CIRCULATION OF ELABORATED SAP. 307 



And the natural commencement of the movement of the ascending sap, 

 which takes place with the returning warmth of spring, has been ex- 

 perimentally shown to occur, in the first instance, not in the neighbour- 

 hood of the roots, but nearest the extremities of the branches ; the 

 exhalation of fluid from the expanding buds being the first process, 

 and a demand for fluid being thus created, which is supplied by the 

 flow that is thus excited in the lower part of the stem, this, again, 

 being supplied from the roots, which are thus caused to recommence 

 their absorbent function. 



542. Thus we see that, in the ascending sap, the movement is en- 

 tirely regulated by the demand for fluid occasioned by the actions of 

 the leaves ; even though it is in great part dependent on the vis a tergo, 

 which has its seat in the spongioles. Not even this force, however, 

 powerful as it has been shown to be, can produce the continuance of 

 the upward flow, when the exhalation from the leaves is checked by 

 darkness, and when the demand occasioned by the action of these organs 

 is consequently suspended. 



548. The movement of the descending sap offers numerous points 

 which deserve to be carefully considered. This fluid is strictly compa- 

 rable to the blood of animals ; having undergone a preparation or 

 elaboration in the leaves, which adapts it to the nutrition and extension 

 of the structure, and to the formation of the various secretions of the 

 plant. A great part of the fluid of the ascending sap has been lost by 

 exhalation : and the remainder, thus concentrated, receives a large 

 additional supply of solid matter through the agency of the green cells 

 of the leafy parts, which take in carbon from the atmosphere ( 83) ; 

 so that it now includes a considerable amount of gummy matter, in the 

 state prepared for being converted into solid tissue, as well as numerous 

 other compounds. Now this elaborated sap seems to be conveyed into 

 the various parts of the system, partly by transmission from one cell to 

 another, but partly through the agency of a network of vessels, which 

 takes its origin in the leaves, and extends along the branches to the 

 stem and roots, chiefly in the bark of those parts. These vessels are 

 strictly analogous to the capillaries or small blood-vessels of Animals ; 

 but they differ from them in this, that the capillary network of Ani- 

 mals communicates on either side with larger trunks, being formed, in 

 fact, by the interlacement or anastomosis of their minutest branches, 

 whilst the network of nutritive vessels in Plants is everywhere con- 

 tinuous with itself, riot having any communication with large vessels, so 

 that the fluid prepared in the leaves commences a circulation there, 

 which is continued on the same plan, until it has found its way to its 

 remote destination in the roots. 



544. The natural movement of the elaborated sap through these 

 vessels may be studied, under favourable circumstances, with the assis- 

 tance of the microscope ; the requisite conditions being, that the part 

 should be sufficiently transparent for the vessels to be distinctly seen, 

 that the sap shall contain globules in sufficient number to Callow its 

 movement to be distinguished by their means, and that the circulation 

 should be observed without the separation of the organ examined from 

 the rest of the Plant, which would produce irregular movements, by the 



