homologous Alternation of Generations in Plants . 363 
(in accordance with the advantage which they ghined from 
escape from competition, and more free exposure to light 
for assimilation) the chance of a frequent recurrence of the 
circumstances necessary for sexual reproduction would be 
diminished, and the dependence upon carpospores for propa- 
gation would increase: consequently the number of spores 
produced by each sexually formed sporophyte must be larger, 
if the race is to survive, and be in a position to compete. 
Any increase in the number of spores entails greater supply 
of external nourishment during their formation ; this in the 
phylum of the Bryophytes is chiefly supplied from the game- 
tophyte, which shows distinct adaptation to sub-aerial habit, 
while the means of nutrition on the part of the sporophyte 
itself are in these plants very limited, and the external mor- 
phological complexity of it very slight. In other distinct 
phyla, however, such as the Filicinae, Lycopodinae, and 
Equisetinae, the sporophyte itself assumed the function of 
nutrition : a higher morphological differentiation of parts 
followed, and a more clear distinction between the organs 
which were to supply the nutriment (stem, leaves, roots) and 
the parts devoted to the formation of spores (sporangia) : 
this for the first time stamped the sporophyte with a character 
of independence and permanence, while the number of spores 
produced might now be practically unlimited : in these 
respects the Vascular Cryptogams are immeasurably superior 
to the Bryophytes. One strange point in the whole story is, 
however, the tenacity with which these plants (under the 
obvious disadvantages which it entails when their habit is 
sub-aerial) retained their aquatic type of fertilization ; it is 
only when we reach the Phanerogams, where the sporophyte 
reaches its climax while the gametophyte is almost abortive, 
that we see the sexual process accommodated to that sub- 
aerial life which had led to the dominant position of the 
sporophyte ; for in them the fertilization is siphonogamic, 
being carried on by the pollen tube: these plants are there- 
fore independent of external fluid water for their fertilization, 
and this fact has doubtless contributed largely to their present 
