1 68 TRANSPORT OF FOODS 



The tracheal tubes must prove very efficient carriers, for the 

 ascending currents of water would sweep the food along much 

 faster than it could be moved in the sieve tubes. While most 

 plants make large use of the tracheal tissues in this way there are 

 others, such as the grape, which make little or no use of them and 

 send practically all of their food into the sieve tubes for trans- 

 portation. The fears that the grapevine may be depleted of its 

 food when bleeding from spring pruning are therefore groundless, 

 since little more than water and a small percentage of salts from 

 the soil are then lost. 



It will be noted that just as the sieve tubes can carry food 

 up or down as needed, so the medullary rays are able to trans- 

 port foods, radially inward for nourishing the xylem or for storage, 

 and outward again when they are to be distributed and put to 

 use. There is nothing about the construction of the food- 

 conducting tissues to prevent movement in them in a certain 

 direction at one time and in the opposite direction at another, 

 and it is quite possible that a flow in opposite directions may 

 occur at the same time in a cell or tube. 



The growth of seedlings in their earlier stages presents another 

 set of conditions in food distribution. Then the food must 

 move from its place of storage in cotyledons and endosperm 

 before the special food-conducting tissues have become differen- 

 tiated; but the distance to be traveled is short and the cell- walls 

 are new and thin and offer relatively little resistance. Under 

 these conditions it seems that food can travel fast enough to 

 maintain rapid growth without the aid of sieve tubes. In seed- 

 lings the direction of flow of food is down into the roots and up 

 into the shoot from the place of storage, but as soon as the green 

 leaves unfold they at once become an additional source of food 

 supply, and by that time the vascular bundles have been formed 

 and the sieve tubes are ready to take up their work, carrying 

 the food down or up as need compels. 



The anastomoses of the vascular bundles described on page 

 42 are of great use in the distribution of food as well as of water. 

 In the elm tree, for instance, the leaves are two-ranked, so that 



