158 PLANT GROWTH SUBSTANCES 



tracheids intermingled with large parenchymatous cells and meristematic 

 zones continued to develop for a long period of time and accounted for 

 much of the overgrowths which continue development for periods longer 

 than six months (20). 



Decapitated stems treated only with lanolin generally enlarged but 

 little. When decapitated and otherwise untreated, calluses derived mainly 

 from cells of the primary and secondary phloem parenchyma might 

 be formed. 



The main responses shown by various tissues of the tomato (7,27), 

 cabbage (17), pea (33), and broad bean (32) to indoleacetic acid were 

 similar to those in the bean, differing from it and from one another 

 chiefly in degree and a few details. 



Decapitated and treated stems of etiolated peas (32) developed a 

 meristem cylinder from division of the cells of the inner cortex, endo- 

 dermis, pericycle, and phloem parenchyma. The root primordia varied 

 in origin in younger and older stems. In younger seedlings the identity 

 of the endodermis became lost in the meristem cylinder complex. The 

 root primordia arose from a group of ray cells within this complex. In 

 older stems the endodermal cells did not divide and the root primordia 

 were pericyclic and intrapericyclic in origin. Iresine (21) and Mirabilis 

 (19) differed from most other species investigated in that the pericycle 

 was more responsive than the endodermis. The phloem of Iresine was 

 strongly reactive while that of Mirabilis was not. Adventitious roots 

 developed mostly from pericycle and ray tissues. 



In Vicia faba (32) adventitious roots were rarely formed, although 

 large, rather unorganized masses of meristematic tissue occasionally 

 developed. These appeared to be potential roots, which had not organ- 

 ized merlstems or orderly systems of derivatives. The cabbage plant (17) 

 was interesting because it developed adventitious shoots from the tumors; 

 apical ones from callus, and lateral ones from inner cortex and ray tissues. 



All of the preceding plants are dicotyledons. The only detailed histo- 

 logical investigation of the responses of monocotyledonous stems to 

 indoleacetic acid was that on three species of L///ww (3). In L.philippinense 

 formosanum and L. longiflorum the first detectable changes in decapitated 

 stems occurred in the cells of the fundamental parenchyma, lateral and 

 centlfugal to the vascular bundles, usually only the outer bundles. These 

 cells became meristematic, and by repeated divisions gave rise to cells 

 which later differentiated as adventitious roots. In L. harrisi (L. longi- 



