4o6 



THE STRUCTURE OF ECONOMIC PLANTS 



Fig. -loj. A-F, stages in development of adventitious roots from cuttings of hypocotyl. 

 A, B, and D, initiation of roots in ray parenchyma on same radius as protoxylem; C, initia- 

 tion of root in pericycle and phloem parenchyma not on radius with protoxylem ; E, median 

 longisection of adventitious root with histogens established; F, transection of hypocotyl, 

 median through an adventitious root, showing cell activity throughout pith region : cal, calyp- 

 trogen ; co, cortex ; dgn, dermatogen ; en, endodermis ; fib, fiber ; phrn, periblem ; ph, phloem ; 

 pi, pith; pi, plerome; atj i, primary xylem; xji ±, secondary xylem. (After Crooks.) 



