Notes . 
585 
and the prothallus of Lycopodium cernuum . The deviations from the 
normal life-history, which will be considered later, may somewhat 
modify this statement. 
We are justified in assuming that the Bryophyta and Pteridophyta 
arose from ancient Thallophytes ; the study of the life-histories of 
the Algae and Fungi, which exist at present, may accordingly be 
expected to aid in arriving at probable conclusions as to the origin 
of the alternation in archegoniate plants. It is naturally among the 
Green Algae that indications of this sort might be expected, nor are 
they wanting, though the precise weight to be attached to them is 
a matter of uncertainty. The higher Fungi and the Red and Brown 
Algae may for the sake of simplicity be left on one side with the 
remark that in Ascomycetes and Florideae we see a development 
which presents analogies with the alternation in Archegoniates. Con- 
fining ourselves to the Green Algae and the simpler Fungi we 
find among them two sorts of phenomena which have been termed 
alternation of generations. Most of these organisms reproduce both 
sexually and asexually, and sexual and asexual individuals, resembling 
one another in their vegetative structure, are often found. The same 
individual may, however, bear both kinds of reproductive organs, 
and Professor Klebs has shown in a number of cases that the mode 
of reproduction is largely determined by the external conditions 
and can be brought under experimental control. There is thus no 
doubt that these sexual and asexual individuals are homologous in 
the full sense of the term. But there are a number of Thallophytes 
in which another stage in the life-history is found, which, by its 
regular recurrence and the position it occupies in the life-cycle, 
suggests a comparison with the sporophyte of the simpler archegoniate 
plants. While in many Thallophytes the fertilized ovum or the 
zygospore develops directly into an independent plant resembling 
the parent, in these it first divides into a number of cells, which are 
usually motile spores, but may form a small mass of tissue from the 
cells of which swarm-spores arise. It is sufficient to mention 
Oedogonium, Cys/opus and Coleochaete as organisms which show this 
clearly. In the life-history of Sphaeroplea only sexual individuals 
and the group of swarm-spores, which results from division of the 
oospore, alternate, independent asexual individuals not being found. 
If we now consider how this second form of alternation in Thallo- 
phytes might have come about, without for the moment extending 
R r 2 
