172 THE LEAF 



standing out upon the face of the leaf at its base, as h in Fig. 465 A. 

 This is the Ligule. The free edges of the Hgule may now pass around the 

 stem, meeting and cohering upon the other side, thus forming a sheath 

 above the basal portion, or true sheath (Fig. 472). Such a sheath is 

 called an Ochrea. If, lastly, it be assumed that the axial portion a-h 

 (Fig. 468) undergo an elongation much greater in proportion than the 

 enlargement of the other parts, we shall have developed a long narrow 

 division between the base and the lamina, as c in Fig. 3, which is the 

 Petiole. It is thus seen that the view here taken will account for the 

 origin of every part of the leaf. The few illustrations here shown refer 

 only to certain combinations in the development of the different parts. 

 As a matter of fact, such combinations found among existing leaves are 

 innumerable, and this variety is increased by the fact that the growth in 

 any one of these parts may be chiefly lateral or chiefly vertical, and 

 that it may be confined wholly or chiefly to some special portion of 

 the part. The student will, nevertheless, be able, by bearing in mind 

 the typical possibilities here considered, to determine the plan of struc- 

 ture of most leaves. This view will also make clear the statement in the 

 opening chapter in regard to the absence of the blade, petiole, or other 

 parts from certain leaves. 



It is interesting to note here that there is ample evidence to prove 

 that the rudimentary or scale-like form of leaves existed upon the 

 earliest flowering plants, so it would appear that the parts of their 

 flowers were developed from such scale-leaves, rather than from the 

 highly developed leaves which we now know. Against this, we have 

 to consider that those floral parts were probably of correspondingly 

 simple development, and that, in the higher plants of today, they 

 have undergone a development which has kept pace with that of their 

 leaves. 



As to what constitutes the typical leaf, we are confronted by two 

 views. Structurally considered, it must be such a leaf as represented 

 by Fig. 467, but such leaves, as we now see them upon plants, do not 

 apparently so well perform all the functions of the leaf as those which, 

 like Fig. 3, have developed the modern leaf-parts. 



The Leaf-Surfaces. — Very rarely has the leaf a terete form and a 

 radial structure as seen in transverse section. Typically it is a flattened 

 body. One flattened surface, the Ventral, faces upward or toward the 

 stem which bears it, and is ordinarily spoken of as the upper surface. 

 The under or outer surface is technically known as the lower or Dorsal. 

 By a partial twist in the petiole, the surfaces may become laterally 



