428 VEGETABLE PHYSTOLOGY 
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 gameto- 
phyte ; 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 gametophyte is always the vegetative body, while the 
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Fic. 174.—PRoTHaLLium (GAMETOPHYTE) OF FERN. 
sporophyte often shows the greater histological differentia- 
tion. 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 ultimately being reduced to microscopic 
dimensions. 
We encounter for the first time in the group of the 
Pteridophyta, the Ferns and their allies, a phenomenon 
which becomes of constant occurrence in all groups above 
