162 



TRANSPORT OF FOODS 



Relation of Phloem Elements to Other Tissues. The 

 food in its downward or upward course through the phloem 

 may be drawn upon at any point by living tissues for growth, 

 repair, etc., or it may be set aside for storage in medullary rays, 

 xylem and phloem parenchyma. Evidently to facilitate this 

 movement of food-stuffs there are often pores between sieve 



tubes and companion cells, and 

 thin places or pits where the 

 medullary rays abut on com- 

 panion cells or phloem par- 

 enchyma. Pits also occur in 

 the tangential walls of the 

 medullary rays to help along 

 the radial movements and 

 storage of foods in the medul- 

 lary rays; and pits in the walls 

 separating the rays from wood 

 parenchyma cells assist in the 

 transmission of foods to, and 

 storage in, the latter (Fig. 88). 

 The medullary rays have for 

 their primary function the radial 

 transmission and storage of 

 food. Their intimate relation 

 with the cells of the phloem at 

 their outer end and with the 

 xylem parenchyma along their 

 inner course, and the fact that we usually find them gorged with 

 food, points to this conclusion. The short vertical extent of 

 the rays, and their isolation from each other, renders them 

 unsuited for the vertical or longitudinal transmission of foods. 

 If they were of value in this respect girdling would not prevent 

 the downward flow of foods. 



The extreme frequency of the rays is one factor of great im- 

 portance to their efficiency in radial conduction and storage. 

 In tangential section, that is, as one would see it when facing 



FIG. 88. Showing pitted connections 

 between medullary rays and xylem paren- 

 chyma, and between contiguous xylem 

 parenchyma cells, m, medullary rays; 

 n, xylem parenchyma. Camera-lucida 

 drawing of tangential section of wood of 

 yellow poplar. 



