50 THE SWEET POTATO. 



No crystals seen in any young tip, cut where the 

 folded leaves are about % inch in length. 



(b) Section through base of old stem (Fig. 15). 



Epidermis with rather few gland cells, with or 

 without hairs, with numerous lenticels. (The writer 

 has not been able to decide whether the lenticel-like 

 patches are made up of cortical tissue, or are pro- 

 liferations of the epidermis.) 



Epidermal cells block-shaped, thin-walled, hypo- 

 dermis more or less well marked, depending on the 

 variety, with or without crystal cells, depending on 

 the varietv; three to five outer lavers of cortex form 

 a collench^Tua sheath, not very strongly thickened, 

 latex canals well preserved, or flattened out, or both 

 kinds in the same section, secreting cells well marked, 

 or hardly recognizable, depending on the variety. 

 With or without crystal cells, depending on the 

 variety. 



Endodermis well marked, sometimes accompanied 

 by slightly-marked accessory endodermis. 



Pericambium a ring of strongly thickened fibers, 

 or occasionally not much thickened, depending on 

 the variety. Phloem thick or thin, unevenly dis- 

 tributed in thickness, with or without crystal cells, 

 depending on the variety, occasionally with a few 

 latex canals, cambium strongly marked, xylem wide 

 or narrow, usually with, sometimes without, large 

 vessels toward external margin, depending on the 

 variety, these vessels often with an abundance of 

 tyloses. Xylem cells showing spiral and reticulated 

 thickenings of various patterns, and bordered pits. 

 Whenever large vessels are present, they are 

 arranged only on the two opposite sides of the stem, 

 one patch on either side, unequal in size. The 



