350 Scientific Proceedings, Royal Dublin Society. 



The upper surface of the square is turned downwards as it lies on the electrodes. 

 In the other arm of the bridge a high adjustable resistance (10,000-400,000 

 ohms) is introduced. In practice two pairs of electrodes were fixed in the 

 frame so that two leaf-squares might be experimented upon simultaneously. 



It was soon found that temperature had a large effect upon the resistance, 

 and it was consequently necessary to have the temperature of the tissue under 

 control. To effect this the frame was fitted into a large test-tube {F, fig. 1) 

 and the whole immersed in a glass vessel containing about two litre^j of water. 

 The water in this vessel could be cooled by the addition of ice, or heated 

 by the passage of an electric current throiigh a submerged wire of German 

 silver supported on a bent glass I'od {R). Adjustment of an external resistance 

 made it possible to maintain the water round the test-tulje at any desired 

 temperature between 0" and 50° C. Before piitting the frame into the test- 

 tube a few drops of water were introduced to keep the space surrounding 

 the leaf-square moist, and thus prevent the drying up of the leaf-square 

 during the observations. 



The resistance of the leaf-squares examined was found to be considerable, 

 amounting to from 200,000 ohms to 600,000 ohms at 0° C. 



The relation of resistance to temperature in the leaves of Hedera helix 

 may be seen in figs. 4, 5, 6, and in the leaves of Syringa vulgaris in figs. 2 

 and 3. Here the ordinates are resistances measured in ten thousands of ohms 

 (thus 12 on the vertical scale indicates 120,000 ohms), and the abscissae are 

 temperatures in degrees centigrade. Fig. 4 contains the records of the 

 behaviour of three leaf-squares of Hedera helix. The curve A shows the 

 change of electrical resistance of a square cut on Febraary 5th from an 

 old last season's leaf of ivy, fixed on the electrodes as described and raised 

 slowly through the range of 0°-50°. Each reading of resistaiiee was carried 

 out after the water round the test-tube was at the temperature recorded for 

 15-20 mins. 



Next day the same leaf-square was heated through the same range, and 

 its resistances are recorded in the cui've A'. 



B and B' are similar curves traced from the behaviour of a similar square 

 cut from another old leaf. The curve Bd gives the resistances of the same 

 square through the same range after it had been killed by exposure to chloroform 

 vapour for somewhat over an hour. 



C, C, and Cd are similar records for a square cut from a young leaf 

 also of the previous season. 



The idiosyncrasies of the three squares are noticeable. There are consider- 

 able absolute differences in resistance of the three, and differences in the form 

 of their curves. 



They all show a marked fall of resistance v\fith rise in temperature. In A, 

 however, the rise between 30°-40° is accompanied by a rise in resistance. 

 So far as my experience goes this is a rare occurrence. 



The reduction after heating is another regular feature. Thus A', B', and 

 C are each lower than A, B, and C respectively. Heating appears to initiate 

 a progressive change which continues to reduce the resistance. Thus in the 

 case of the series of experiments recorded in G and C" it was found that 

 the resistance of the leaf-square on being cooled to 0°, immediately after 

 being raised to 50°, rose to 43-3 X 10* ohms from 11-5 X 10* ohms. Next 

 day its resistance at 0° was only 260 X 10* ohms. It had been kept during 

 the night at a temperature of about 10°-11°. 



This progressive and lasting reduction of resistance caused by exposure 



