6o 



PHYSIOLOGY OF STREAMING MOVEMENTS 



this form of activity exists, although slow changes produce no such result. 

 The same author ] states that the minimum temperature for streaming is 

 oC., the optimum 37 C. to 38 C, and the maximum 41 C. to 42 C., 

 although the leaf of Elodea may show active streaming at 43 C. and slow 

 streaming at 52 C. 



Attempts to determine the cardinal points of single plants to a degree 

 or a fraction of a degree are largely futile, since not only do the cardinal 

 points vary according to the plant or cell examined, but they also depend 

 upon (i) the age and condition during experimentation, (2) the external 

 medium, (3) the duration of the exposure, (4) the supply of oxygen, (5) 

 the rapidity with which the temperature alters. 



FlG. 9. Combined hot stage and gas chamber. 



Streaming may permanently cease in cells suddenly raised to a tem- 

 perature at which it continues for a longer or shorter time, if the rise is 

 more gradual. Hence a cell may be killed by immersal in hot water, 

 though not if plunged into air at the same temperature. Similarly, a cell 

 which is kept moist can withstand a slightly higher temperature in dry air 

 than in air saturated with water-vapour, owing to the cooling effect of 

 evaporation. Again, at high temperatures the cell consumes more oxygen, 

 and, owing to the decrease in the solubility of this gas as the temperature 

 rises, it is absorbed in sufficient quantity with greater difficulty from hot 

 water than it is from cold water or hot air. The same applies even to 

 green cells exposed to light, for at high temperatures the power of photo- 

 synthesis is suppressed. Hence in all test experiments the preparations 

 were placed in thin films of water enclosed in a hot chamber 2 filled with 

 air kept saturated with moisture. A slight interval for adjustment was 

 allowed between each change of temperature, and in order to avoid any 



1 Hauptfleisch, Jahrb. f. wiss. Bot, 1892, Ed. xxiv. 



2 The hot stage devised by the Cambridge Scientific Instrument Company was at first used, but 

 the above modification of it (Fig. 9) yields more accurate results, since the loss of heat by radiation 

 through the cover-glass is almost entirely avoided. 



