150 PHYSIOLOGY OF NUTRITION 



time before a break occurs tension may be removed by applying water to the 

 exterior of the cylinder, when the mercury column quickly falls. The column 

 may thus be made to rise and fall repeatedly. 



To avoid the use of an inconveniently long tube, while still retaining the 

 possibility of tensions of an atmosphere or more, the external pressure on the 

 mercury (H) in the reservoir may be reduced just before a test, by applying 

 suction (at e) to an extent indicated by a suitable gauge or manometer. In this 

 case the gauge reading is of course substituted for the current barometer reading 

 in calculating the pressure at any given level in the tube. — Ed.] 



These experiments indicate the great magnitude of the force of cohesion 

 existing between the molecules of water; the water column is not broken 

 even when it is subjected to a considerable stress. They also give some 

 idea of the magnitude of the imbibition force resident in cell walls of plants and 

 also in plaster of Paris, porous porcelain, etc.; this force is so great that when 

 water is removed from the cell wall by evaporation more water is immediately 

 withdrawn from the interior of the cell in spite of the osmotic force that opposes 

 such movement. Transpiration from the leaves, the force of imbibition in the 

 cell walls, and the cohesion of liquid water, are therefore the main causes 

 underlying the movement of water in plant stems. The so-called root pres- 

 sure, which causes bleeding in plants, may also be involved here to some extent." 



The amount of water passing through the plant is important in the distribu- 

 tion of mineral substances throughout the organism, as well as in their absorp- 

 tion. Schlosing's studies with tobacco plants may serve as an illustration 1 

 of this. A portion of a plant was allowed to grow in a water-saturated atmos- 

 phere, under a bell-jar, while the remainder was exposed to natural condi- 

 tions. The ash content in the leaves grown in the moist atmosphere was 

 lower than that of the other leaves, the former being only 13 per cent., while the 

 latter was 21.8 per cent., of the total dry weight."' 



1 Schloesing, Th., Vegetation compared du tabac sous glocke et a l'air libre. Compt. rend. Paris 69: 

 353-356. 1869. 



* The discussion here given of the physics of the rise of the transpiration stream is fragmen- 

 tary and incomplete, but it has not seemed advisable to attempt to render it much more thor- 

 ough in the limited space to which editorial notes should be restricted in a translation such 

 as the present volume. The notes that have been added to this section aim to place before the 

 student the main points omitted by the author, and to give references to the literature, so that 

 the best treatments of the modern phase of this much-discussed problem may be read. The 

 writings of Dixon, Renner, and J. B. Overton, cited in note r, p. 144, should be referred to, at 

 any rate. The existing text-books are all unsatisfactory in regard to this subject, the Dixon 

 theory not yet having been adequately incorporated into any of them. — Ed. 



w But see: Hasselbring, Heinrich, The relation between the transpiration stream and the 

 absorption of salts. Bot. gaz. 57: 72-73. 1914. Hasselbring's conclusion is the direct 

 opposite of the one reached by Schlosing. The question as to what rates of transpiration 

 are necessary to elevate the requisite amount of salts in tall plants deserves further atten- 

 tion at the hands of experimenters. It appears clear enough, on a priori grounds, that some 

 transpiration must generally give better growth than none at all, but the rates generally 

 experienced by ordinary plants are probably much higher than the optimum. See also: 

 Muenschner, W. C, The effect of transpiration on the absorption of salts by plants. Amer. 



