MOVEMENT OE MATERIALS IN THE PLANT 



147 



cell walls of plants and also in plaster of Paris; 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 withdrawn from the interior of the cell in spite of the osmotic force that opposes 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. 1 ' 



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. Schlösing's studies with tobacco 

 plants may serve as an illustration 1 



Pig. 83. — Arrangement to show rise of a 

 mercury column caused by evaporation of water 

 from the leaves of a cut twig. 



Pig. 84. — Evaporation of water through 

 a membrane, causing rise of mercury in 

 tube below. 



1 Schloesing, Th., Vegetation comparee du tabac sous glocke et ä lair 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 

 thorough 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. 



