CONDUCTING POWER OF SAP-WOOD. 231 



could conduct the necessary supply of sap to the foliage of a 

 arrowing tree, and also whether the bark alone could furnish 

 the requisite water to prevent the leaves from wilting. A 

 specimen of Hibiscus splendens, standing in the ground and 

 having three stems from the same root, was selected for trial. 

 The shrub was growing rapidly, and was prepared for the 

 experiment as follows : Two of the stems were tied firmly to 

 stakes, and the third left undisturbed. The first specimen 

 had all the bark removed from one inch of the stem, and then 

 the wood was cut away till there remained only a small piece 

 of the outside layer of sap-wood, which was one inch long and 

 seven-sixteenths of an inch in circumference. This exposed 

 surface was immediately covered with grafting-wax, to protect 

 the tissues from the action of the air. The amount of stem 

 remaining was just one eighty-fourth of the original, which 

 was about four inches around. The healthy leaf-surface was 

 fully twenty-five hundred square inches, from both sides of 

 which exhalation went on to some extent, making five thou- 

 sand square inches of exhaling surface. The result was, that 

 the foliage remained perfectly fresh and vigorous for ten days, 

 until, on the tenth of November, the specimen was cut for 

 the museum. (Figs. 20, 21.) 



The other stem was used to determine whether by osmose, 

 or in any other way, the crude sap could ascend in the bark 

 find supply the leaves with water. All the wood and one- 

 third of the bark were removed from a portion one-half inteh 

 in length, the exposed tissues protected by wax, and the 

 branches so pruned as to leave only five hundred square 

 inches of leaf-surface. The foliage all drooped in a single 

 hour and never recovered. This experiment showed that the 

 bark was altogether incompetent to furnish the requisite 

 supply of crude sap to the parts above it, although it was 

 thick and succulent, and much greater in quantity, when com- 

 pared with the exhaling surface, than the piece of sap-wood 

 which showed such marvellous conducting power. If osmose 

 were the cause of the ascent of sap, it would seem that the 

 abundant parenchyma of the bark, intimately united as it is 

 with the wood by the medullary rays, must freely transmit 

 the amount required in this case. But the leaves wilted and 

 perished as quickly as if the entire stem had been severed. 



