THE WALLFLOWER 55 



Each vessel consisted at first of a whole row of super- 

 posed cells, each with its own protoplasm and nucleus. 

 Then the cross walls separating these cells from one 

 another were dissolved away, and so the whole row 

 came to form one open passage. The cross walls, how- 

 ever, do not disappear entirely. A ring is left at the 

 edge, and in the Wallflower we can easily see these 

 rings, even in the mature vessels, marking the limits 

 between the original cells which have fused to form a 

 vessel (see Fig. 36, p. 94). These rings are found in 

 all vessels, and. must not be confused with the rings of 

 thickening, which are found in the annular vessels of 

 the protoxylem only. 



We have already seen that the cell-wall is formed 

 by the protoplasm. Where, as in vessels, certain parts 

 of the cell-wall are specially thickened, these thicken- 

 ing masses also are deposited by the protoplasm, 

 which at last becomes almost entirely used up in the 

 process. What little protoplasm remains is absorbed 

 by the neighbouring cells, and the vessel is left quite 

 empty. Thus the vessels when fully formed are dead 

 structures ; they contain no living matter, and are 

 incapable of further growth. They serve simply as 

 open channels through which the sap can pass. 



2. The Fibrous Cells 



The wood of the Wallflower also contains fibrous 

 cells, which are scattered about among the vessels, 

 and are more common in the later-formed parts of the 

 wood. They are very long, thick-walled cells, with 

 sharply pointed ends. The wall is thickened almost 



