CHAP. II. FUNCTION OF NUTRITION. 181 



plant. If this view of the subject should prove correct, 

 then the intercellular passages must be considered ana- 

 logous to the stomachs of animals, mere recipients of a 

 crude material, which is afterwards modified and ren- 

 dered available for the purposes of nutrition. 



(165.) Cause of Progression. The progression of 

 the sap appears to be influenced by several causes. De 

 Candolle supposes it to be carried forward through the 

 intercellular passages by successive contractions and dila- 

 tations of the cells. But there appears to be no warrant 

 for the supposition ; on the contrary, it seems impos- 

 sible that such an effect could be produced in cells 

 which are replete with an incompressible fluid. If 

 contraction were to take place, an expulsion of the con- 

 tained fluid must ensue, and every dilatation of the cells 

 would require that the ambient fluid should enter them. 

 Whether therefore the sap rises or not through the in- 

 tercellular passages, the hypothesis which he has framed 

 to explain its progression appears to be inadmissible. 



(166.) Propulsion of the Sap. The first and most 

 important cause of the rise of the sap, resides in the 

 spongioles. The water imbibed by them, is also by 

 them propelled forward with considerable force, and 

 the effects are strikingly analogous to those exhibited 

 by the endosmometer (art. 144.). Hales cut off the 

 stem of a vine in the spring, when the sap rises with 

 the greatest velocity, and luted a tuba to the top of the 

 stump, bent in the manner we have described in the 

 construction of the endosmometer. As the sap rose into 

 the tube, mercury was introduced at the open end ; and 

 a measure of the force of the rising sap was thus ob- 

 tained, and found to equal the pressure of an atmosphere 

 and a half. If a piece of bladder be tied over the sur- 

 face of a v^ne-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 

 indicate the presence of a powerful force, a ' ' vis a tergo." 

 N 3 



