346 Bower. — A Theory of the 
been contemplating could not be carried out without special 
means of nourishment of the spores during their development, 
and some mechanism for their dissemination when mature. On 
ascending from the simplest Bryophytes to the more complex 
Ferns, Lycopods, and Equiseta we see various devices for 
meeting these and other difficulties : these devices are more 
or less directly connected with the second factor, viz. sterili- 
zation of some of the potentially sporogenous cells ; it will thus 
be seen that these two factors of progress have worked side 
by side, increase of spore-production bringing in its train the 
necessity of a partial sterilization. Frequently the results of 
the one factor are complicated by the occurrence of the other ; 
while, further, the appearance of continued apical or intercalary 
growth may make it difficult, or even impossible, to form 
a definite opinion as to the part which the two factors have 
individually taken in the evolution of the organisms as we 
now see them. 
Our next step will be to consider examples of the evidence 
that sterilization of potentially sporogenous cells has occurred ; 
to recognize the various ends which the sterile cells serve, 
and to note how widespread the examples of sterilization are 
from the lower archegoniate plants upwards to the Angio- 
sperms. 
Starting with the simplest Bryophyta, it may be seen that 
differentiation of the cells of the sporophyte, though absent 
from their Algal prototypes, is always present in the Arche- 
goniatae. In Riccia a peripheral layer of cells forms the 
protective wall of the sporophyte ; from the lowest of arche- 
goniate sporophytes upwards such protective tissues of the 
wall surround the spores, and the sporogonial or sporangial 
wall may thus be regarded as the most archaic part of the 
sterile system of the sporophyte. There is no direct evidence 
to show that the wall of Riccia has been the result of steriliza- 
tion of sporogenous cells, but the analogy of the peridium of 
the Uredineae would suggest that view, which is strengthened 
by comparison with Algal forms. Further steps in advance 
among the Bryophytes exemplify two types of sterilization of 
