SOLANUM TUBEROSUM 539 



interpretation of de Vries, is relatively inactive and produces 

 little secondary tissue. 



The Ontogeny of the Tuber. — Following the elongation of 

 the rhizome, which normally reaches a length of 3 to 4 inches, 

 the first change incident to tuber formation is the radial enlarge- 

 ment of its tip. The first region to grow actively is the pith; and, 

 as a result of its increasing size, the elements of the vascular 

 cylinder are inclined obliquely outward from the course which 

 they follow in the axis of the rhizome. 



To compensate for medullary growth, tangential enlargement 

 and radial divisions of cells take place simultaneously in the 

 cortical, perimeduUary, and vascular regions. As tuberization 

 proceeds, the cortical cells become filled with starch and the storage 

 character of the organ is definitely indicated. Following cell 

 division in the pith and the coincident compensating growth in 

 the cortex, the pericyclic and perimeduUary zones become the 

 regions of greatest growth activity. The pericyclic cells surround- 

 ing the outer groups of primary phloem divide, enlarge rapidly, 

 and, like the cells of the cortex, become filled with starch. They 

 appear to be in direct contact with the cortical parenchyma and 

 also form radial wedges of tissue between the outer phloem groups. 



In early ontogeny, the endodermis does constitute a line of 

 demarcation between cortex and outer pericyclic zone and this 

 helps to eliminate the possibility of misinterpreting the outer 

 pericyclic storage region as a centripetal extension of the cortical 

 parenchyma. (Fig. 185, B, C.) The failure of the endodermis 

 to form Casparian strips as the girth of the stele increases finally 

 results in its disappearance as a distinct layer. The activity of 

 the pericycle causes the original groups of outer phloem to become 

 widely separated, and additional ones are formed in the intervening 

 areas which consist of a few sieve tubes, companion cells, and 

 parenchyma. 



Changes similar to those taking place in the pericycle occur in 

 the inner pericyclic and perimeduUary zone, where rapid and 

 continuous cell division results in the spreading and centripetal 

 deflection of the inner phloem groups. There is a limited amount 

 of cell division in the vascular ring which causes a tangential 

 dispersion of the xylem elements and a radial separation of the 

 protoxylem from the metaxylem. (Fig. t.S^, A.') The epidermis 

 of the tuber primordium is ephemeral because of the activity of 



