436 VEGETABLE PHYSIOLOGY 
as far as the Thallophytes are concerned. ‘here are indi- 
cations of its origination 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 Cidogonium 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 Coleochwie; 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 spore which is furnished with cilia. 
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 
is Important to notice, however, both in their case and in 
that of Coleochete, that only part of the structure in most 
cases is derived from the contents of the fertilised cells, 
the rest coming from other cells of the tissue of the game- 
tophyte. 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 
