124 .! F. MCCLENDON. 



One defect in the plasmolytic method is the fact that the cel- 

 lulose cell wall, if not very thick, is elastic, and a slightly hyper- 

 tonic solution may cause the cell to decrease in volume without 

 pressing the protoplasm away from the cell wall. This source 

 of error may be eliminated by substituting calculations of the 

 volume of the cells (as necessary for animal cells) for observations 

 on plasmolysis. 



It is well known that movement, and in many cases increase 

 in size of plants is due to changes in turgor of the cells. If we 

 exclude the turgor changes in aerial plants produced by variations 

 in the ratio of the water supply to the transpiration, turgor 

 changes may be due to changes in the osmotic pressure of the 

 external medium, or of the cell sap (due to metabolic changes) 

 or to changes in the permeability of the plasma membrane. 

 Lepeschkin 1 has confirmed Pfeffer in showing that changes in 

 permeability of stipule cells accompany (or immediately precede) 

 changes in turgor. By chemical analysis of the medium he has 

 shown that an outward diffusion of dissolved substances, from 

 the cells, accompanies loss of turgor, and by plasmolytic ex- 

 periments, that the permeability to certain substances increases. 



It is interesting to note the force that may be exerted by such 

 changes in turgor. From measurements of the pull of a stamen 

 hair of Cynara scolumus or Centaurea jacea on loss of turgor fol- 

 lowing stimulation, it seems not improbable that the change in 

 turgor amounts to 2-4 atmospheres (Hober). This also indicates 

 the strength of the cell wall necessary to prevent rupture of the 

 plasma membrane'. The osmotic pressure of the juices pressed 

 out of plants varies from 3.5-9 atmospheres. 2 The pressing 

 out of the juices causes an error due to chemical changes; on 

 the other hand, in taking the freezing point or pieces of plant 

 tissues, an error arises from lowering of the freezing point by the 

 walls of the capillary spaces. Miiller-Thurgau 3 found the A 

 (corrected freezing point lowering) of plant tissues =.8-3.1. 

 Many plants respond to light by definite movements, produced 



1 Her. deutsch. hot. Gesell.. XXVI. (a). 725. 



2 DeVries, Pringsheime Jahrbuchcr wiss. Bot., 1884. XIV., 427; Pantanelli, ;'/</,/.. 

 1904. XL., 303. 



3 Landu'irtschaftl. Jahrb.. 1886, XV., 490. 



