THE WORK OF STEMS 255 



tubes with openings in their end -walls through which 

 the proteids may pass ; the importance of these open- 

 ings lies in the fact that most proteids cannot pass 

 through the cell -wall and hence could not be trans- 

 ported to the growing regions were it not for the sieve- 

 tubes. What causes the proteids to move in the 

 sieve -tubes is riot definitely known, but the pressure 

 on the surrounding cells on the sieve -tubes, which 

 causes the proteids to flow out when the stem is cut, 

 must help to force proteids into the young and grow- 

 ing portions of the plant, and the bending of the plant 

 in the wind probably assists this. 



The smaller cells of the bast are of two kinds, 

 those which are closely connected with the sieve -tubes 

 and whose end -walls correspond with theirs, hence 

 called the companion -cells (cc), and shorter cells, 

 called the bast parenchyma (bp) . The function ot 

 these two kinds of cells is not known, but it is 

 conjectured that they assist in some way in the 

 transportation of the proteids. 



The openings in the end -walls of the sieve-tubes 

 may be easily studied in the cross -section (sp, Fig. 

 133), where they are seen to be so numerous as to 

 give the wall a sieve -like appearance, hence the name 

 sieve -plate is applied to these walls. The bast may be 

 traced, in connection with the wood, clear up into the 

 leaf and also down into the 'root (see Fig. 90) . Most 

 plants are not so well provided with sieve -tubes as 



