8 



Nature-Study Agriculture 



The return- 

 ing sap 



The differ- 

 ence 

 between 

 rising and 

 descending 

 sap 



The course 

 sap 



a tree 



Breathing 

 pores in 

 the leaves 



The sap, after being changed in the leaves, streams 

 away from them to all parts of the plant. It goes back, 

 however, through a different set 

 of channels from those through 

 which it passed on its way up. 

 Moreover, the returning sap is no 

 longer crude; it is laden with 

 sugar and other foods that have 

 been manufactured in the leaves. 

 It supplies nourishment, on its 

 journey, to all parts of the plant, 

 including the roots, and some of 

 u. s. D. A. ft i s stored, in the form of starch, 

 FIG. 9. Droplets collect in- until it may be needed. 



5* ^ an ordinary tree the sap runs 

 up through the sap wood, which 

 lies near the outer part of the trunk but still well 

 below the bark. (Exp. 6.) The returning stream of 

 sap comes down through the inner layer of the bark 

 (Fig. 8). If a tree is girdled (cut all around) so deeply 

 as to cut through the sapwood, it wilts and dies very 

 quickly ; but if it is girdled only deeply enough to cut 

 through the bark, the tree may live for months. It will 

 die eventually, however, for the roots cannot live long 

 without the nourishing sap that is sent down from the 

 leaves, since the crude sap which has not been to the 

 leaves does not contain the food that the roots need. , 

 The leaves. The surface of each leaf contains thou- 

 sands of little openings, called " stomata " (Greek : 

 mouths), into which oxygen and carbon dioxid from the 

 air enter. It is through these openings, also, that most 



