58 SUNFLOWER. [CH. IV 



The word phloem on the other hand points to a 

 likeness to the bark of trees, and here again it will appear 

 that the term is well used, since the phloem part of the 

 bundle in the sunflower is a tissue allied to the external 

 tissues of trees. 



The most essential character common to the xylem 

 and phloem is that which gives to both the quality 

 of vascular tissue, namely, the fact that they consist 

 largely of tube-like elements or vessels built up of long 

 cells placed end to end. In the xylem as well as in 

 the phloem there are also non -vascular tissues made up of 

 cells not fitted together into tubes. So that xylem and 

 phloem are made up of: xylem vessels and xylem 

 cells, phloem vessels and phloem cells. 



The points of difference are perhaps more striking 

 than the points of resemblance. The xylem vessels (like 

 the vessels in the root which were briefly examined) 

 are hollow tubes empty of protoplasm, whereas the vessels 

 of the phloem contain protoplasm. The xylem vessels 

 have but few partitions, the cross-walls of the constituent 

 cells having mostly disappeared. The cross-walls in 

 the phloem-vessels have not disappeared, and moreover 

 present a peculiarity which is especially characteristic, 

 and which has given rise to the name sieve-tubes by 

 which these vessels are known. The cross- walls of the 

 sieve-tubes are perforated by numerous holes (like a 

 sieve), and through these holes one constituent cell 

 communicates with the next in the row. Not that the 

 cavities communicate, for the minute holes in the sieve- 

 plates (as the perforated cross-walls are called) are filled 



