THE WALLFLOWER 57 



altogether, or in its phloem portion, though this is not 

 the case in the Wallflower. The xylem, therefore, does 

 not of necessity form the whole skeleton of the plant. 

 It is especially the thick parts of the cell-wall 

 which are lignified. In the vessels these hard, thick, 

 stiffening bands have a special function, for the cavity 

 of a vessel is often almost a vacuum, only containing 

 air at a very low pressure, and a very small quantity 

 of water. Vessels in this state would very readily 

 yield to the pressure of the surrounding tissues, and sc 

 collapse, if their walls were not specially strengthened. 



The Bast or Phloem 



In the Wallflower the bast consists entirely of cells 

 with soft walls of pure cellulose. The most important 

 cells are of three kinds (see Fig. 23). 



1. The Sieve-Tubes or Bast- Vessels 



These are made up of long cells placed end to end. 

 Their great peculiarity is that their end-walls are 

 perforated that is, bored through in a number of fine 

 holes, like a sieve. From this fact they obtain their 

 name. They are called vessels because the cells of 

 which they are built up are in open communication 

 with one another, but while in many wood-vessels the 

 cross walls are almost entirely absorbed, in all bast- 

 vessels they are only perforated. The sieve- tubes 

 retain their protoplasm, and also contain a slimy 

 substance which is rich in nitrogenous compounds ; 

 sometimes starch grains are also present. The sieve- 



