150 PROGRESSION OF THE SAP. 



wind, which, as Mr. Spencer has shown, exercises considerable in- 

 fluence not only propulsive on the main ascending and descending 

 currents, but also extravasating into the lateral flows. It may be 

 said that there is a, vis a tergo, without the presence of leaves, as shown by 

 the experiments of Hales (fig. 243), combined with a vis a fronte, 

 depending on the suction power of the leaves. 



When cut twigs or flowers are put into water, their functions are 

 kept up for some time by endosmose and capillarity. The latter power 

 has great influence in such a case, and hence the cleaner the cut the 

 better, so that no lacerated or ragged edge may interrupt its operation. 

 In these circumstances, also, small solid particles and colouring matters 

 will enter the tubes. Boucherie found that felled trees, the extremities 

 of which were immediately immersed in various solutions, continued 

 to. imbibe them with great force and japidity for many days. A 

 Poplar, 92 feet high, absorbed in six days nearly sixty-six gallons of a 

 solution of pyrolignite of iron. 



Heat and light have a powerful influence on the movements of the 

 sap, by promoting transpiration and the action of the cells. After the 

 winter's repose the first genial sunshine of spring stimulates the sap 

 to activity, and after the leaves are expanded the circulation goes on 

 with vigour. The efiect of leaf-buds in promoting the movement of 

 sap, may be exhibited by introducing a single branch of a vine grow- 

 ing in the open air into a hothouse during winter, thus exposing it to 

 the action of heat as well as light. In this case the leaves are de- 

 veloped, and the fluids are set in motion from the roots upwards, so 

 as to supply this single branch, although in the other branches there 

 is no increase in the circulation. 



In spring, the first effect of light and warmth is to stimulate the 

 leaf-buds. These enlarge, and the osmotic action commences in their 

 cells. The matter stored up during the winter undergoes changes ; 

 certain substances are dissolved, and thus the sap is thickened, so that 

 the endosmotic process is powerfully increased, and the whole plant 

 exhibits an active and vigorous circulation. The starch deposited in 

 the previous season becomes converted into sugar and dextrin, it is 

 thus readily acted on by the ascending fluids, and in a state of solu- 

 tion admits of being generally diffused. Towards the latter part of 

 the season when the heat and light decrease, the leaves perform their 

 functions more languidly, and there is a near approach to equUibriiun 

 in the density of the fluids, and ultimately there is a cessation of the 

 circulation. 



The height to which the sap rises in the case of lofty trees with 

 spreading roots is very great. The force with which it ascends has 

 been measured by Hales, and is found to vary according to the state 

 of the weather and the vigour of the plant. By fastening a bent tube, 

 containing mercury, on the stem of a vine, he found in one of his 



