102 COMPOUND ORGANS OP PLANTS. 



bark is evident from the following simple experiment. If a 

 ring of bark be removed from a tree in spring, the sap will 

 rise just the same as usual, but when the sap begins to descend, 

 a protuberance will be formed just above" the ring, which is 

 occasioned by the accumulation of sap there, its farther descent 

 being stopped by the removal of the bark. The same effect 

 will be produced if we make a simple ligature or annular com- 

 pression of a young stem. At the end of a year or two a cir- 

 cular swelling will form itself immediately above the ligature. 

 This swelling is evidently produced by the sap, which descending 

 through the thickness of the bark from the summit of the 

 stem, and finding an obstacle which it cannot pass, accumulates 

 above that obstacle. All must have observed the distortions 

 which twining stems thus produce on the trunks of the trees 

 about which they entwine themselves. 



So also we see the reason why the branch of a fruit tree, 

 when sterile, may be made to flower and fruit abundantly by 

 being girdled. This consists in removing a narrow ring of 

 bark from the branch, sufficient to arrest the downward course 

 of the elaborated sap, which is thus accumulated in the branch 

 sufficient in quantity to produce this desirable result. 



The ascending and descending sap are very different both in 

 appearance and qualities. The ascending sap in all plants is 

 nearly the same, containing no noxious qualities even in the 

 most poisonous. We are told, by Berthellot, that the natives 

 of the Canary Islands tear off the bark from the poisonous 

 Euphorbia Canarensis, and find the ascending sap which they 

 obtain from the alburnum a refreshing drink, whilst the des- 

 cending sap is of so acrid a nature that it acts as a caustic, 

 burning the flesh ofi' such as happen to touch it. In the maple, 

 and some other plants, the ascending sap is so sweet that sugar 



