538 



THE STRUCTURE OF ECONOMIC PLANTS 



constant to be of diagnostic value in describing types. In most 

 cases, however, individual variations under a wide range of cultural 

 conditions indicate that the character of the periderm is too plastic 

 to be of use in this connection. 



The cortex consists of a narrow band of storage tissue within 

 the periderm, and its peripheral cells contain tannins, protein 

 crystals, and a limited amount of starch in addition to pigment 

 in the colored varieties. The pith forms a small central core from 

 which radiate arms of medullary parenchyma so that this region 

 appears irregularly stellate in transection. The cells are relatively 



low in starch content, 

 have a greater propor- 

 tion of water, and 

 appear more translucent 

 than adjacent tissues. 



The vascular cylinder 

 lying between the cortex 

 and the perimedullary 

 zone is narrow, contain- 

 ing secondary xylem and 

 phloem outside of which 

 the outer primary 

 phloem forms a rela- 

 tively limited storage 

 region. Gen trad to the 

 vascular ring is the 

 major region of storage 



inner phloem; «.x.metaxylem; .;,^. outer phloem ;?x, parenchyma. This is 

 protoxylem ; xy i, secondary xylem. CArter Artsch- ^ . ■' 



wager, Jour. Agr. Res.^ derived from the inner 



portion of the procam- 

 bium and constitutes the perimedullary region in which numerous 

 groups of inner phloem are located. (Fig. 186.) 



The relative activity of the tissues which enter into the develop- 

 ment of the tuber may be indicated in the following order; the 

 initial activity occurs in the pith and cortex, but the perimedullary 

 zone is the most active region and produces the largest proportion 

 of the tissue of the mature tuber. The parenchyma of the outer 

 phloem is relatively inactive, but divisions take place so that 

 this zone keeps pace with the enlargement of the tuber without 

 greatly increasing its width. The cambium, contrary to the 



Fig. 2.86. Transection through portion of vascular 

 tissue of mature tuber: ca, cambium; co, cortex; / pf>. 



