THE AXGIOSPERMAE : LEAVES 



949 



Fig. 936. — Orobanche ramosa. Seedlings showing leafless tuberous stems. {After Gnehel.) 



Leaf Form. 



Both in external form and in internal structure leaves are exceedingly 

 variable, more so perhaps than any other plant organ, 'iliis is partly due to 

 physiological necessity, for the leaf, on account of its relatively large surface 

 and exposed position, is intimately affected by climatic variations ; indeed 

 it is in this respect the most sensitive part of the plant. Moreover the 

 structure of every leaf embodies a compromise between two opposed require- 

 ments, namely, the necessity for the effective exposure of as large a surface 

 as possible to sunlight, which is required for photosynthesis, and the necessity 

 for protection against excessive evaporation, to which its large surface and 

 thin texture make it peculiarly liable. 



On the other hand a great deal of leaf variation has nothing to do with 

 physiological causes but is functionally indifferent and is referable to heredity. 

 Such are the manifold variations of leaf outline and appearance, which are 

 often so closely connected with the systematic relationships of the plants 

 concerned, that is to say, with their genetical history, that they are of the 

 greatest value as aids in identification. A useful distinction has been made 

 in calling physiologically determined plant forms ecads and hereditary 

 forms phyads, and the difference is genuine and deep-seated. 



Variations which are clearly related to special environmental factors, as 

 in the case of aquatic or of desert plants, we shall deal with under Bionomics 

 in Volume IV. Here we shall consider only the systematic variations. 



In earlier days when Botany was largely a classificator^- science, the forms 

 of leaves were recognized as important guides to affinities. They were, 

 therefore, intensivelv studied and classified under a large number of types 



