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A TEXTBOOK OF THEORETICAL BOTANY 



promontories of leaf tissue are formed, the shape and arrangement of which 

 will depend on the venation. 



The cases mentioned above are only a few conspicuous examples of 

 variations in the leaf form which are readily traceable to local variations of 



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Fig. 971. — Monstera deliciosa. Plant showing large 

 perforate leaves. {Pliotogroph taken in Brazil.) 



growth during the later phases of leaf development, and it is probable that 

 nearly all the normal as well as the abnormal forms of leaves owe their 

 characteristics to similar causes, though direct information on the matter is 

 lacking. Such variations are, however, not entirely due to internal or heritable 

 factors and the environment undoubtedly has a modifying influence. Sinnott 

 and Bailey have pointed out the ecological relationship of certain leaf types 

 to climate, in that certain forms of leaf are predominantly characteristic of 

 certain habitats. For example, leaves or leaflets with entire margins are 

 characteristic of low-lying tropical regions, while leaves or leaflets with 

 non-entire margins are predominant in mesophytic, cold temperate areas, 

 though dry temperate regions usually produce entire margins. The effect 

 of submersion in causing the development of filamentous leaves in aquatics 

 is also well known. 



The first appearance of primordia at the stem apex is frequently far in 

 advance of the time for their full development, and, after reaching a certain 

 size, a period of dormancy ensues. This is conspicuously the case in the 



