TRANSLOCATION OF SUBSTANCES IN THE PLANT 321 



'^ascending stream," though m particiUar cases, for instance, m 

 drooping branches, it may move downward. At any rate, its 

 chief direction is from the base to the tip of any organ of the 

 plant. The other stream carries chiefly organic substances. It 

 originates in the leaves, which elaborate these substances, and 

 is directed mainly downward along the stem to the roots, where 

 these substances are utilized in growth and respiration. That 

 is why it is termed the '* descending stream." But its direction 

 is not so definite as that of the ascending stream. It may also 

 move upward from the leaves to the growing points, to branch 

 ends, blossoms, and developing fruit. That is why it is fre- 

 quently termed the ''stream of organic, or food, substances" 

 without reference to its direction. 



The existence of these two streams in the plant may be demon- 

 strated by the classical method of ringing, which may be done 

 in the following manner. Cuts one above the other are made 

 completely through the bark and around the trunk or branch, 

 and the bark between them is removed. Care must be taken to 

 remove all of the bark without injuring the young tender outer 

 layers of wood. To protect the exposed wood from desiccation, 

 it is covered either with thin rubber or with wax paper, or it is 

 coated with liquid wax. Leaves above the trunk treated in this 

 way remain alive and do not show any signs of wilting. Growth 

 of the flower buds and fruit proceeds unimpeded. On ringed 

 large branches, the fruit sometimes even reaches unusual size 

 and quality. This proves that the absorption of water and 

 mineral substances proceeds quite normally, and consequently 

 that the ascending stream moves along the wood. 



The descending stream, moving through the bark, is stopped 

 by the ringing. Nutritive substances accumulate at the upper 

 edge of the cut, resulting in an excessive outgrowth of the tissues 

 of the bark, forming a callus, which grows over the denuded 

 wood (Fig. 101). If the ringed region is not broad, this callus 

 may reach down to the lower edge and join mth the lower bark. 

 The wound then heals, and the downward movement of the sap 

 is resumed. But if the ring is broad enough and not a single 

 branch but the main trunk is ringed, the roots cease to obtain 

 nutritive substances from the leaves and gradually die from 

 starvation. This interrupts water absorption, the ascending 

 stream is Hkewise stopped, and finally the ringed tree dies. 



