CUCURBITA 



593 



--ep 



from a common meristematic zone. Whiting (40), in a study of 

 Cucurbita maxima, confirms Janczewski's (xo) work and describes 

 this generative zone. The meristem consists of about seven layers 

 of cambiform cells, the number diminishing toward the periphery 

 of the growing point. Terminally, the outermost cells of the gen- 

 erative zone become the cells of the root cap, while the marginal 

 cells function as a dermatogen and produce the epidermis. The 

 joint production of root cap and epidermis by the peripheral 

 portions results in the charac- 

 teristic "stair-step" arrange- 

 ment referred to by Janczewski 

 in the description of his third 

 type of root ontogeny. (See 

 Linum, Chapter XIII.) 



The remaining tissues are 

 derived from the inner face of 

 the generative zone. The cells 

 undergo several divisions, 

 some of which are irregularly 

 periclinal; and then the cen- 

 trally located cells differentiate 

 as stelar tissues, while adja- 

 cent cells produce the cortical 

 region. In this type, where 

 there is general meristematic 

 activity, it is difficult to fol- 

 low the differentiation of the 

 various tissues; and the situation is further complicated by the 

 rapid elongation of the root and the delay in the maturation of 

 the primary tissues. It is possible to determine the approximate 

 limits of the cortex and stele relatively early, since the cells of the 

 innermost layer of the cortex continue to divide tangentially for 

 a longer period of time than do the adjacent cells of the stele. 



The first stelar tissue to differentiate is the protophloem, which 

 consists of four strands. No sieve tubes develop in the proto- 

 phloem, which is comprised only of parenchymatous elements 

 that are small in diameter and considerably elongated. In the 

 center of each protophloem group, and usually adjacent to two 

 large pericyclic cells, a single cell differentiates, forming a duct 

 which can be distinguished by its less dense contents. Other 



Fig. 310. Transection of sector of young root 

 showing development of xylem prior to differ- 

 entiation of large central metaxylem : co, cortex ; 

 eti, endodermis ; ep, epidermis ; mx, metaxylem ; 

 pel, pericycle ; ph, phloem ; px, protoxylem. 



