of course, a well-marked alternation of genera- 
tions, the gametophyte and the sporophyte, and each generation 
usually leads an independent existence for at least a part of its 
life. The conspicuous plant is the sporophyte or nonsexual gen- 
eration. It consists of a more or less dorsi ventral leafv shoot 
Fig. 1. Diagram of Life Cycle of Selaginella. 
from the lower side of which dichotomously branched roots grow 
out. The roots strike ground usually after growing an inch or 
more in length. In this plant there is no increase in thickness of 
the stem but some Heterosporous Pteridophytes do grow consid- 
erabh’ in thickness by the development of the general tissue but 
not by a true cambium layer. The vascular bundles are con- 
centric with the xylem in the centre. The stem usually has two 
bundles side by side each contained in a tubular cavity or air 
space and connected with the walls of the cavity by means of 
numerous slender filaments. Other types have a different stem 
structure. 
After attaining a considerable size the sporophyte begins to 
reproduce by developing cones or primitive flowers at the ends of 
some branches. The cones are bisporangiate, having two sets of 
sporophylls which are but slightly differentiated from the ordi- 
nary foliage leaves. The two kinds of spore-bearing leaves are 
