798 PHYSIOLOGY. 



as Tliallogens, which have no steins, there is no regular course 

 of the sap, but the fluids may be noticed flowing in all direc- 

 tions through their cells, and to be more especially evident in 

 those parts which are of a lax nature. 



The cause of the ascent of the sap is, as Herbert Spencer has 

 well expressed it, a disturbance of equilibrium creating a demand 

 for liquid. This is produced mainly by the evaporation or tran- 

 spiration going on in the leaves, but also by abstraction of the 

 sap by the growing tissues and by extravasation from the vessels 

 by pressure. The circulation is helped by osmotic and capillary 

 action, and also, when it occurs, by any swaying motion of the 

 branches causing intermittent pressure on the vessels. In the 

 winter no transpiration takes place, and the wood of the stem 

 and root is filled with watery matters holding starch and other 

 insoluble 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 dextrin and sugar, 

 development and transpiration immediately follow, and a conse- 

 quent 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 

 wlien 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 vessels as well as the prosenchymatous cells being filled with 

 sap, so that the whole plant is, as it were, gorged with it : much 

 of the sap which flows is indeed little more than water rapidly 

 pumped up from the soil to supply the drain of fluid. But as 

 soon as the leaves are in full activity, or the flowers, if they be 

 developed before the leaves, the sap becomes rapidly absorbed, 

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

 the younger 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, rather 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 plant in a perfectly normal state, the amount of 



