62 CAUSE OF ASCENT OF SAP. 



sap is principally accelerated by the plentiful perspiration of 

 the leaves." 



The sap then ascends in consequence of an attracting force 

 exercised from above downwards by the foliage of plants. But 

 it is evident that this is only a partial explanation of the 

 phenomenon ; for it does not account for the ascent of sap in 

 winter when leaves are absent. In order to explain that fact 

 we must have recourse to the action of endosmose, a force the 

 effect of which is to produce propulsion. A tree may be 

 assumed to be a combination of hollow tubes freely communi- 

 cating with each other, and enclosed in a skin through which 

 fluids are capable of being absorbed on the one hand and 

 expelled on the other. If we conceive a body of this kind, in 

 which the tubes are nearly empty, to have its lower extremity 

 plunged in water, the absorbing power of the skin at that part 

 will begia to introduce the water into the interior, and this 

 continuing to go on for a sufficient time, the tubes must neces- 

 sarily become at last filled with water rising upwards from 

 below. To effect this, no attracting force at the upper end of 

 the cylinder was necessary ; every particle of water which was 

 absorbed by the lower end, having driven before it a corre- 

 sponding volume of the water previously existing in the appara^ 

 tus. Under the influence of this operation the tubes would in 

 time become full, and if unelastic the introduction of more 

 water would be impossible. But if such tubes and the skin 

 that encloses them were elastic and extensible, then any such 

 further quantity of water might be introduced as the apparatus 

 could receive without bursting. If we then suppose that the 

 one end of the apparatus were cut open the sides of the tubes 

 would collapse, and the water would be forced out till there was 

 no more left than the tubes held in their original unstretched 

 condition. A tree is just such an apparatus. Its tubes are 

 nearly empty at the fall of the leaf. During winter the roots 

 absorb water from the soil and fiU the tubes again. By the 

 arrival of spring they are filled almost to bursting, and then if 

 the stem is cut it bleeds ; or if the roots are cut they bleed. 

 Bleeding ceases as the leaves unfold. The Vine, the Walnut 

 the Birch, are all as incapable of bleeding as other trees when 



