54 THE SWEET POTATO. 



there may be cells containing the same kinds of 

 crystals (Fig. 8), varying much in size. Loose 

 parenchyma o-i cells thick, with stomatic chambers 

 (Figs. 6, 7). 



Veins (Fig. 9) protruding on l)oth surfaces; the 

 upper epidermis modified into narrow, papillate 

 cells, somewhat more thickened than ordinary, 

 underlaid by a patch of more or less thickened 

 collenchyma cells, which may be entirely or partially 

 sejDarated from the pith by the palisade cells, which 

 are then usually shorter and at least three deep. 

 Fundamental tissue with numerous latex canals, the 

 secreting cells of which are not well differentiated 

 from the other cells. Bundle crescent-shaped, con- 

 cave side up, the horns of the crescent of the xylem 

 connected by a string of separate small patches of 

 internal phloem, in which there may or may not be 

 a small patch of xylem, entirely distinct from the 

 main xylem mass. Fundamental tissue underlaid hj 

 a band of collenchyma cells, less strongly thickened 

 than those near the upper surface. Then follows a 

 single row of small, round hypodermal cells, and then 

 a modified epidermis, less markedly papillate than 

 that of the upper surface. 



Very small veins enclosed in the loose parenchyma 

 and surrounded by large non-chlorophyllous cells 

 (Fig. 7), sometimes accompanied l)y one or two 

 rows of collenchyma and modified epidermal cells, 

 even if entirely cut off from epidermis by cliloro- 

 phyllous parenchyma. 



Tip of midril) forming an awn-like tongue built of 

 non-chlorophyllous cells. 



Petiole. Petiolar nectaries (Fig. 10) consist of 

 invaginations of the epidermis, to form cavities 

 which are thickly lined with glandular hairs similar 



