306 CIRCULATION OF NUTRITIVE FLUID. 



in the lowest tribes of Animals ; as in this way alone can the forces 

 which are concerned in the movement of the fluid, be rightly appre- 

 ciated. In all the higher Plants, the ascending or crude sap is to be 

 distinguished from the elaborated or descending sap. The former of 

 these fluids should be compared rather with the chyle than with the 

 blood of Animals ; for it is not yet fully prepared to take part in the 

 nutrition and extension of the structure. But there are some circum- 

 stances attending its movement, which throw light upon other more 

 complicated phenomena. The ascending sap consists principally of 

 water; which is imbibed, together with various substances which it 

 holds in solution, by the delicate tissue at the soft extremities of the 

 root-fibres, or spongioles. The power of forcing upwards a column of 

 sap, which exists in these bodies, and which seems due to Endosmose 

 ( 491), is shown by very simple experiments. If the stem of a Vine, 

 or of any tree in which the sap rises rapidly, be cut across when in full 

 leaf, the sap continues to flow from the lower extremity ; and this with 

 such force, as to distend with violence, or even to burst, a bladder tied 

 firmly over the cut surface. If, instead of a bladder, a bent tube be 

 attached to this, and mercury be poured into it so as to indicate the 

 pressure exerted, it is found that the rise of the sap takes place with a 

 force equal to the pressure of from one to three atmospheres (from 15 

 to 45 Ibs. upon the square inch) or even more. Thus the ascent of 

 the sap is partly due to a powerful vis a tergo, or impelling influence, 

 derived from the point where the absorption takes place. 



541. But, on the other hand, if the upper extremity be placed with 

 the cut surface of the stem in water, a continued absorption of that 

 fluid will take place, as is evidenced by the withdrawal of the water 

 from the vessel; the fluid which is thus taken up, however, is not 

 retained within the stem and branches, but is carried into the leaves, 

 and is thence dissipated by exhalation. It is obvious, then, that the 

 vis a tergo is not the sole cause of the ascent of the sap ; but that a 

 vis a f route also exists, by which the fluid is drawn towards the parts in 

 which it is to be employed. This is further made apparent by a few 

 simple experiments. If a branch, when thus actively absorbing fluid, 

 be carried into a dark room, the absorption and ascent of fluid imme- 

 diately cease almost completely ; and are renewed again, so soon as the 

 leaves are again exposed to light. Now we know, from other experi- 

 ments, that light stimulates the exhaling process ( 87), whilst darkness 

 checks it ; and the cessation of the demand in the leaves thus produces 

 a cessation in the absorption at the lower extremity of the stem. And 

 this is the case, also, in the natural condition of the plant ; as is easily 

 shown by immersing the roots in water, and observing the respective 

 quantities which are removed by absorption during sunshine, shade, and 

 darkness. On the other hand, the movement of the sap may be excited, 

 when it would not otherwise take place, by the production of a demand 

 at the extremities of the branches ; thus if a branch of a vine growing 

 in the open air, be introduced into a hot-house, and be subjected to 

 artificial heat during the winter, its buds will be developed, its leaves 

 will expand, and these will draw fluid to themselves through the roots 

 and stems, which are still inactive as regards the remainder of the tree. 



