7S2 PHYSIOLOGY. 



tion takes place, and the cells of the young buds are moreover 

 then filled with watery matters holding starch and other in- 

 soluble substances in suspension. The fluids of the plant are 

 therefore in a nearly quiescent state, as there are no changes 

 then taking place to produce their distribution. When the 

 increased heat and light of spring commence, the insoluble 

 starch, &c., become converted into soluble dextrine, sugar, &c., by 

 which the contents of the cells become thickened, development and 

 transpiration immediately follow, which give rise to endosmotic 

 action and a consequent ascent of the sap. This flow continues 

 throughout the summer months, when the causes favourable to 

 it are in full activity ; but towards the autumn, as heat and light 

 diminish again, the force of the ascent also diminishes, and the 

 flow of sap is again suspended in the winter months from the 

 reasons above alluded to. 



The force with which the sap ascends is probably greatest in 

 the summer months, when heat and light are most intense, and 

 when vegetation is consequently most active ; and least in the 

 winter. At first sight it would appear, that the most rapid flow 

 of the sap was in the spring months, at which period alone, 

 plants will give off" much fluid, or bleed as it is commonly termed, 

 when their stems are wounded. But this bleeding arises from 

 the plants being then in an abnormal state, for the vessels as 

 well as the prosenchymatous cells are filled with sap, so that the 

 whole plant is, as it were, gorged with it, and thus in conse- 

 quence of the leaves being not then fully formed, and unable, 

 therefore, to absorb for their use all the sap which the stems 

 contain, such stems will bleed when wounded. But as soon 

 as the leaves, &c., are in full activity, or when the flowers are 

 developed before the leaves, the sap becomes rapidly absorbed, 

 and the current is soon confined to its proper channels — the 

 prosenchymatous cells — and the stems no longer bleed. It by 

 no means follows, therefore, that when the plant is most gorged 

 with fluid matters, and bleeds, that the force of the circulation 

 is most active, but the force is greatest when the stem is least 

 gorged with sap, as in the summer months, when vegetation is 

 most active, and the sap consumed as fast as it can be transferred 

 upwards through the stem. 



In a healthy state of the plant, as already noticed (p. 779), 

 absorption of fluid matters by the roots and their transpiration 

 by leaves are directly proportionate tha one to the other, and 

 hence all stimulants to transpiration are in hkc manner stimu- 

 lants to absorption. In like manner, as we have just observed, 

 the force of the circulation is promoted by transpiration, and 

 thus in a healthy plant in a perfectly normal state, the amount 

 of fluid absorbed by the roots, the force with which it ascends 

 to the stem, and tlic amount transpired by the leaves, are 

 directly proportionate to one another. 



