MOTION OF THE SAP THROUGH PLANTS. 521 



presence of sap in the vessels of cut and bleeding branches ; but granting 

 they contain sap for a short time, yet, as it appeared to the latest and 

 strictest observer, E. Briike (Poggendorff 's Ann. 1844), the sap passes 

 only passively out of the neighbouring cells into the vessels. But can it 

 be thus in the uninjured plant ? I think not ; for before the sap begins 

 to ascend all the vessels are filled with air. What becomes of the air ? 

 Briike says it escapes or is absorbed. Does it escape through the cica- 

 trices of the leaves ? But the cicatrices do not bleed ; and it seems more 

 than improbable that they should allow of an escape of air, for under a 

 pressure of 2 atmospheres they resist the escape of a fluid of the 

 density of distilled water. That it is absorbed, is not less improbable. 

 The air in vessels is rich in oxygen (Bischoff). In the Vine (not in- 

 cluding the pith, which is filled with air) it is certain that the volume of 

 vessels is equal to the volume of fluid in the cells. Pure water absorbed 

 only 6'5 vol. per cent, of oxygen, and of nitrogen 4*2 vol. per cent. ; fluids 

 which hold in solution sugar, gum, &c., yet smaller quantities (Saussure). 

 It is impossible, therefore, that the air contained in some vessels can 

 be absorbed by the fluids contained in others. By the most careful 

 observation, I have found uncut vines to contain only air in their vessels. 

 I believe that I have now made it out to be at least probable, if not 

 entirely proved, that the effects produced upon plants that have been cut 

 or pierced in the spring are only pathological phenomena, and that con- 

 clusions from them cannot be drawn for uninjured vegetating plants: 

 this, at least, appears to me, at all events after what has gone before, to 

 be very near the truth. It is very probable that there may not exist any 

 other than a rapid stream of spring sap in the uninjured plant, but it is 

 certainly much more inconsiderable than the summer stream. A mid- 

 dling-sized sun-flower receives daily more than a pound of water (Hales). 

 Its leaves have not certainly half the superficies which the leaves of the 

 vine-branch had, on which I made my experiment in the summer. These 

 branches gave out in the spring, at the time of their greatest bleeding, 

 almost 189'48 CC., therefore about 0'508 Ib. p Now^in summer, according 

 to all correct observers, the vessels are filled with air, while at the same 

 time the stream of sap is doubly stronger than when the cut vessels are 

 bleeding in the spring, and consequently every possible condition of a 

 normal motion of the sap in the vessels fails. The phenomenon of the 

 assumed spring sap has misled observers. 



The power with which, according to Hales and others, the fluid is 

 poured from the cut branch exists also in the uninjured plant only as an 

 endosmotic expansion of the extremity of the root, and is very probably 

 greater in the spring than in the summer, because in the summer the sap 

 in the cells is less concentrated, and contains less of the fluids of the soil, 

 and plants at that season vary more in density than in the spring. If the 

 stream of sap is more considerable in the summer than at other times of 

 the year, the cause is that the evaporation is then larger in amount, thus 

 creating space for the stream, and then absorbing it. 



Now, after these preliminary observations, we must keep the following 

 points in view : 



1. That as no vessels with continuous tubes exist in the plant, for 

 absorbing and conveying fluid, so the possibility of the motion of the sap 

 rests on the penetrability of the cellular membranes by fluids. 



2. The moving power which effects the entrance of the sap into the 

 plant, and into each single cell, is endosmosis, assisted by absorption in 

 consequence of evaporation. 



