8o Morphology BOOK i 



ments differ ab initio, although the lines of demarcation 

 between the two series of organs as distinct categories may 

 vanish. 



The morphological conception of leaf and stem being 

 taken as a starting-point, two different theories were 

 advanced to explain the facts of metamorphosis in the 

 several categories. Of these the first was put forward by 

 Frank in 1892 and was known as the theory of differentia- 

 tion. His position was that when we see the rudiment of 

 a leaf or a leaf segment, which would normally become 

 a foliage leaf, developed instead as a scale leaf or a sporo- 

 phyll in consequence of some change in the conditions of 

 its life, we are face to face only with a possibility of varied 

 development in it, and need not hold that we are en- 

 countering an actual transformation of one organ into 

 another one. The rudiments which are capable of going 

 through various developments are composed of undifferen- 

 tiated embryonic tissue, but when they have assumed the 

 form of one definite structure they are incapable of changing 

 into any others. 



Goebel, writing in 1895, rejected this hypothesis of Frank. 

 He said that this differentiation theory assumes that at 

 the vegetative point of the shoot indifferent primordia arise 

 which are capable of development in various ways according 

 to the need of the plant, but that they have in common 

 the fact that they are leaves. He objected to this assump- 

 tion and pointed out that the idea of ' leaf ' is a purely 

 abstract one, ' an artificially constructed category which 

 has no concrete existence. What these organs have in 

 common, which we endeavour to fix by a general idea, 

 must be something else than their origin from leaf prim- 

 ordia.' 



Goebel claimed, in opposition to Frank, that in such cases 

 we are face to face with such a change in the progress of 

 development that the rudiment of a particular organ 



