176 THE LEAVE8. 



exhalation is very great, while a further supply of moisture can 

 hardly be extorted from the parched soil ; — as we observe also in 

 a leafy plant newly transplanted, where the injured rootlets are not 

 immediately in a fit condition for absorption. Ordinarily, how- 

 ever, exhalation by the leaves and absorption by the roots are in 

 direct ratio to each other, and the loss sustained by the leaves is 

 immediately restored (by endosmosis, 40) 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, at a rate corresponding to 

 the amount of exhalation. The action of the leaves is, therefore, 

 the principal mechanical cause of the ascent of the sap. This is 

 well illustrated when a graft has a different time of leafing from 

 that of the stock upon which it is made to grow, the graft Avholly 

 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 Avithout 

 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 ac- 

 cumulation can take place : but in autumn, when the leaves perish, 

 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 in- 

 cisions mafle into the wood. This sap undergoes a gradual change 

 during the winter, and deposits its solid matter in the cells of the 

 wood. The absorption recommences in the spring, before new 

 leaves are expanded to consume the fluid ; chemical changes take 

 place ; 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 

 functions, 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 prob- 

 ably most rapidly ascending. 



