178 BOARD OF AGRICULTURE. 



chemical or physical forces, but their very multiplicity ex- 

 poses their unsatisfactory character. 



Grew, in his "Anatomy of Plants," gives an illustration to 

 explain the ascent of sap, which reminds one of the attempt 

 of a man to lift himself over a fence by pulling on his boot- 

 straps. He represents a number of cells surrounding a tube 

 or duct, and states that water, being absorbed by the cells, 

 passes into the duct to a given height. The cell membranes 

 then swell so as to compress the duct, which forces the water 

 a little higher. It now passes out into the empty cells above 

 those first named, their walls are swollen by the absorption 

 of the fluid, the duct is again compressed, and so on to the 

 top of the tree. 



Malpighi was of the opinion that the contraction and ex- 

 pansion 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 grow 

 as w^ell with one end up as with the other. Moreover, 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. 



De Saussure gratuitously supposed the sap-vessels to be 

 endowed with a capacity for contraction and dilation under 

 the influence of appropriate stimulants, and thus to force up 

 the fluid, which had been absorbed by the ordinary imbibition 

 of the spongy rootlets. 



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

 extending from the centre to the circumference of the stem, 

 to possess irritability, and by their contraction and ex- 

 pansion to compress and dilate alternately the fibro-vascular 

 tissue and so cause it to act somewhat like a force-pump. 



Du-Petit-Thouars, rejecting all mere physical forces, ad- 

 vanced the hypothesis that the original force is a vital one, 

 but that in the spring, after a period of repose, the buds, un- 

 der 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, 



