Chap, ii The Theory of Metamorphosis 71 



are not to be regarded as leaves according to the conception 

 based on relative position. According to Goebel they are 

 members sui generis. 



Nor do anatomical details afford us a criterion. Though 

 the bilateral leaf and the radially symmetrical stem seem 

 to have little in common, Bower has shown how inadequate 

 is internal structure to supply grounds of identification, 

 which shall be indisputable or conclusive. 



These and other considerations show us that the concep- 

 tion of leaf and stem, even in the groups of plants in which 

 we find it convenient and even necessary to recognise them, 

 must remain arbitrary, and their individualities in many 

 cases almost impossible to define. 



The essential or fundamental character of the leaf as 

 a distinct conception was discussed by Eichler in 1861. 

 In the Entwickelungsgeschichte des Blatter, published in 

 that year, he claimed that there is a primordial leaf that 

 is distinguishable from the stem. It exists as such before 

 internal or external differentiation, and becomes differen- 

 tiated later into two chief parts, a stationary zone, or 

 foliar base— which takes no further share in the formation 

 of the leaf — and a vegetative part, which forms the petiole 

 and the lamina and its branches. He laid great stress on 

 the existence of a radical difference of these two parts 

 from one another. 



The difficulty of forming an adequate conception of the 

 leaf presented itself very forcibly to the mind of Sachs, 

 who discussed the matter with some completeness in the 

 Lehrbuch. He claimed that absolute distinction between 

 stem and leaf, and between thallome and leafy shoot, 

 cannot be maintained. He said ' the expressions stem 

 and leaf denote only certain relationships of the parts of 

 a whole— the shoot ; the greater the differentiation the 

 more clearly are stem and leaf distinguished ', and again, 

 ' certain lateral outgrowths in some Algae may be termed 



