THE STRUCTURE OF THE STEM 



::i71 



of pericycle, touching its internal face, a number of protoxyleui 

 groups are differentiated, between wliich are an equal number 

 of protopliloems. The primary wood develops centripetally 

 from these protoxylem groups, and the separate masses so formed 

 unite irregularly in the centre of the stele, forming a number 

 of bands which are separated by similar bands of phloem. 



Fig. 756. 



/VV. 756. Section of stem of Lijcopudium. ep. Epidermis, en. Endodei-mis. 

 pe. Pseudo -pericycle. px. Groups of protoxylem. ph. Phloem. After 

 Sachs. 



This type of structure is found also in Psilotiim and a few other 

 cases. 



A similar centripetal formation of the primary wood is 

 found in the separate steles of several of the polystelic 

 Selaginellas, and also in the hypocotyledonary portion of the 

 axis in some of the Phanerogams, where the transition from 

 the structure of the stele of the stem to that of the root can 

 be observed. 



Schizostelic Stems. — In tuis type of stem the central stele 



B B 2 



