homologous Alternation of Generations in Plants . 355 
thus inasmuch as there is a differentiation as regards the 
mode of reproduction, we may in a sense distinguish an 
alternation of generations ; but since such generations are 
similar to one another in every other respect, they are all 
to be considered as potential gametophytes, and homologous 
one with another : Vaucheria may thus be taken as a simple 
example of homologous alternation of non-sexual and sexual 
gametophytes. 
In addition to the above phases of life a resting stage has 
also been described by Stahl 1 , the Gongrosira stage, which 
appears to result from conditions of drought: the separate 
protoplasmic masses, produced by fragmentation, are each 
surrounded by a gelatinous cell-wall, and may undergo a 
period of rest : they germinate on exposure to suitable con- 
ditions, either by direct formation of new Vaucheria tubes, 
or by division of the protoplasmic body, and escape of 
the portions as separate amoeboid bodies, which ultimately 
develope into new Vaucheria tubes. Here is a peculiar adap- 
tation to peculiar circumstances, but there is no reason to see 
in it more than a variant upon the structure of the ordinary 
gametophyte, and it may be ranked with other developments 
of the gametophyte. If the Gongrosira form be dignified by 
being styled an alternate generation, it is simply a generation 
homologous with the gametophyte. 
If we compare other examples of the Siphoneae, even where 
an alternation has been recognised by others, it will be seen 
that it is nothing more than an homologous alternation : thus 
in Botrydium , in which Rostafinski and Woronin have de- 
scribed such various forms of reproduction 2 , the life -history, 
though complicated, falls into the same plan. The gametes 
(here similar to one another) after conjugation form the resting 
zygote, which corresponds to the zygote of Vaucheria : this 
on germination, as in Vaucheria , produces the vegetative 
Botrydium plant. Under divers circumstances this may re- 
produce itself vegetatively in very different ways : by the 
formation of swarm-spores, which may be produced in four 
1 Bot. Zeit., 1879, p. 129. 2 Bot. Zeit., 1877, pp. 663, &c. 
