CHAPTER XVI 



THE LEAF 



Importance of Leaf-study. — To the pharmacognosist a thorough 

 knowledge of the leaf is a necessity. Of its cellular structure, little can 

 be learned without the aid of the compound microscope. Its gross 

 parts were briefly referred to in our opening chapter, and these must 

 now be studied in detail. 



Development of the Leaf. — The varied forms of structure which leaves 

 present can best be understood by considering them as modifications of 

 an original or primary leaf and noting the changes in the latter which 

 have occurred to produce them. It is apparent that such a primary 

 leaf was a mere scale of small size, as indicated in Fig. 466. It then 

 appears that any modern foliage leaf must have resulted either from 

 the uniform growth and development of all the parts of such a scale, or 

 from the greater relative growth of some one or more of its parts. The 

 result of its luiiform growth would be a leaf of the same form, but larger, 

 its base sheathing the stem, as represented in Fig. 467. But leaves of this 

 exact character are rare, from which it would appear that modern leaves 

 generally represent unequal degrees of development of the different 

 parts of the original leaf. Their attentive examination shows that the 

 following parts of such a primary leaf have in different cases undergone 

 independent enlargement and development. In Fig. 468, the portion a, 

 cut off by the dotted line at the top, may represent the Apical region; 

 that at h the Central-basal; the strip a-h the axial; and the remaining 

 portions upon either side (c and d) the lateral. Let us assume first that 

 the enlargement is confined chiefly to the central-basal portion. The 

 base will then become converted into the form represented in Fig. 469, 

 without the enlargement of the other parts there shown, this leaf being 

 a mere sheath around the stem, bearing the original scale at its tip. 

 Leaves of this form are rather common upon undeveloped or partly 

 developed stems. They are called Leaf-sheaths, or often, for emphasis, 

 Naked Sheaths. The edges of such a sheath may cohere after passing 

 around the stem, giving us the Closed Sheath, as in the sedges {a, in Fig. 

 465 B), or they may remain free, giving us the Open Sheath of the 



