428 



VEGETABLE PHYSIOLOGY 



on the chief vegetative functions, while the gametophyte 

 is the small prothallium (fig. 176). 



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 



FIG. 176. 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 



