CH. Ill] VESSELS. 43 



The eridodermis is the innermost layer of the cortex : 

 the pericycle the most external layer of the central 

 cylinder. 



Within the bundle -sheath 8 or 10 patches of tissue, 

 4 or 5 being of one kind, 4 or 5 of another, alternate 

 with each other as shown in fig. 13, where phi and x 

 alternate as the eye travels round the circumference of 

 the axial cylinder. The patches marked x are known as 

 xylem, the alternate ones phi are called phloem. Xylem 

 and phloem are the constituents which, in vascular plants, 

 i.e. plants with vessels, make up the vascular tissues. At 

 present we are only concerned with the xylem ; it is made 

 up of vessels, a vessel being a pipe or tube built up of 

 cells placed end to end, the constituent cells of a vessel 

 being excessively long in proportion to their diameter. 



The striking feature about them is that they have no 

 cell-contents ; the protoplasm which they originally con- 

 tained, and which regulated their behaviour whilst they 

 were developing, dies and disappears. Moreover the 

 cross walls, which are the end walls of the constituent 

 cells, become disorganised, and disappear, either in part or 

 completely, so that the vessels finally come to be elongated 

 tubes without protoplasmic contents. The walls of the 

 vessels undergo moreover a peculiar change, they are no 

 longer ordinary cellulose ; they have been lignified, 

 changed in such a way that they no longer react chemi- 

 cally like cellulose. 



The root is thus seen to be characterised by the 

 presence of elongated tubes running along its whole 

 length, which might suggest the transference of fluid 



