THE CONDUCTION OF WATER. II 63 



140-152 m., and Sequoia gigantea, with a height of 79-142 m., stand out as 

 giants in the plant world ; Abies pectinata (75 m.), Picea excelsa (60 m.), Fagus 

 sylvatica (44 m.), Platanus and Fraxinus (30 m.), may also be cited as examples 

 of trees of great height. 



These observations, unfortunately, lead us to a very perplexing result. 

 The ascent of water cannot be treated merely as a physical problem, for some 

 of the most critical data required for the solution of the question are wanting ; 

 nor is it possible for the moment to speak of a theory of the movement of water 

 in the plant, since not one of the numerous investigations to which the problem 

 has given rise has up till now provided us with an estimate, proving that the 

 amount of water raised by the process assumed is actually commensurate with 

 that raised in the plant. So long as such quantitative evidence is wanting we 

 may, in our opinion, speak of hypotheses only, not of theories. 



With these preliminary remarks we may now attempt to study more 

 closely the nature of the forces which operate in the ascent of sap. We are 

 unable to do more than present a critical analysis, which itself makes no pre- 

 tension at completeness. An historical enumeration of the voluminous researches 

 which have been made on this problem since the time of HALES would be indeed 

 of interest, but for this we cannot afford space (compare COPELAND, 1902). 



In the first place we might imagine that the water was forced up by root- 

 pressure, and it is apparent that bleeding-pressure must operate to some extent 

 in this way. But the question at once arises as to whether this force is suffi- 

 cient to force water to the tree top, and whether the amount of water supplied 

 by the root is approximately enough to replace what is lost in transpiration. 

 Some experiments carried out by SACHS (1873) are worthy of consideration on 

 this latter point. He compared the amount of sap given off in a definite time 

 from the root of a herbaceous plant with the amount sucked up by a shoot 

 whose cut end had been submerged. A root-stock of Nicotiana latissima gave 

 off about 16 ccm. of sap in five days, but its shoot absorbed 200 ccm. A simi- 

 lar disparity was exhibited in other cases also. Further, it is very improbable 

 that the secretory capacity of the root is sufficient of itself to compensate for the 

 loss of water due to transpiration. It must be remembered, however, that varia- 

 tions may be set up in the root owing to the amputation of the shoot. [Compare 

 also DARBISHIRE, 1905 (Bot. Gaz. 39, 356).] So far as the pressure which causes 

 bleeding is concerned (apart altogether from local pressures, which must ob- 

 viously be entirely disregarded in this connexion), we may accept the statement 

 of WIELER (1893) as indicating the extreme pressure which has been demon- 

 strated with certainty. WIELER found that in the case of the birch a pressure 

 of 139 cm. of mercury, or about two atmospheres, was developed. If we 

 disregard the frictional resistance presented by the vessels, such a pressure 

 might force water up to a height of 20 m., but it could not raise water to 

 the top of a growing tree 25 m. high. Let us take another example. The 

 silver fir may reach a height of 75 m., and if water is to be elevated to the top 

 of such a tree by means of bleeding-pressure only, that pressure must amount 

 to at least seven and a half atmospheres. Bleeding-pressure, however, in all 

 Coniferae is extremely feeble, as very many estimates clearly prove. HOF- 

 MEISTER (1862), indeed, states that, as a general rule, Coniferae do not ex- 

 hibit the phenomenon of bleeding ; and though WIELER (1893), at a later date, 

 was able to show that bleeding could be demonstrated in these plants, still it 

 is impossible to draw any other conclusion than that it was quite insignifi- 

 cant in amount. Quite apart from Coniferae there are many other plants 

 whose maximum root-pressure is quite inadequate to bring about the filling of 

 the vessels of the leaves and branch apices. Thus, according to WIELER s 

 statements (1893, p. 122), the root-pressure in Morus reaches only to 12 mm., 

 in Fraxinus to 21 mm., and in Acer pseudoplatanus from 169-313 mm. of 



