The Origin of Land Plants 113 



of the sporophyte as a vegetative bud upon the 

 gametophyte, have been recorded for various ferns, 

 and it has also been found that the gametophyte 

 may develop directly from portions of the sporo- 

 phyte other than the spores. These phenomena have 

 led to the theory that there is no essential difference 

 between the two generations, and that the sporo- 

 phyte is not to be considered as homologous with 

 the sporogonium of the bryophytes, but as the 

 direct modification of some gametophytic structure. 



The writer has pointed out more than once that 

 these phenomena, which are often pathological, may 

 be very properly compared with numerous cases of 

 adventive budding or regeneration common to so 

 many of the higher plants. Of course there is an 

 essential structural similarity in the cells of the 

 gametophyte and sporophyte, each of which is nor- 

 mally derived from special cells of the other, egg 

 or spore respectively. It is not surprising then that 

 under special conditions, in view of the great power 

 of regeneration exhibited by most plant tissues, that 

 the phenomena of apogamy and apospory should 

 occur. No more remarkable than the production of 

 the whole plant from the root of a poplar, or from a 

 fragment of the leaf of a begonia, where it has 

 not been claimed that the bud is in one case homol- 

 ogous with the root and in the other with the 

 leaf. 



Antiquity of the Pteridophytes. It is clear, from 

 the study of Paleozoic fossils, that all the existing 



