FORCE OF THE ASCENT OF THE SAP. 821 
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 
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. At this period gallons of fluid will 
come, in some cases, in a few hours, from a wounded tree before 
the leaves have expanded ; and the fact that the leaves have not 
expanded is the explanation of the matter. For at this season 
of the year, before the leaves are fully developed, the reserve 
materials of the tree are largely stored up in the root, and from 
chemical changes there actively going on, the fluids in that part 
become very dense, and the consequence is that an excessive 
osmotic action takes place. There is far more fluid absorbed 
from the earth than the plant can use, and root-pressure then 
takes place, and this pressure forces the fluid up the stem. (See 
page 791.) ‘This is the explanation of what is called bleeding. 
But this bleeding arises from the vessels as well as the prosen- 
chymatous cells being then filled with sap, so that the whole plant 
is, aS it were, gorged with it: much of the sap which at that 
period flows is indeed little more than water rapidly pumped 
up from the soil to supply the drain of fluid. The process does 
not take place at any other time of the year, for as soon as the 
leaves are in full activity, or the flowers, if they be developed 
before the leaves, the fluid which is absorbed by the roots is 
naturally carried up the plant, and becomes transpired, and 
thus carried off. It by no means follows, therefore, that when 
the plantis most gorged with fluid matters, and bleeds, the force 
of the circulation is most active; but rather that it is greatest 
when the stem is least gorged with sap, as in the summer 
months, when vegetation is in full vigour, 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 fluid absorbed by the roots, the force with which it ascends 
to the stem, and the amount transpired by the leaves, are 
directly proportionate to one another. 
