460 



Bulletin 77. 



to grow. The same may also be said of the cells of the wood (W), 

 of the medullary rays (M), and of the pith (P). Those forming 



the pith are perfectly inac- 

 tive and have no practical 

 value in the economy of 

 the plant. The wood 

 of the grape serves as a 

 medium for the passage of 

 large quantities of water or 

 G crude sap. In these cells 

 the general tendency of the 

 movement is upward. 

 The office of the bark and of the 

 corky layer is probably mainly one 

 of protection to the more delicate 

 parts beneath. The cells compos- 

 ing them, excepting the soft bast 

 cells (s b), are also inactive, and 

 possess apparently no life. The 

 flow of liquid, if it ever takes place 

 in them, is very limited.' The same 

 may also be said of the medullary 

 rays. The use of these woody cells, 

 extending from the pith to the out- 

 er bark, seems to be largely me- 

 chanical, for they serve as a frame- 

 work which is designed to give solid- 

 ity to the stem, and also as a medium 

 of connection between the inner and 

 outer parts of the stem. 



One part of the diagram, that 

 marked (C), still remains to be dis- 

 cussed, and it is this portion which 

 is of the greatest interest to those 

 who graft plants. This layer of cells or cambium, as it is called, 

 is that part of the stem which contains the living or growing cells. 

 In the main part of the drawing it is represented simply by two 

 parallel lines. But at the end of these lines to the right the struc- 



. Cross section of a grape cane, 

 much magnified, to show the 

 structure, c k, cork, s b, soft 

 bast, h by hard bast. B. bark, 

 C, cambium. IV, wood, P, 

 pith. M, medullaiy ray. 



