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SCIENCE. 



[N. S. Vol. XII. No. 300. 



whicli decompose glucosides (emulsin, my- 

 rosin, etc.) ; those which act on proteids 

 (trypsins) and on fats (lipases) ; the oxi- 

 dases, which cause the oxidation of various 

 organic substances ; and the zymase, re- 

 cently extracted from yeast, which causes 

 alcoholic fermentation. 



The old distinction of the microorgan- 

 isms as ' organized ferments ' is no longer 

 tenable; for, on the one hand, certain of 

 the chemical changes which they effect can 

 be traced to extractable enzymes which 

 they produce ; and, on the other, as Pasteur 

 has asserted, every living cell may become 

 an ' organized ferment ' under appropriate 

 conditions. The distinction now to be 

 drawn is between those processes which are 

 due to enzymes and those directly effected 

 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 proc- 

 esses leads naturally to that of the func- 

 tion of transpiration and of the means by 

 which water and substances in solution are 

 distributed in the plant. This is perhaps 

 the department of physiology in which prog- 

 ress 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-pres- 

 sure, Hofmeister (1858-82) discovered that 

 ' bleeding ' — 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 cur- 

 rent 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 re- 

 mains 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 cur- 

 rent 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 con- 

 curred. 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), Unger 

 (1864, 1868), and Sachs (1878) ; but it has 

 since been strongly asserted by Boehm, 

 Elfving, Vesque, Hartig, and Strasburger 

 thab 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 sup- 

 port 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 differ in that the 

 one involves pressure from below, and the 

 other suction from above. In the one, sug- 

 gested by Godlewski and by Westermaier 

 (1884), the cells of the medullary rays and 



