TUEGIDITY ■ 77 



this liquid does not cease altogether at 0° C, as was thought 

 by Schwendener.' At 0° C. protoplasm is not only perme- 

 able to water, but also to KNO3, glycerin, urea, methylene 

 blue, caffein, and ammonium carbonate. Thus Krabbe's 

 idea that below 5° C. nothing but water penetrates is entirely 

 unfounded. Temperature variations in permeability to 

 solutes were also observed in many cells by the same author. 

 These, too, seem to follow the rule for water as stated on the 

 preceding page. 



However, Copeland^ showed that decrease in temperature 

 caused a rise in tde turgor pressure of moss leaves. This 

 may not be a direct efPect of the temperature upon the pro- 

 toplast, for the same author found that various agencies 

 which checked growth also caused a rise in turgor; there is 

 surely a close relation between growth and turgor, what- 

 ever this relation may ultimately turn out to be. In this 

 connection it may be noted that De Vries' found that, as 

 growth proceeds, turgor rises, to fall again after the curve 

 of growth begins to decline. 



The extrusion of liquid from the cells of the pulvini of 

 "sensitive" organs, such as the leaves of Mimosa and the 

 stamens of Berberis, may be due to a change in permeability 

 also. There seems to be a question, however, as to whether 

 the exuded liquid is water or a solution. Pfeffer* considers 

 this subject at some length, and concludes that solutes prob- 

 ably do not pass out. He believes that the salts found by 

 Janse^ in the extruded liquid from pulvini in Mimosa are 



1 S. ScHWENDENBR, " Zur Kritik der neuesten Untersuchungen 11. d. Saf tsteigen," 

 SitzuTigsber, d, k.preuss. Akad. d. Wiss. zu Berlin, Vol. VI (1892), p. 911. 



2 E. B. CoPELANU, Ueber den Emflvss von Licht u. Tempiratur auf den Turgor, 

 Halle a. S., 1896. 



8 H. De Veibs, " Ueber die Ausdehnnng wachsender Pflanzenzellen duroh ihren 

 Turgor," Bat. Zeitg., Vol. XXXV (1877), p. 1. 



*W. Pfefpee, "Zur Keuntuiss der Plasmahaut u. d. Vaouolen, etc.," Abhandl, 

 d. k. sacks. Ges. d. Wiss., math.-physik. Klasse, Vol. CLXI (1890), pp. 187-344. 



5 J. M. Janse, " Die Permeabilitat des Protoplasma," Verslag. en Mededeel, d. 

 k. Akad. v. Wetensch. te Amsterdam, 1888. 



