KEPBODUCTION 



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 garnet o- 



Fio. 174. PEOTHALLTUM (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, 



