LOCATION AND EXTENT OF STORAGE TISSUES 



189 



to greater or less distances out into the bark between the phloem 

 bundles, and they are therefore in position to take on food as 

 it travels from the leaves through the phloem, and to store it 

 within themselves, or to deliver a 

 part of it to the wood parenchyma 

 for storage. And when the period 

 of storage is over, the rays are in 

 position to deliver the food to the 

 tracheal tubes for quick transporta- 

 tion toward the crown where un- 

 folding buds and flowers and fruits 

 are in need of it (Fig. 94). 



The medullary rays in the wood 

 store non-nitrogenous foods in much 

 greater abundance than the nitrog- 

 enous, and usually in the form of 

 starch, and these ray cells must 

 therefore possess many active leuco- 

 plasts which seize upon the food as 

 it comes in solution and change it to 

 the insoluble condition before it, or 

 more than a fraction of it, has a 

 chance to enter the tracheal tubes 

 and be swept back toward the leaves 

 whence it came. It will be seen 

 that the physical conditions would 

 impel the dissolved food after enter- 

 ing the ray cells to pass on into the 

 adjoining tracheal tubes by diffusion 

 (see page 94), unless it were rendered 



insoluble, or the plasma membrane imposed its interdiction 

 (see page 92). It may well be that the plasma membranes 

 of the ray cells play an important part in this way; but if they 

 do they alter their behavior when growth is resumed in the 

 spring, for they then allow the digested food to pass freely into 

 the tracheal tubes (see Fig. 103). 



FIG. 103. Diagram to show food 

 from the leaves descending through 

 the sieve tubes and being stored in 

 the medullary ray cells and xylem 

 parenchyma, in A; and the digestion 

 of the stored food and its ascent 

 through the tracheal tubes when 

 growth is resumed in the spring, at 

 B. In both diagrams the black 

 bodies indicate stored food; that 

 at the points of the arrows is being 

 stored, and that at the base of the 

 arrows is being digested and carried 

 away. 



