CONTINUITY OF SECONDARY HADROME 



671 



We must next turn our attention to the question of the close 

 relation between xylem-parenchyma and medullary ray tissue. A 

 few remarks must, however, be interpolated regarding the dimensions 

 of medullary rays. The simplest form of medullary ray is only one 

 cell in width vertically and horizontally, or, in other words, consists of 

 a single row of cells. More frequently, however, the rays only appear 

 to be uniseriate when seen in tangential or transverse section, but in 

 reality consist of several vertically superimposed cell-series (e.g. in the 

 majority of Conifers). Generally speaking, both the height and the 

 width of medullary rays vary within 

 wide limits; rays which attain any 

 considerable breadth, naturally com- 

 prise several or even numerous rows 

 of cells, while a large number of cells 

 may also be involved in the vertical 

 direction. The number of rays is 

 usuallv greatest, where the individual 

 rays are narrow and of inconsider- 

 able height, a relation which requires 

 no further consideration from the 

 physiological point of view. 



The medullary rays and more 

 especially the procumbent cells of 

 the rays represent radial water 

 channels which are linked together 

 both tangentially and longitudinally 

 by means of more or less extensive 

 masses of xylem-parenchyma. Tan- 

 gential commissures composed of this 



tissue connect every medullary ray with its neighbours (Fig. 277) ; the 

 presence of closely crowded pits on the radial walls of the commissure- 

 cells indicates that the flow of translocated material along these channels 

 is not always longitudinal, but that it may, if necessary, follow a tan- 

 gential course. In transverse sections of the woody cylinder, some 

 of the shorter commissures may only appear to abut against medullary 

 rays at one end (Fig. 277), or they may even seem to be entirely 

 isolated; but, in view of the fact that the medullary rays occupy 

 different radii at the various levels, it may be safely assumed, in such 

 cases, that the apparently isolated commissures bridge the interval 

 between adjacent rays at some level which is either above or below 

 the plane of the section. In the same way, individual rows of xylem- 

 parenchyma cells are never really isolated, although they often appear 

 to be so ; if their longitudinal course is carefully followed up, as has 



Fig. 277. 



Diagram illustrating the continuity of the 

 hadrome-tissue in the woody cylinder of 

 Casuarina. g, vessels ; m, medullary rays ; 

 p, strips of xylem parenchyma. All the 

 intervening space is occupied by mechanical 

 tissue (wood-fibres). After Wiesner. 



