XYLEM 



83 



from which it was cut off. The large nucleus can generally' 

 be seen embedded in the dense cj'toplasm which completely fills 

 the cell. The phloem-parenchj-ma cells (Fig. 13, p.) are readity 

 distinguished h\ their shape from the other elements. The cells 

 of the cambium (Fig. 38, Ca.) appear much elongated in the 

 longitudinal section and have abruptl}^ pointed ends. 



In the transverse section of the u-ood one can recognise, 

 as in the Sunflower, small protoxylem -elements towards the 

 interior (Fig. 37, P.xy), large metaxylem-vessels beyond [M.xy.), 



Fig. 38. — Longitudinal section through tlie stem of the Vegetable Marrow 

 (Cucurbita). Ca., cambium; Coll., collenchj-ma ; Ep., epidermis; 

 M.xy., metaxylem ; Par., parenchymatous cortex; Ph.' and Ph.", 

 outer and inner phloem ; P..\y., protoxjrlem ; Scl., sclerenchyma. 



and numerous intermingled wood-parench\TQa cells. Where two 

 vessels are in contact, the middle lamella between the pitted 

 thickening lavers of the common wall can be clearly made out. 

 In longitudinal sections the broad metaxylem-vessels (Fig. 38, 

 M.xy.) show reticulate thickenings which are in marked contrast 

 to the spiral thickenings of the protoxylem-vessels [P.xy.) ; the 

 shght constrictions occurring at short intervals mark the limits 

 of the cells from which the vessel was built up, and, in thick 

 sections, the ring-like remnants of the original septa can often 



