Movement of Materials in Stems 163 



the most favorable conditions it is not possible to draw out of a 

 tree more than 5 per cent of the sugar that it contains. 



A flow of sap somewhat similar to that in the maples occurs 

 in the spring in some other species of trees, as in the birch, butter- 

 nut, and hornbeam. In the birch the flow is more regular and 

 continues until May ; but the rate of flow and sugar content are 

 less than in the maple. 



The movement of sugar in the water-conducting tissues of 

 stems is rather exceptional ; its usual path hes in the food-con- 

 ducting tissues. 



Movement of foods. In previous chapters we learned that the 

 vascular bundles of stems contain food-conducting tissue. This 

 tissue is composed of thin-walled elongated cells and extends 

 from the veins of the leaves, through the stem, into the extrem- 

 ities of the roots. The larger of the vessels are the sieve tubes. 

 We also learned that dissolved substances alone can move from 

 one cell to another, and that the movement is by diffusion from 

 regions of greater concentration to regions of less concentration. 



Applying this information to the movement of food in stems, 

 it is evident that in annuals the general direction is from the leaves 

 to the stems and roots during the vegetative period. When 

 flowering and the development of seeds begin, a considerable 

 part of the excess food moves into the reproductive structures. 



In trees food moves into the branches and trunk during the 

 summer and autumn, and accumulates in the food-conducting 

 tissues, in the pith rays, and in young stems in the pith also. 

 In the spring there is a great increase in the amount of soluble 

 foods, and these move both into the roots and into the twigs. 

 At this time new roots are developing, and buds are growing into 

 new leafy shoots. When growth has stopped and photosynthesis 

 is active, the movement is again from the twigs toward the trunk. 



Digestion. Before insoluble foods may be moved from one 

 part of a plant to another, they must be changed to soluble sub- 

 stances. This process may be illustrated by the changes that 



