i9*4l ECKERSON— THERMOTROPISM OF ROOTS 257 



temperatures from 8° to 15 C. gave positive curvatures. At 



after 9 hours. 



from 17 to 29 C. there were no curvatures even 



Table I gives the thermotropic curvatures of five species, 

 selected to show the various "types" of reaction. Where the data 



have been obtained by other workers, the name is given in paren- 

 theses. 



In attempting to locate the mechanism of thermotropic curva- 

 tures, one must bear in mind the various possibilities, or "types" 

 of reaction, shown in the table. The rapidity of the reaction indi- 

 cates that it is not a growth phenomenon. It is known that the 

 permeability of protoplasm increases with increase of temperature. 

 The range of permeability change was not known, nor was it known 

 whether there is such great variation with species as would be 

 necessary to explain the curvatures given in table I. However, 

 permeability changes and consequent turgor changes were con- 

 sidered a possible factor; accordingly the permeability of the roots 



at temperatures from 10 to 40 C. was determined for the five 

 species. 



II. Permeability 



EFFECT OF TEMPERATURE ON PERMEABILITY 



In 



I 



permeability 



asm of epidermal 



ssium nitrate, and urea) increases 



temperature from o° to 30°C. 

 ; the volume of liquid extruded 

 multiflorus increases from o° t 



found 



to 35 C. During the extrusion the osmotic pressure of the cell 

 sap decreases, indicating an increased permeability to solutes as 

 well as to water. In 1908 Lepeschkin (4, 5, 6) established the fact 

 that pulvinal movements of leaves are due to the effect of light on 

 the permeability of the protoplasm of the pulvinal cells. Light 

 increases the permeability, with consequent decreased turgor pres- 

 sure (decrease of volume). Darkening decreases the permeability, 

 with consequent increase of turgor pressure (increase of volume). 

 In 1 9 10 Trondle (o) found that the leaves of Buxus sempervirens 



permeable to sodium 



6 



