Dixon — Variations in the Permeahilitij of Leaf- Cells. 



351 



to high temperatures explains the fact that, generally, the resistances observed 

 in snmmer are less than those of the same plant found in winter and spring. 



The amount of the reduction in resistance is by no means regular, as 

 inspection of fig. 4 will show. Fig. 5 also gives a striking example. The 

 squares A and B were cut from the same mature leaf of IJedera helix, and 

 included symmetrically-situated pieces on opposite sides of the mid-rib. Their 

 resistances throughout the first heating were almost identical. On the second 

 heating a marked difference was noted. It is recorded in the curves A' and B'. 



©R 



W 



w 



R© 



■ CiA-rre for ht Aea.ti^g- Jul-. tSroL 



oon. hzS ulpK^de. 



t'a.poit.r 



Fig. 1. 



JO IS 20 IS 30 35 40 Ai M 



TeTrvperature.. 



Fig. 2. 



The changes induced by the heating do not always appear to be lethal. 

 This is evidenced by the fact that sometimes the recovery of resistance 

 after the heating may be almost complete, e.g. B', fig. 5. Also a similar 

 change is obsei'ved when the heating is carried only as high at 35°. Thus 

 a leaf-square of Hedera helix heated from 0° to 35° on February 28th had 

 a resistance of 41-7 X 10* ohms at 0°, and 108 X 10* ohms at 35°. On the 

 next day its resistance at 0° was 308 X 10* ohms and 11-8 X 10* ohms at 35°. 



The curves show a rapid fall in resistance between 0° and 15°. They are 

 less steep at the higher temperatures, but often show a steeper bend between 



