REPRODUCTION 427 
as far as the Thallophytes are concerned. There are indi- 
cations of its crigination in that group, but they are ex- 
tremely rudimentary, and occur in families which are widely 
separated from each other. The gametophyte was doubtless 
the primitive form of the plant, and in some way or other 
the sporophyte took its origin from it. Certain phenomena 
which may represent stages in the process can still be 
observed. In Gidogoniwm the fertilised cell does not grow 
out into a new filament, but produces in its interior four 
zoospores which escape from it, and after a period of rest 
germinate and produce new plants. The fertilised cell here 
may perhaps represent the sporophyte, reduced, however, to 
a single sporangium. An even simpler stage of develop- 
ment may perhaps be recognised in Spirogyra, where the 
nucleus of the fertilised cell divides into four, though no 
definite cells are formed. On germination of the zygote, 
however, only one filament grows out. A more complex 
structure is formed in Coleochete; the zygote becomes 
invested with a covering derived from the adjacent cells, 
and after sinking to the bottom of the water, it germinates, 
producing inside its coating a small mass of cells, each one 
of which liberates a zoospore. Other complex structures 
are found as the result of the growth and development set 
up by fertilisation in the Rhodophycee. These are known 
as cystocarps, and they have been held to represent the 
sporophytes of those plants. It isimportant to notice, how- 
ever, both in their case and in that of Coleochete, that only 
part of the structure in most cases is derived from the con- 
tents of the fertilised cells, the rest coming from other 
cells of the tissue of the gametophyte. As we have seen, 
the sporophyte in the higher plants is entirely derived from 
the zygote. 
The antithetic alternation of generations is seen most 
clearly in the groups of the Mosses and Ferns. In the 
former the Moss plant is the gametophyte, the so-called 
capsule or theca with its stalk is the sporophyte. In the 
Ferns the sporophyte is the predominant form and takes 
