86 CAriLLAUY ATTRACTIONT. 



to an amount which probably diflfer with every species of plant. The 

 remainder of the sap and of tiie substances it holds in solution must be 

 diffused through the cellular spongy terminations of the roots, and, with 

 the new suj)ply of liquid imbibed from the soil, returned again to tlie 

 stern with the ascending current. 



But what causes the sap thus to ascend and descend? By wliaf 

 power is it first sucked up through the roots, and afterwards forced down 

 again from the leaves? Several answers have been given to this ques- 

 tion. 



1°. When the end of a wide tube, either of metal or of glass, is 

 plunged into water, the \u\md will rise within the tube sensibly to the 

 same level as that at which it stands in the vessel. But if a capillary* 

 tube be employed instead of one with a wide bore, the liquid will rise, 

 and will permanently remain at a considerably higher level within than 

 without the tube. The cause of this rise has been ascribed to an attrac- 

 tion which the sides of the tube have for the liquid, and which is suffi- 

 ciently strong to raise it and lo keep it up above the proper level of the 

 water. The force itself is generally distinguished by the name o^ capil- 

 lary attraction. 



Now, the wood of a tree, as we have seen, is composed of a mass of 

 fine tubes, and through these the sap has been said to rise by capillary 

 attraction. But if the top of a vine be cut off" when it is juicy and full 

 of sap, the liquid will exude from the newly formed surface, and if the 

 air be excluded, will flow for a length of time, and may be collected in 

 a considerable quantity [Lindley's T/i'eory of Horticulture, p. 47, note]. 

 Such a flow of the sap is not to be accounted for by mere capillary at- 

 traction — the sides of tubes cannot draw up a fluid beyond their own 

 extremities. 



2°. To supply the defect of this hypothesis, De Saussure supposed 

 that the fluid at first introduced by capillary attraction into the extremi- 

 ties of the root, was afterwards propelled upwards by the alternate con- 

 traction and expansion of the tubes of which the wood of the root and 

 stem is cotnposed. This alternate contraction and expansion he also 

 supposed to be caused by a peculiar irritating property of the sap itself, 

 which caused each successive part of the tube into which it found ad- 

 mission to contract for the purpose of expelling it. Mr. Knight also as- 

 cribed the ascent of the sap to a similar contraction of certain other parts 

 of the stem. Being once raised, he supposed it to return again or de- 

 scend by its own weight — but in droo[)ing branches it is obvious that the 

 sap must be actually driven or drawn upwards from the leaves on iisre- 

 turn to the root. These explanations, therefore, are still unsatisfactory. 



3^. If one end of an open glass tube be covered with a piece of mois- 

 tened bladder or other fine animal membrane, tied tightly over it, and a 

 strong solution of sugar in water be then poured into the open end of the 

 tube, so as to cover the membrane to the depth of several inches, and if 

 the closed end be then introduced to the depth of an inch below the sur- 

 face of a vessel of pure water, the water will after a short time pass 

 through the bladder inwards, and the column of liquid in the tube will 

 increase in height. This ascent will continue, till in favourable circum- 



* Glass tubes perforated by a very fine bore, like a human hair, are called capillart/ tubea. 

 Such are those of which thermometers are usually made. 



