PISUM SATIVUM 



345 



-ep 



'- -CO 



- -en 



centripetal differentiation of the metaxylem results in the formation 

 of a solid primary xylem strand; and, early in the ontogeny of the 

 primary phloem, three groups of fibers are differentiated adjacent 

 to the single-layered pericycle. A broad zone of interstitial paren- 

 chyma remains between the phloem and the metaxylem regions; 

 and, later in development, a cambium arises in this zone. The 

 cambium is relatively inactive, however; and there is little lateral 

 extension of cambial activity beyond the initial points of origin. 

 Consequently, the production of 

 secondary tissue is limited to a 

 few large xylem vessels; and a 

 small number of phloem elements. 

 (Fig. 176.) The cortex is rela- 

 tively large, consisting of paren- 

 chymatous cells with intercellular 

 spaces at their angles. It is 

 limited outwardly by the epi- 

 dermis, and the innermost layer 

 differentiates as an endodermis 

 with well-defined Casparian thick- 

 enings. 



Root Nodules. — The structure 

 of the nodule has been investi- 

 gated in several species of legumi- 

 nous plants. These include the 



work of Pierce (X3) on MedicagO p,^, ,^^ Transection of sector of young 

 denticulata. Whiting (31) chiefly root of pea: co, cortex; en, endodermis; 



with Soia and Vigna, Brenchley '^\'P''*'ri' ' ,7' ""T^'""!' ^'^' ^"' 



' . . cycle ; ph fib, phloem nbers ; px, protoxy- 



and Thornton (i) on Vicia faba, lem. 



Prazmowski (14), who worked 



with species of Trifolium, Medicago and Phaseolus, and Nemec 



(ii) on Ornithopus. 



The bacillus causing nodule formation in the pea is Rhizobium 

 leguminosarum which, according to Fred, Baldwin and McCoy (9), 

 can induce this reaction in 18 species of legumes. The genus is 

 distinguished from other genera of bacteria by its ability to form 

 nodules on legumes; and, while there seems to be a group specific- 

 ity, with respect to species of Rhizobium, Wilson (33) has pointed 

 out that the question of cross-inoculation groups is still a con- 

 troversial one. 



— px 



- - pel 



— mx 



ph fib 



