74i AKATOMY AND PHYSIOLOaY OF 



witli sap, and this was retained in tlie cut plant by the pressure 

 of the atmosphere. 



If we take into consideration that the vessels, save in the said 

 exceptions, convey air, that in the Vine and other woody plants, 

 before the bleeding begins, tho cells are filled with sap, which is 

 only afterwards taken up by the vessels, that after the unfolding of 

 the leaves and the great evaporation resulting from this, the ves- 

 sels are again emptied of sap, we cannot doubt that the cellular 

 tissue of the plant is the primary and principal system to which 

 the conveyance of the sap is committed, and that the vessels take 

 part in the function only under special circumstances, when the 

 plant is temporarily overfilled with sap, or in some very succulent 

 plants perhaps throughout the whole period of vegetation. 



All parts of the plant do not play an equally active part in 

 the conveyance of the sap, for many experiments go to shew that 

 the organs situated at the two ends of the plant are especially 

 active at least in the ascent of the sap, the root fibres on the 

 one hand driving sap upwards, and on the other hand the leaves 

 attracting it. 



That the ascent of the sap in Spring, before the unfolding of 

 the leaves, is chiefly caused by the roots driving the sap up- 

 wards, might be partly deduced from the fact, that the force with 

 which the sap flows from a wound in the stem of the Vine, is de- 

 pendant on the temperature in which its roots are placed (Dassen, 

 ^^ Froriep's Neuen Notizen^" B 39, p. 129), partly also from the 

 fact that the sap does not flow merely from the cut stem of a 

 bleeding Vine, but the same phenomenon is displayed in the roots 

 down to their most slender ramifications. But that in many leafy 

 plants, in which the attraction of the sap by the leaves is active 

 as a second cause of the motion of the sap, the impulse exercised 

 by the roots upon the mass of the sap is also frequently necessary, 

 for the conveyance of a sufiicient quantity of sap to the leaves, 

 follows from the experiments of Dassen, according to which, in 

 N'yonphcea alba and other plants, the leaves dry up, when they, 

 or the stems to which they belong, are placed with their cut sur- 

 faces in water, but they remain fresh, under similar surround- 

 ing conditions, when the fibrils of the roots are uninjured. Yet 

 that the leaves, even when only a comparatively small number 

 of them are left at the top of a plant, are in a condition to lift 

 fluids to a very considerable height in the stem, independently of 

 the influence of the root, follows from the experiments made by 

 Boucherie (^'Gompt rendus,'' 1840, ii. 894) upon trees, in which a 

 solution of pyrolignite of iron was applied to the lower ends of the 

 sawn-off stems. 



Observations on bleeding woody plants, especially on the Vine, 

 prove that the activity of the roots is capable of causing the sap 

 not only to ascend in the cells of the stem, but also to enter into 

 the vessels. In like manner the activity of the leaves causes 



