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III. THE LEAF. 



Goethe was the first to recognize that the numerous 

 and varied foliar organs of the plant are all modifica- 

 tions of the same original organ. He says : " The 

 secret relation subsisting between the different ex- 

 ternal organs of plants, such as leaves, calyx, corolla, 

 and stamens (which are developed in succession, and, 

 as it were, out of one another), has been long acknow- 

 ledged by naturalists in a general way ; indeed, much 

 attention has been bestowed upon it, and the title 

 Metamorphosis of Plants has been given to the opera- 

 tion by which one and the same organ presents itself 

 to us under various disguises." He imagined a typical 

 leaf of which all leaves are variants, which surely is the 

 root-idea of modern comparative morphology. But 

 he had no conception of a transformation of foliar 

 organs by descent from some common ancestor, or 

 that his type-leaf had a real existence in the past. 

 He describes only the modifications which appear in 

 the foliar organs during the development of the plant 

 from the cotyledons up to the flower. Alex. Braun 

 put forward much the same kind of view. But the 

 treatises of both authors are profound and valuable so 

 far as they go. Groebel's view is that every foliar 

 rudiment is that of a foliage-leaf, but according to 

 circumstances it may develop into any other type of 

 foliar organ. He gives no reasons for holding such a 

 view, and it is difficult to know exactly what his 

 meaning is. If, however, it is meant that all other 

 foliar organs have been derived from foliage-leaves in 

 the past, he is no doubt correct. 



In the light which recent palaeontological data have 

 afforded, it would seem that the factor of reduction, 



