TUEGIDITY 63 



ness. Plasmolysis occurs, but is of short duration ; Overton' 

 found that it took from two to five hours for equilibrium to 

 be re-established in solutions of these substances. 



There are exceptions here also, however, for De Vries 

 found that the cells of the bud scales of Begonia manicata 

 were almost impermeable to glycerin and urea. Jennings' 

 states that paramoecia are permanently plasmolyzed in 

 glycerin. 



By an extensive investigation of the plasmolytic behavior 

 of various organic compounds Overton' found that a great 

 number of these do not produce plasmolysis at all, so rapidly 

 do they penetrate the protoplasm. Among these substances 

 may be named : ethyl alcohol, ethyl ether, formaldehyde, 

 chloral hydrate, acetone, methyl cyanid, furfurol, cafEein, 

 etc. The list includes practically all of the aliphatic alco- 

 hols and related compounds which are soluble in water, and 

 also a number of soluble aromatic compounds, such as 

 anilin, acetanilid, phenol, phloroglucin, etc. Although the 

 author does not express himself on this point, it seems 

 probable that the apparent plasinolysis which occurs when 

 plant cells are placed in strong alcohol is due, not to osmotic 

 pressure, but to an increased permeability in the osmotic 

 membranes (due to the poison) and also to an active con- 

 traction on the part of the protoplasm. The same author 

 shows that there are all gradations in rapidity of penetra- 

 tion, from those substances which fail to plasmolyze at all 

 to those which produce permanent plasmolysis. In all these 

 cases he found that the protoplasmic sac is as readily per- 

 meated outward as inward. Practically all liquids which 



1 E. OvEETON, " Ueber die osmotischen Eigenschaffcen der lebenden Pflanzen- 

 und Tierzelle," Vierteljahrschr. d. Naturf.-Ges. in Zurich^ Vol. XL (1895), pp. 159-84. 



2 H. S. Jennings, " Studies on the Beactions to Stimuli in Unicellular Organ- 

 isms": I, "Reactions to Chemical, Osmotic, and Mechanical Stimuli in the Ciliate 

 Infusoria," Jour. Physiol., Vol. XXI (1897), pp. 258-322. 



3 E. OvEBTON, loc. cit. The table of compounds occurs on p. 181. 



