144 EXPLANA TION OF FORMS OF LEA VES. [cHAP. 



there was some definite form told off for each species, 

 then, surely, a similar rule ought to hold good for 

 each genus. The species of a genus might well differ 



Fig. 94- — Passifiora ccerulea. Seedling, 



more from one another than the varieties of any 

 particular species ; the generic type might be, so to 

 say, less closely limited ; but still there ought to be 

 some type characteristic of the genus. Let us see 

 whether this is so. No doubt there are many genera 

 in which the leaves are more or less uniform, but in 



