180 THE LEAVES. 



immediately restored (by endosrnosis, 37) through the ascent of 

 the sap from the branches, the latter being constantly supplied by 

 the stem ; so that, during active vegetation, the sap ascends from 

 the remotest rootlets to the highest leaves, with a rapidity corre- 

 sponding to the amount of exhalation. The action of the leaves is, 

 therefore, the principal mechanical cause of the ascent of the sap. 

 This is beautifully illustrated when a graft has a different time of 

 leafing from that of the stock upon which it is made to grow, the 

 graft wholly regulating the season or temperature at which the sap 

 is put in motion, and controlling the habits of the original stock. 

 Also by introducing the branches of a tree into a conservatory 

 during winter ; when, as their buds expand, the sap in the trunk 

 without is set unseasonably into motion to supply the demand. 



315. During the summer's vegetation, while the sap is consumed 

 or exhaled almost as fast as it enters the plant, no considerable 

 accumulation can take place : but in autumn, when the leaves per- 

 ish, the rootlets, buried in the soil beyond the influence of the cold, 

 which checks all vegetation above ground, continue for a time 

 slowly to absorb the fluid presented to them. Thus the trunks of 

 many trees are at this season gorged with sap, which will flow 

 from incisions made into the wood. This sap undergoes a gradual 

 change during the winter, and deposits its solid matter in the tubes 

 and cells of the wood. The absorption recommences in the 

 spring, before new leaves are expanded to consume the fluid ; the 

 soluble matters in the tissue of the stem are redissolved, and the 

 trunk is consequently again gorged with sap, which will flow, or 

 bleed, when wounded. But when the leaves resume their func- 

 tions, or when flowers are developed before the leaves appear, as 

 in many forest-trees, this stock of rich sap is rapidly consumed, 

 and the sap will no longer flow from an incision. It is not, there- 

 fore, at the period when the trunk is most gorged with sap, in 

 spring and autumn, but when least so, during summer, that the sap 

 is probably most rapidly ascending. 



