482 



TRANSLOCATION OF SOLUTES 



epidermis 

 chlorenchyma 



endodermis 

 ■pericycle 



■phloem 



tips or lateral organs and in the root tips. Although in most species solutes 

 move for the greatest distances through stems it is important to realize that 



the conductive tissues of plants constitute a 

 complicated but unit system which ramifies 

 to all parts of the plant body. 



The general arrangement of the xylem 

 and phloem tissues in representative types of 

 stems has already been considered (Fig. 53, 

 Fig. 54). In the majority of dicot stems the 

 YoiienciiL/ma phloem usually occurs as a continuous cylin- 

 der of tissues just external to the cambium 

 layer. In a few species some strands of in- 

 ternal phloem are present inside of the xylem 

 (Fig. 105). In roots the primary phloem 

 and xylem are present, as seen in cross sec- 

 tion, in a radial arrangement, but the sec- 

 ondary conductive tissues are oriented in 

 essentially the same pattern found in stems 

 (Chap. XVII). 



Five principal types of cells are found in 

 the phloem: (i) sieve tubes, (2) companion 

 cells, (3) phloem fibers, (4) phloem paren- 

 chyma, and (5) phloem ray cells. The pro- 

 portions of these various types of cells present 

 are different in every individual species and 

 in some species not all are present. 



Mature sieve tubes consist of linear series 

 of elongated, rather thick-walled cells which 

 are joined together end to end (Fig. 106). 

 The individual cells are also often referred 

 to as sieve tubes, but a better term for them 

 is sieve tube elements. The cross walls sepa- 

 rating the cells composing the sieve tubes are 

 often oblique. Strands of cytoplasm pass 

 from one sieve tube element to the next 

 through pores in the end walls. These pores 

 are grouped into definite areas called sieve 

 plates. Sieve plates often are also present in 

 the side walls of the sieve tubes. In many 

 species the end walls of the sieve tube elements are one large sieve plate. In 



vessel 



internal 



TOM '"°''° 



Fig. 105. Cross section of a 

 small portion of a tomato stem 

 showing internal phloem. 



