SOLANUM TUBEROSUM 



533 



form a nearly continuous band limited externally by pericyclic 

 parenchyma which in turn is bounded by the endodermis. The 

 inner phloem groups are separated from the xylem by thin-walled 

 cells of the perimedullary zone. Between the outer phloem and 

 the primary xylem, there is a single layer of procambial tissue 

 which later functions as a cambium 

 and gives rise to secondary xylem 

 and phloem. 



In the young rhizome, the primary 

 xylem consists of a few scattered 

 annular and spiral elements sur- 

 rounded by parenchyma. (Fig. x8x.) 

 These initial elements occur very close 

 to the apex of the rhizome, and the 

 elongation of this portion of the axis 

 results in a stretching of the protoxy- 

 lem elements until the lumen of the 

 vessel may be closed owing to the 

 flattening of the lateral walls. The 

 later formed primary xylem consists 

 of vessels with close spiral thicken- 

 ings or shorter, scalariform elements. 

 The phloem cells are similar in form 

 and development to those described 

 for the aerial stem. The cambium 

 first appears in the vascular regions 

 of the procambial cylinder; but, 

 later, it forms a nearly continuous 

 zone which produces a limited amount 

 of secondary vascular tissue. 



In that part of the rhizome which forms the connecting link 

 between the stem and the tuber, the endodermis is very well defined 

 with prominent Casparian strips, and there also may be a suberin 

 layer deposited over the inner tangential walls of its cells. Unlike 

 the portion which tuberizes, the epidermis persists, and its outer 

 walls as well as the epidermal hairs may become lignified. The 

 accompanying diagram (Fig. 183) indicates the proportionate 

 amount of tissue found in each zone of the mature rhizome, and 

 Artschwager (i) has computed the relative areas of each type of 

 tissue. 



Fig. i8i. Transection of tip of 

 young rhizome showing a few annu- 

 lar and spiral elements of protoxylem 

 surrounded by parenchyma : co, cor- 

 tex ; en, endodermis ; e^, epidermis ; 

 /' ^h, inner phloem ; o ph, outer 

 phloem; fcl, pericycle; px, protoxy- 

 lem. (After Artschwager, Jour. Agr. 

 Kes.') 



