TRANSACTIONS OF SECTION K. 927 



Pasteur has asserted, every living cell may become an ' organised ferment ' under 

 appropriate conditions. The distinction now to be drawn is between those pro- 

 cesses which are due to enzymes and those directly eSected by living protoplasm. 

 Many now definitely included in the former class were, until lately, regarded as 

 belonging to the latter ; and no doubt future investigation will still further increase 

 the number of the former at the expense of the latter. 



The consideration of the metabolic processes leads naturally to that of the 

 function of transpiration and of tlie means by which water and substances in 

 solution are distributed in the plant. This is perhaps the department of physiology 

 in which progress during the nineteenth century has been least marked. We have 

 got rid, it is true, of the old idea of an ascending crude sap, and of a descending 

 elaborated sap, but there have been no fundamental discoveries. With regard to 

 transpiration itself, we know more of the detail of the process, but that is all that 

 can be said. As for root-pressure, Hofmeister (1858-62) discovered that 'bleed- 

 ing ' — as the phenomena of root-pressure were termed by the earlier writers — is not 

 confined, as had hitherto been thought, to trees and shrubs ; but the current theory 

 of the process, allowing for the discovery of protoplasm and of osmosis, has 

 advanced but little upon that given by Grew in the third book of his ' Anatomy 

 of Plants' (1675). Again, the mechanism of the transpiration-current in lofty 

 trees remains an unsolved problem. To begin with, there is still some doubt as to 

 the exact channel in which the current travels. Knight (1801-8) first proved that 

 the current travels in the alburnum of the trunk, but not, he thought, in the 

 vessels, for he found them to be dry in the summer, when transpiration is most 

 active; a view in which Dutrochet (1837) subsequently concurred. Meyen (1838) 

 then suggested that the water must travel, not in the lumina, but in the substance 

 of the cells of the vessels, and was supported by such eminent physiologists as 

 Hofmeister (1858), linger (1864, 1868), and Sachs (1878) ; but it has since been 

 strongly asserted by Boehm, Elfving, Vesque, Havtig, and Strasburger that the 

 young vessels always contain water, and that the current travels in the lumina and 

 not in the walls of the vessels. 



Now as to the force by which the water of the transpiration-current is raised 

 from the roots to the topmost leaf of a lofty tree. From the point of view that 

 the water travels in the substance of the walls the necessary force need not be great, 

 and would be amply provided by the transpiration of the leaves, inasmuch as the 

 weight of the water raised would be supported by the force of imbibition of the walls. 

 From the point of view that the water travels in the lumina the force required to 

 raise and support such long columns of water must be considerable. Dismissing 

 at once as quite inadequate such purely physical theories as those of capillarity and 

 gas-pressure, there remain two theories as to the nature of this force which 

 resemble each other in being essentially vitalistic, but difier in that the one involves 

 pressure from below, the other suction from above. In the one, suggested by 

 Godlewski and by Westermaier (1884), the cells of the medullary rays and of the 

 wood-parenchyma are supposed to absorb liquid from the vascular tissue at one 

 level and force it back again by a vital act at a higher level : this theory was 

 disposed of by the fact that the transpiration-current can be maintained through a 

 considerable length of a stem killed by heat or by poison. In the other, sug^sted 

 by Dixon and Joly (1895-99 ), and also by Askenasy (1895-96), it is assumed that there 

 are, in the trunk of a transpiring tree, continuous columns of water which are in a 

 state of tensile stress, the tension being set up by the vital transpiratory activity of 

 the leaves. Some idea of the enormous tension thus assumed is given by the fol- 

 lowing simple calculation relating to a tree liiO feet high. Not only has the liquid to 

 be raised to this height, but in its passage upwards a resistance calculated to be equal 

 to about five times the height of the tree has to be overcome. Hence the tran- 

 spiration-force in such a tree must at least equal the weight of a column of water 

 720 feet in height ; that is, a pressure of about twenty-four atmospheres, or 360 lb. to 

 the square inch. But there is no evidence to prove that a tension of anvthino- like 

 twenty atmospheres exists, as a matter of fact, in a transpiring tree ; on|the con- 

 trary, such observations as exist (e.ff., those of Hales and of Boehm) indicate much 

 lower tensions. Under these circumstances we must regretfully confess that yet 



