CHAPTER XI 

 LEAVES 



118. General features. It is not necessary to define 

 what is meant by foliage leaves, for no structure of 

 plants is more familiar. They are thought of by botanists 

 as expansions of green tissue exposed to light and air, and 

 especially concerned in the manufacture of food. Every- 

 thing important connected with a foliage leaf is to be ex- 

 plained by this fact, for arrangement, position, form, and 

 structure are all related to the work of food manufacture. 

 A consideration of leaves has been deferred until the Angio- 

 sperms can be included, for it is in this great group that 

 the largest display of leaves is found. 



The variation in the form and structure of leaves is so 

 great that they are useful in classification, and for this rea- 

 son numerous technical terms have been devised to indicate 

 these variations with precision. However, to learn the defi- 

 nitions of all these terms is not to know a leaf and its work, 

 and therefore in this presentation they are disregarded except 

 so far as some of them may be of service. 



119. General structure. It is a matter of common ob- 

 servation that the green expansion called a leaf may arise 

 either directly from the stem or it may have a stalk of its. 

 own (petiole). The presence or absence of a petiole (Fig. 

 154) is related to the exposure of the leaf, and has nothing 

 to do with the structure of the leaf for the work of food 

 manufacture. 



If the leaf is examined superficially, it will be seen that 



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