282 Strasburger . — The Periodic Reduction of 
certain Chlorophyceae, in which the sexual act consists in the 
coalescence of swarming gametes. These gametes are ob- 
viously derived from asexual swarm-spores, which they quite 
resemble except in that they are smaller and often have fewer 
cilia. 
The sexually differentiated plants manifest certain differ- 
ences in their ontogeny, from which it is possible to infer what 
was the course along which the phylogenetic differentiation 
proceeded after sexual differentiation had taken place. The 
simplest case is that in which the product of fertilization gives 
rise to an individual similar to those which gave rise to the 
product of fertilization ; and which closes its own life-history 
with the development either of sexual organs or of asexual 
organs homologous with them. This occurs in many Chloro- 
phyceae, where, from the zygospore (the product of the 
coal-escence of similar gametes) or the oospore (the product of 
the coalescence of dissimilar spermatozoids and ova), a genera- 
tion is developed which resembles the preceding and gives 
rise either to swarm -spores or to sexual cells homologous 
with them. Generally, any one sexual generation follows 
after a number of asexual generations, the relation being, how- 
ever, dependent on external conditions, so that, as Klebs has 
shown, the development of a sexual or an asexual generation 
can be determined by the observer. In such cases there is 
a homogeneous sequence of generations which does not include 
any other kind of sequence or alternation beyond the develop- 
ment either of asexual reproductive organs or of sexual organs 
homologous with them. The asexual reproductive organs are 
especially concerned with the rapid multiplication of the indi- 
viduals under favourable external conditions ; whilst sexual 
reproduction is of importance in maintaining the existence of 
the species under circumstances which are unfavourable to the 
vegetative existence of the individual. At the same time, 
sexual reproduction ensures certain advantages which arise 
from the coalescence of distinct sexual cells. 
In proportion as the asexual mode of reproduction was 
replaced by the sexual, the numerical conditions of multipli- 
