^THE VEGETABLE CELL. 73 



cyanide of potassium, and afterwards with a solution of sulphate of 

 iron, had prussian blue precipitated in the vessels and not in the 

 wood-cells. If this result proved constant, the experiment must 

 be acknowledged as a conclusive evidence for the conveyance of 

 the sap by the vessels, but although these experiments were con- 

 firmed by Eoininger (" Bot Zeit" 1843, 177), and also have been 

 made repeatedly by myself with the same results in many other 

 cases, with Hoffman Q'kk cle Organe d. Safibewerjung/' — Bot Zeit 

 ISoO ; Scient Memoirs, Series 2, Vol. J.), they furnished diametri- 

 cally opposite results, without our being able at present to deter- 

 mine with certainty the cause of the difference, which po&sibly 

 may have depended on accidental injmies in the plant where the 

 saline solutions penetiated into the vessels. 



The defenders of the idea that the vessels carry air, as the chief 

 of whom in recent times Schleiden is to be named (" Grundz,'' 

 2nd ed. II. 505), stand simply upon microscopic investigations, 

 since in these air is always found in the vessels. This statement, 

 special exceptions excluded, is undoubtedly correct. 



In the first place, in regard to these exceptions, our woody 

 plants furnish them during the time preceding the unfolding of 

 the leaves in Spring. During the winter a portion of the cells of 

 the wood are filled with sap, the vascular system with air. Dur- 

 ing the rising temperature of Spring the cells become gradually 

 fuller and fuller of sap, and this subsequently enters the vessels 

 also ; noA^ the sap flows freely from wounds in the wood, which is 

 not the case so long as this is contained in the cells alone ; after- 

 wards, when the unfolding of the leaves increases very much the 

 evaporation of the plant, the wood is again partially emptied of 

 its sap, and air re-enters more particulaiTy into the vessels. This 

 condition of a special fulness of sap, in which the vessels also con- 

 vey it, seems to be a constant condition in certain tropical climb- 

 ing plants, especially in Phytocrene and certain species of Cissus 

 (see Gaudichaud, " Obscrv. sur V Ascension de la s^ve dans uns 

 Li/me f — Ann. des.sc. nat 2nd ser. VI. 138 — Poiteau, "Sur la 

 Liane des Voyagcurs;" — Ann. des. sc. nat VII. 233), The sap is 

 exposed to a more or less considerable pressure in the vessels, so 

 that it mostly flows with force out of a wound ; the force with 

 which this takes place was first determined by Hales, in his cele- 

 brated experiments on the Vine, which afterwards were fully con- 

 firmed by other experiments, more particularly by those of Briicke 

 Q' Fogg Ann." 1844, No. 10). Hales found that the presence of 

 the sap flowing out under favourable circumstances balanced a 

 column of mercury twenty-six inches high. In the observations 

 made by Gaudichaud on Cissus hydrophora^ and by Poiteau on an 

 unknown Oissus, the sap did not flow free from either the upper or 

 lower piece of the cut stem, but only out of pieces of stem which 

 were separated completely firom the parent plants, so as to present 

 two open ends ; here evidently the vessels were not over-filledi 



