336 FORMS OF LEA VES. [March 



heart-shaped and lobed leaves, the former may represent the earlier or 

 ancestral condition. He then pointed out that if there were some 

 delinite 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 more from one another than the varieties of any parti- 

 cular species ; the generic type might be so to say less closely limited ; 

 but still there ought to be some type characteristic of the genus. 

 He took, then, one genus, that of Scnccio (the groundsel). 



Now, in addition to Senecios more or less resembling the connnou 

 groundsel, there were species with leaves like the daisy, bushy 

 species with leaves like the privet and the box, small trees with 

 leaves like the laurel and the poplar, climbing species like the 

 tamus and bryony. In fact, the list is a very long one, and showed 

 that there is no definite type of leaf, but that the form in the 

 A'arious species depends on the condition of the species. From these 

 and other considerations, he concluded that the forms of leaves did 

 not depend on any inherent tendency, but to the structure and 

 organization, the habits and requirements of the plant. Of course 

 it miglit be that the present form had reference to former and 

 not to present conditions. This rendered the problem all the 

 more complex; and difficult. The lecture was illustrated by 

 numerous diagrams and specimens, and Sir John concluded by 

 saying the subject presented a very wide and interesting held of 

 study ; for [^if he were correct in his contention, every one of 

 the almost infinite forms of leaves must have some cause and 

 explanation. 



