i6o THE STRUCTURE OF ECONOMIC PLANTS 



respects. The cortical cells are more persistent; and there may be 

 a hypodermal zone, two or three cells in thickness, which becomes 

 sclerotic and protects the parenchymatous cells centrad to it. 

 Percival (lo) reports that in the stronger adventitious roots 

 arising from aerial nodes, the cortical cells frequently have chloro- 



FiG. 69. Transection of the lower portion of the first internode of a four weeks old plant : 

 CO, cortex ; co hu, cortical bundle ; end col bu, endarch collateral bundles ; ep, epidermis ; ph, 

 phloem; atj, xylem. (After Avery.) 



plasts so that the stele appears to be surrounded by a zone of 

 chlorophyllose tissue. Other differences in the stele are the 

 development of several large metaxylem vessels instead of a 

 single one; and the presence of a correspondingly smaller amount 

 of stelar parenchyma which becomes lignified early in ontogeny so 

 that it is difficult to distinguish protoxylem from parenchyma 

 in transections. 



