130 PROGRESSION OF THE SAP. 



the process of endosmose and exosmose depends on the chemical affinity 

 between the fluids on each side of a membrane, the porosity of the 

 membrane, "and the attraction existing between it and either of the fluids, 

 it follows that the nature of the parietes of the cells and vessels of plants 

 must have a marked effect on their contents and secretions. 



269. The observations of physiologists and chemists thus lead to the 

 conclusion, that the movement of the sap in plants is due partly to the 

 changes effected in the leaves and other green parts, by light and air ; 

 partly to capillary attraction, the continuous influence of which is kept 

 up by the constant loss of fluids ; and partly to endosmose and exos- 

 mose. It may be said that there is a vis a tergo, without the presence 

 of leaves, as shown by the experiments of Hales (fig. 224), combined 

 with vis afronte, depending on the suction-power of the leaves. 



270. 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 colour- 

 ing matters will enter the tubes. Boucherie found that felled trees, 

 the extremities of which were immediately immersed in various solu- 

 tions, continued to imbibe them with great force and rapidity for many 

 days. A Poplar, 92 feet high, absorbed in six days nearly sixty-six 

 gallons of a solution of pyroMgnite of iron. 



271. 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 effect of leaf-buds in promoting the movement of 

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

 in the open air into a hot-house during winter, thus exposing it to heat 

 and light. In this case the leaves are developed, 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 circulation. 



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

 the leaf-buds. These enlarge, and the endosmotic process commences 

 in their cells. This is communicated to other cells, and gradually 

 extends to the root, which draws up a continued supply of fluids from 

 the soil. 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. 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 equili- 

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

 of the circulation. 



