NUTRITION". 49 



with which the sap rises has been proved by 

 several curious experiments. Hales introduced 

 the root of a vigorous pear tree into a glass tube 

 hermetically sealed at the top, with a lute quite 

 impervious itself to air ; this tube was filled with 

 water, and placed in a cup of mercury ; in six 

 minutes the mercury had risen eight inches in 

 the tube, to replace the water that had been 

 absorbed. From other experiments on the force 

 with which the sap rises, Hales drew the con- 

 clusion that it is five times greater than that 

 with which the blood is thrown into the crural 

 artery of a horse. " If a piece of bladder be 

 tied over the surface of a vine stump when the 

 sap is rapidly rising, it soon becomes tightly 

 distended, and will ultimately burst. These 

 effects manifestly bespeak an action very different 

 from the ordinary results of capillarity, and indi- 

 cate the pressure of a powerful force, a ' vis d 

 tergo,' residing in the lowest extremities of the 

 roots, by which the propulsion of the sap is regu- 

 lated. Although these results so closely resemble 

 those of endosmosis (20), there still exists a 

 difficulty in connecting the two phenomena ; for 

 whilst we may admit the possibility of an inter- 

 change between the contents of the vesicles com- 

 posing the spongioles, and the water in the soil 

 E 



