. 152 



\ 

 forces, but their very multiplicity exposes tbeir unsatisfactory cbarac- 

 ter. Malpighi was of tbe opinion that the contraction and expansion of 

 air in the ducts under the influence of heat and cold pumped up the 

 sap, but this could not be without valves to obstruct its reflex action 

 which do not exist, since they cannot be found, and since willow or 

 rose cuttings will do as well with one end up as with the other. More- 

 over at the period of greatest pressure, there is often no air in the tree, 

 but every cell and duct is gorged with sap, as has been fully shown in 

 the experiments at the college. 



Knight, without any good reason, assumed the pith-rays, extending 

 from the center to the circumference of the stem, to possess irritability, 

 and by their contraction and expansion to compress and dilate alter- 

 nately the fibro-vascular tissue and so cause it to act somewhat like a 

 force-pu mp. 



Du-Petit-Thouars,rejectingall mere physical forces, advanced thehypo- 

 thesis that the original force is a vital one, but that in the spring, after 

 a period of repose, the buds under the influence of the sunshine, begin 

 to expand, and by the absorption of sap, which they exhale, create a 

 vacuum or suction which puts the fluids in motion throughout the entire 

 plant. Exhalation and chemical changes, then occurring, keep up the 

 flow till the fall of the leaves in autumn. This, however, entirely fails 

 to account for the familiar fact that the sap is often pressing into trees, 

 like the birch, with tremendous force, several weeks before there is 

 the slightest activity in the buds. 



Dutrochet discovered the principle called osmose, which causes unlike 

 fluids separated by a thin septum to flow together with different degrees 

 of rapidity. Thus, if a solution of sugar be separated, by a thin 

 membrane from pure water, the water will pass through into the sugar 

 freely, while a minute portion of the sugar will enter the water, the 

 result being a large increase in the volume of the sugar solution. This 

 force, under favorable circumstances, will overcome the force of gravita- 

 tion so as to cause the rise of water in a tube to a considerable height. 



The general principle of osmose has been almost universally adopted, 

 without any considerable attempt at demonstration by j^hysiological ex- 

 periments, as the chief cause of all the motions which occur in the con- 

 tents of vegetable cells, such as the absorption of water by the rootlets, 

 the ascent of the crude sap to the leaves, and the general transferrence 

 of all nutrient matters to the parts where they are deposited and assimi- 

 lated. There are many difficulties in the way of accepting this charm- 

 ingly simple hypothesis. Among these may be named the fact that 

 there are found in tbe different adjoining cells of plants entirely distinct 

 substances which do not mingle, as in tbe brilliant petals of flowers, 

 where superimposed layers of cellular tissue contain fluids of unlike 

 colors. . The cambium, also, which evidently does not penetrate the sap- 

 wood, readily finds its way through hundreds of feet of its proper con- 

 ducting medium. Again, the organic contents of plant cells are almost 

 exclusively colloids, and the prooif of tbeir easy and rapid transmission 

 through imperforate membranes is yet to be discovered ; neither is there 

 sufficient evidence of any such exudation of organic matter from tbe 

 rootlets, where osmose is imagined to occur, as is required by all thatis 

 known of this principle in its operation upon lifeless matter. 



Ordinary absorption and capillary attraction have been thought to 

 assist in producing tbe phenomena of the motions of sap, though no one 

 regards them as sufficient of themselves, since they not only lack the requi- 

 site power, but also that peculiar ability, manifested by tbe living plant, 



