76 METABOLISM 



shown without doubt that fuchsin had been carried to the top of the tree 

 without the co-operation of hving cells, because parts of the stem investigated as 

 high as 21-8 m. from the ground were tinged with the pigment. 



The experiment just described is undoubtedly among the most interesting 

 of those which have been carried out on the subject of water conduction, but 

 it cannot be regarded as absolutely convincing, for it raises many doubtful 

 issues. First of all there is the question why the absorption of water decreases 

 so rapidly. Probably because evaporation from the leaves ceases, as a con- 

 sequence of withering ; but we do not know whether the withering of the 

 leaves is due to death by the poison or is a result of reduced water supply. 

 It is quite possible to assume that water still rises in the dead stem, but no longer 

 with the requisite rapidity, or in amount equal to what the transpiring leaves 

 demand. A completely convincing experiment must show that the leaves which 

 are supplied with water by a dead stem are able to remain for a long time alive. 

 [Ursprung (Beihf. z. bot. Centrbl. 1904, 18, 147) has advanced certain im- 

 portant criticisms tending to disprove Strasburger's experiments, but his own 

 experiments do not convince us that living cells conduce to the ascent of sap. 

 Compare Dixon, 1905 (Proc. R. Dublin S., 11, No. 2) ; Ursprung, 1905 

 (Bot. Ztg. 63, II Abt. 241), and Josx, 1905 (Bot. Ztg. 63, II Abt. 243).] 



When we spoke above of an indirect action of the living cells on the con- 

 duction of water, we meant an action not dependent on a sucking and pumping 

 upwards of air, but referred, rather, to the influence of living cells on the air 

 in the vessel. Noll (1897), in a preliminary treatise, has noted the fact 

 that gases which are injected into the vascular system undergo changes, and 

 Devaux (1902) has observed a negative pressure in the air of the vessel on 

 the stoppage of transpiration. This he attributes to the withdrawal of oxygen 

 from the vessels owing to the respiration taking place in the living cells. Fur- 

 ther investigations have now to show whether negative pressures in larger areas 

 are really produced in the wood by this means. If that be the case, then it 

 would be impossible to doubt the suctional action of the expanded air-bubbles, 

 and the living cells would then play an important part in the ascent of water. 



The summary we have given above shows that our knowledge up to the 

 present of the causes of the ascent of water is in many respects very imperfect. 

 A complete exposition of the voluminous literature (Copeland, 1902) is out 

 of the question here ; it would lead us to no definite results, for we are ignorant 

 even yet of the answers to the most elementary questions. It may be difficult, 

 perhaps, to clear up entirely the darkness which surrounds this question, 

 although we may still look for future researches to throw light on the problem, 

 if these, as we have often already emphasized, pay more attention than hitherto 

 to the quantity of the water to be raised as contrasted with the quantity 

 actually raised. 



Bibliography to Lecture VI. 



AsKENASY. 1895. Verhandl. d. naturhist.-med. Vereins Heidelberg, N. F. 5. 



ASKENASY. 1896. Ibid. 



BoHM. 1879. Bot. Ztg. 37, 225. 



Claussen. 1 90 1. Flora, 88, 422. 



Copeland. 1902. Bot. Gaz. 35, 161. 



CzAPEK. 1899. Zeitschr. f. phys. Chemie, 17, 143. 



Darwin, Vines, Joly. i 896. Report of a discussion on the ascent of water. Annals 



of Bot. 10, 630. 

 Devaux. 1902. Compt. rend. 134, 1366. 

 Dixon and Joly. 1895 a. Proc. Roy. Soc. 57, 3. 

 Dixon and Joly. 1895 b. Phil. Trans. B. 186, 563. 

 Dixon. 1896. Proc. R. Irish Acad. 4, 61. 

 Elfving. 1882. Bot. Ztg. 40, 713. 



