CAISK OF THE DKSCEXT OF THE SAP. 31 



RV such a mysterious affair, for which they could offer no explanation. Physiologists 

 baye here stepped in with their phantom vitality, and have explained the descent of 

 the elaborated sap on visionary hypotheses, that it was alive, or had obtained some 

 vital qualities. It had long been perceived that gravitation could have little or nothing 

 to do with the motion, for the descending sap flows upward in a dependent branch. 



95. What, then, is the reason that the light of the sun controls the rapidity of imbi- 

 bition the speed with which the ascending current comes ? Because it controls the 

 amount of carbonic acid gas which is reduced, and, therefore, the amount of elaborated 

 sap that is formed. Why is it that the flow from the roots diminishes when changes 

 are befalling the leaves, and why does it stop in the winter ! Because the mucilagi- 

 nous solution which is made by the light diminishes in quantity, or ceases to be formed 

 altogether. How is it that, when parts are rapidly developing, the latex moves fastest, 

 and the ascending sap comes with most force ! Because the consumption and the con- 

 sequent formation of the mucilaginous matter are then at a maximum. 



96. We see, therefore, that the two sources of force in a flowering plant, the spon- 

 giole aid the leaf, derive their power from ordinary physical principles. And these 

 considerations also furnish us with another instance of that unity of plan so often met 

 with in the works of Nature ; the same law which determines the action of the spon- 

 giole determines also the "action of the leaf. The same idea is concerned in throwing 

 the sap upward into the stem, and forcing it down again from the leaf. And the rays 

 of the sun. which, by forming that mucilaginous body, gives rise to these concurrent 

 and harmonious actions, equally set in operation the tissues of the leaf which is freely 

 exposed to their influence, and the absorbing mechanism of the spongiole, which is 

 buried, perhaps, many feet deep in the ground. 



97. Whatever has been here said respecting the movements of the nutritious juice 

 in exogenous plants, applies also to the case of endogens, the essential mechanism be- 

 ing the same in both instances. 



98. It has been clearly established, by the researches of comparative anatomists, that 

 the presence of a circulating machinery is determined by the centralization of the nu- 

 tritive and respiratory apparatus. In exogenous and endogenous plants, from the cir- 

 cumstance that the liquid and solid material are introduced at distant points, the one 

 through the root, the other through the leaf, channels of communication from one 



, to the other, and, indeed, to every part, are required, and hence the introduction of a 

 circulatory apparatus. In lower tribes of vegetable life, where this separation of func- 

 tion does not exist, the circulatory mechanism is correspondingly absent ; the sea-weeds 

 absorb on their whole surface, and nutrition is directly carried forward at the points of 

 reception. In lichens there is the first appearance of a transfusory mechanism, arising 

 from the circumstance that on those parts which are shaded from the light absorption 

 most rapidly takes place ; here, probably, however, the channels of movement are the 

 interspaces between the cells, and the cause, simple capillary attraction. In mush- 

 rooms there is a more close approximation to the mechanism more fully developed in 

 the higher plants, for in them the rootlets absorb nutrient matter from the soil, from 

 which it passes, by capillary action, to every part of the system. 



