REPRODUCTION 437 
on the chief vegetative functions, while the gametophyte 
is the small prothallium (fig. 174). 
Even without going beyond the Ferns we can notice as 
we pass through the several divisions of the vegetable 
kingdom that the predominant form of the plant has changed. 
In the Thallophyta it is always the gametophyte; the 
sporophyte is not universal there, and is never more than 
a small structure, which nearly always remains attached 
to the gametophyte. In the Bryophyta the two phases are 
more nearly alike in degree of development ; the gameto- 
Fie. 174.—PRotTHaLLium (GAMETOPHYTE) OF FERN. 
phyte is always the vegetative body, though the sporophyte 
often shows the greater histological differentiation. It 
is always parasitic upon the gametophyte and never 
attains a higher degree of morphological value than a 
thallus. In the Pteridophyta the predominance of the 
sporophyte is very marked, and as higher and higher groups 
of plants are reached it becomes still more pronounced, 
the gametophyte continuously retrograding and ultimately 
being reduced to microscopic dimensions. 
We encounter for the first time in the group of the Pteri- 
dophyta, the Ferns and their allies, a phenomenon which 
becomes of constant occurrence in all groups above them, 
