CHAP, ii The Theory of Metamorphosis 75 



extent. The anatomy of the ribs bears out the idea, 

 the structure of the petiole being traceable with sufficient 

 clearness through the length of the midrib. There is a 

 fundamental similarity in anatomy between the leaf-axis 

 and the stem-axis, though this is modified in the region 

 of the epipodium by the flattening associated with the 

 development of wings. 



The leaves of the higher plants, therefore, may be com- 

 pared with the flattened appendicular outgrowths in the 

 Algae ; both resemble at first the main axis in organization 

 and structure, only later showing specialization for func- 

 tional purposes, both in form and construction. The degree 

 of specialization is greater in the higher plants, and is 

 mainly dependent on the need for an appropriate relation 

 between surface and bulk. The need of proper communica- 

 tion between the atmosphere and the interior of the plant 

 is imperative, and is met by subdivision, with subsequent 

 flattening or winging. 



This view of the morphology of stem and leaf harmonizes 

 completely with Sachs' claim that the differentiation of 

 form is fundamentally into root and shoot ; indeed, it 

 is only a development of his theory. 



With a certain school of thinkers the entrance of physio- 

 logical or adaptive considerations led to a still greater 

 departure from the Wolffian theory, which was making itself 

 known in the closing years of the century, and which may 

 be traced back to the Darwinian position of the struggle 

 for existence. This was the view that the determination 

 of form does not depend on fundamental categories of 

 members, but may be traced back to physiological influences 

 combined with, or superposed upon hereditary tendencies 

 in minor directions. If the primitive or original plant 

 was a unicellular organism, possessing a power of, or ten- 

 dency to the acquirement of, increasing complexity, its 

 gradual development can be traced by studying the existing 



