CHAPTEE XYIII. 

 THE LEAF. 



EXERCISE LXXIV. 

 The Minute Structure of Leaves. 



STEMS bear nothing but leaves of some kind or 

 other, for branches are only secondary stems. As 

 leaves are developed upon the stem, we should expect 

 to find them composed of the same elements as the 

 stem. By means of the microscope you may easily 

 determine whether this is so. First examine the 

 structure of the framework of a leaf. Observe a 

 thin horizontal or oblique section, taken from the 

 petiole, midrib, or veins. You will find it composed 

 of fibrous and vascular tissue. Examine a similar 

 slice of the pulp. It consists of cells filled with 

 chlorophyll. The framework is fibro-vascular, while 

 the meshes of the framework are filled with paren- 

 chyma. If you should trace the elements of veins 

 and ribs back into the stem, you would find the upper 

 part of this framework connected with the medullary 

 sheath, and you would note that this upper portion, 

 like the medullary sheath, is largely composed of 

 spiral ducts. The lower portions of the framework 

 that appear on the under side of the leaf, you would 

 find to arise from the bark, and to be continuations 

 of the liber. The ribs of most dicotyledons contain 

 much liber, which makes them project on the lower 

 surface. 



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