SOLANUM TUBEROSUM 519 



outer phloem. In the differentiation of the primary vascular 

 system, six major bundles are formed which are separated by 

 medullary rays consisting of fundamental parenchyma; and, later, 

 an interfascicular cambium develops across them, although this 

 does not take place simultaneously at all points. While the inter- 

 fascicular cambium is being formed, there is also a lateral extension 

 of groups of outer and inner phloem. The outer groups are small 

 and rather closely arranged while the inner groups are more widely 

 separated from one another. (Fig. zSo.) 



At this time, changes also occur in the epidermis and cortex. 

 The epidermal cells become cutinized and guard cells are formed. 

 Within the epidermis are one or two layers of vertically elongated 

 parenchymatous cells, and centrad to these is a zone of collen- 

 chymatous elements with thickenings at their angles and on the 

 radial and tangential walls. Centrad to the collenchyma, the 

 cells are large and there are numerous intercellular spaces. The 

 cortex is limited centripetally by an endodermis, and Casparian 

 strips develop at about the time that the collenchyma matures. 



Among the last tissues to differentiate are the pericyclic fibers 

 which arise immediately adjacent to the endodermis, and fibers 

 also develop in the perimedullary zone inside the inner phloem 

 groups. These have been regarded by some investigators as 

 phloem fibers; but others, who interpret the stele as an amphi- 

 phloic one with an inner pericycle, have considered them to be 

 pericyclic in origin. The perimedullary zone is a band of paren- 

 chymatous tissue lying between the inner phloem and the primary 

 xylem which consists of small compact cells that are derived from 

 the procambial ring. 



The Mature Stem. — The mature stem is subtriangular or quad- 

 rangular in transection. This results from the development of 

 three large vascular bundles and the wing-like projections of the 

 leaf which extend down the stem from each node because of the 

 decurrent habit of the petiole. Between each pair of major bundles 

 are three smaller ones; and, in the mature stem, a continuous 

 cylinder of vascular tissue is formed by the development of an 

 interfascicular cambium. The stelar region is limited outwardly 

 by the endodermis and inwardly by scattered groups of pericyclic 

 fibers, which become more or less irregularly oriented on its inner 

 margin owing to the inequalities of cell division and enlargement 

 in this region. (Fig. i8o.) 



