CHAPTER XIV 



THE LEAF 



A general definition of the leaf or foliar organ has been supplied 

 on an earlier page. The leaf is important, not only on account 

 of its features of structure as an organ of vegetation, but also 

 because of the primitive and intimate relation between it and the 

 organs of reproduction. The latter parts will be discussed in two 

 chapters following 

 the present one, 

 while the main 

 features of organi- 

 zation of the leaf 

 itself will occupy 

 attention in the 

 present connec- 

 tion. The anat- 

 omy of the foliar 

 organs of the cryp- 

 togamic forms, 

 living or extinct, 

 need not be con- 

 sidered at this 

 stage, as any refer- 



FIG. 146. Leaf bundle of Cycas revoluta. Explana- 

 tion in the text. 



ences to them 

 which are neces- 

 sary will appear in 

 later chapters dealing with particular groups. 



We may conveniently begin the discussion of the leaf with the 

 most primitive type of living gymnosperms, the Cycadales. This 

 group, much reduced in numbers under modern conditions, has a 

 type of organization in the leaf which is of great interest from the 

 evolutionary standpoint. Fig. 146 presents the transverse view of 

 one of the bundles of the main axis or rachis of the leaf in Cycas 



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