124 PLANT LIFE. 



from cell to cell, but air generally travels in the 

 spaces between the cells. The presence of air in the 

 vessels might indeed altogether prevent the rise of the 

 water or sap. 



From the leaves the food material travels downwards 

 by the phloem tubes and eventually reaches the patches 

 of phloem already mentioned, in the root ; this food 

 material will therefore be close to the pericycle on its 

 outside ; and, from the phloem, everything outwards 

 (endodermis, cortex, and root hairs) obtain s its 

 nourishment. The cells inside the phloem, between 

 it and the xylem, are also supplied with food material 

 from it. 



As is shown in a longitudinal section, and the 

 transverse section of a very young root, the formation 

 of the xylem vessels is inwards^ that is, new vessels 

 are continually added on the inside of the original 

 patches such as ,t', until all these cells become 

 xylem, and unite to form the distinct chariot wheel- 

 like arrangement which has in this case four spokes. 

 After this, it does not at first seem possible for any 

 new water-vessels to be formed. That, however, is not 

 the case, for the crescents of cells between xylem and 

 phloem, which, as we have seen, are well supplied with 

 food, begin to divide and increase : some of them, those 

 to the inside, form new xylem, and others, to the outside, 

 form new phloem. The root, in fact, begins to increase 

 in thickness by the action of a cambium as in the stem 

 (see pp. 1 60 and 161). At this period of its life the root 

 is probably no longer itself absorbing, it is simply 

 carrying the water from the younger roots which have 

 developed from it. The cortex cells are therefore no 

 longer required ; the pericycle cells will probably begin 

 to form a cork ring or cylinder round the central con- 



