XYLEM 83 



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

 be seen embedded in the dense cytoplasm which completely fills 

 the cell. The phloem-parenchyma cells (Fig. 13, p.) are readily 

 distinguished by their shape from the other elements. The cells 

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

 longitudinal section and have abruptly pointed ends. 



In the transverse section of the wood 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 the stem of the Vegetable Marrow 

 (Cucurbita). Ca., cambium; Coll., collenchyma ; Ep., epidermis; 

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

 outer and inner phloem ; P.xy., protoxylem ; Scl., sclerenchyma. 



and numerous intermingled wood-parenchyma cells. Where two 

 vessels are in contact, the middle lamella between the pitted 

 thickening layers 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 

 slight 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 



