THE TISSUES OF THE STEM 41 



position ; but additional sieve-tubes are present also on the side 

 adjoining the pith. Cambial activity is evident between xylem and 

 the outer phloem, and a few divisions are also seen between the xylem 

 and the medullary phloem. 



The stems of climbing plants, and especially of the Cucumber and 

 Vine, have been habitually used to demonstrate the structure of 

 sieve-tubes, because there they are specially large. In Fig. 25 the 



FIG 25. 



Transverse section of a vascular strand of the Cucumber, showing plentiful 

 phloem both on peripheral and central sides of the xylem (bicollateral). The 

 vessels of the xylem are few, but very large. s<= sieve- tubes, p.v. =pitted vessels. 

 *=protoxylem. tc = interfascicular cambium. ( x 75). 



sieve-tubes appear nearly circular in transverse section ; they are 

 limited by a cellulose wall, and a cytoplasmic lining invests the wall 

 during life. This readily contracts from the wall when the tissues are 

 cut across, and the internal pressure relieved. That is the state in 

 which they are usually observed. Associated with each is a small 

 companion-cell, often triangular in transverse section, and with dense 

 nucleated cytoplasm. Where a sieve-plate is included in the section 

 it will present a surface perforated by dot-like pores. The contents 

 are densely aggregated round the plate. Under a high power, when 



