INTRODUCTION 31 



organ. As a rule we may say that structure is 

 intimately correlated with function. And since 

 unction is expressed in structure it is more closely 

 connected with the specific characters. The latter 

 are preserved by heredity, and their limits defined 

 by the evolution of the species. Any adaptations 

 that may result in adjustment to the environment 

 are less permanent. 



In a word, the more plastic elements of the plant 

 are those that are related to variation in form, and 

 in these cases, more especially to the vegetative 

 organs of a plant, rather than to the reproductive 

 organs. The external characters, or form, as expressed 

 in the visible anatomy of the plant, thus respond to 

 environment and vary more than the more gene- 

 ralised parts (which make up the internal anatomy, 

 the internal characters or structure), which respond 

 to the ordinary processes of evolution, or to the 

 inherent tendency to vary, which is less marked. But 

 there is, of course, a reaction between each, and the 

 one influences the other, to a greater or less extent. 



The modifications which may arise in the internal 

 anatomy or histological structures are thus expres- 

 sions of the adaptation of each structure to its needs, 

 and vary along the lines laid down by the evolu- 

 tionary history of the species."^ 



* The adaptations due to environment and functional requirements 

 which find expression in plant form are thus less distinctive, though 

 often more apparent or obvious. The internal structural adaptations 

 are perhaps less variable comparatively speaking than the external 

 adaptations expressed in the specific form of plants viewed as a whole. 



