ANATOMY OF PHLOEIM TISSUES 



483 



potato stolons the sieve tube elements range in length up to 100 /x, while in 

 cucurbits some of them attain lengths of 1000 /x, but in most species they are 



shorter. 



Most of the protoplasm of sieve tube elements is present as a thin layer 

 between the central vacuole and the cell wall. In mature elements fine strands 



s'lQve tube 



companion cell 



phloem 

 parenchyma cell 



'■. — sieve plate 



Fig. 106. Longitudinal section of a sieve tube and associated cells from the stem of 

 tobacco. Redrawn from Holman and Robbins (1938). 



of protoplasm cross the vacuole. Cytoplasmic connections are established be- 

 tween adjacent elements through the intervening end walls as the element 

 matures. Young sieve tube elements contain one or more nuclei, but as the 



