IPOMOEA BATATUS 



505 



extent in the blade, and there is a progressive reduction in the 

 amount of phloem tissue in succeeding branches until the ultimate 

 veinlets consist of a single spiral tracheid. 



The primary xylem, which is tangentially oriented in the petiole, 

 is differentiated in a manner equivalent to the endarch condition 

 in stems with the protoxylem elements toward the upper surface of 

 the cotyledon. 



The cells of the upper epidermis of the lamina are larger than 

 those of the lower, irregular in size, and their outer walls are some- 

 what convex so that the surface is roughened. Stomata occur in 



Fig. 2.64. /I, outline drawing of mature cotyledon showing venation of lamina and petiole; 



B, same of foliage leaf. 



both epidermal layers but are more abundant in the lower one. In 

 the mesophyll, the palisade parenchyma is loosely organized, and 

 usually consists of two rows of cells, but interspersed with them 

 are cells whose length equals that of the double palisade. The 

 spongy tissue is four or five cells in thickness, except at points 

 where the principal veins traverse it; and the isodiametric cells 

 are so arranged that large lacunae and substomatal cavities are 

 formed. In the region of the principal veins, the mesophyll is 

 more compact and the palisade tissue and spongy cells are replaced 

 by collenchymatous cells above and below the vein. (Fig. 165.) 

 Glandular scales or hairs occur on the lower epidermis and each 

 consists of a basal cell subtending several elongated ones that are 



