THE WALLFLOWER 59 



sieve-tube and companion-cell are formed by the 

 division of the same mother-cell (see Fig. 23, A B 

 and C, z). 



3. The Phloem-Parenchyma 



The rest of the phloem is made up of rather large 

 parenchymatous cells, which have the typical structure 

 of living cells, each containing a layer of protoplasm 

 lining the wall and a single nucleus. This tissue 

 undergoes no special modification, and is called simply 

 the phloem parenchyma. 



The cells at the extreme outer edge of the phloem 

 are the first to be developed, and are therefore called 

 the protophloem. 



Unfortunately, the phloem of the Wallflower is not 

 well adapted for showing the minute structure, as all 

 the cells are very small in this part. It has therefore 

 been necessary to introduce Fig. 23, taken from another 

 plant (the Pumpkin). 



The phloem, like the xylem, is a conducting tissue ; 

 its special function is to conduct nitrogenous food-sub- 

 stances from the leaves to the parts where growth is 

 going on. 



Between xylem and phloiim there are some layers of 

 cells which do not as yet show the characters of either 

 tissue. On the inner side these layers pass over 

 gradually into xylem, and on the outer side into 

 phloem. The layer in the middle is itself constantly 

 dividing, and thus contributing new elements to wood 

 and bast respectively. The thinnest walls indicate 

 the most recent divisions (see Fig. 21). The divisions 

 take place chiefly in the tangential direction that is 

 to say, in the direction of the line separating xylem 

 from phloem, which is parallel to a line tangential 



