354 Scientific Proceedings, Royal Dublin Society. 



The probable explanation of this rather surprising observation seems to 

 be that the increase of thickness with age is due to the increase in size of the 

 intercellular system, and does not involve an increase in the cross-section of 

 the conducting cells, or an increase in the surfaces of contact of these cells. 

 The resistance due to the protoplasm of the constituent cells evidently depends 

 directly on this total surface. Expansion of the individual cells and of the 

 intercellular spaces would tend to reduce this surface, and hence explain the 

 growth of resistance with age. 



The contrast between the resistances of the living and of the dead leaf 

 is in every case very marked. That of the living leaf has been found to 

 vary between twenty-two times and six times that of the dead leaf. The form 

 of the curve for the dead tissue generally resembles that for the living tissue. 

 This is made plain by iig. 2, in which Ad X 10 is obtained by plotting the 

 temperature-resistance curve for the dead tissue to ten times the vertical scale. 

 The large difference between the dead and the living tissue is in agreement 

 with the view that the electrical resistance of a tissue is controlled by the 

 semi-permeable propex'ty of the cells composing it. 



Figs. 2 and 3 summarize experiments on the leaves of Syringa vulgaris 

 similar to those On Hedera helix already described. The same features, already 

 noticed in the case of Hedera helix, are apparent in the curves for the leaves 

 of Syringa vulgaris. 



Preliminary experiments on the effect of light in altering the penneability 

 have not given coiiclusive results, inasmuch as the effects observed might be 

 attributed to the heating produced by the illumination. 



The salient fact brought out by the foregoing experiments is the reductioji 

 of resistance or the increase of protoplasmic permeability produced by a rise 

 of temperatux'e. Thus we may expect the permeability of leaf-cells to become 

 about doubled by a rise of temperature from 10° to 30°, and at 20° the 

 permeability of the cells will be 50 per cent, greater than at 10°. 



A rise in the temperature of the surroundings when the atmosphere round 

 the leaves is saturated will of course produce a rise in the general temperature 

 of the plant including its leaves. Under these conditions all the cells of the plant, 

 if they behave like the leaf-cells, will become more pemieable, and probably 

 important effects result from this change. 



In 1905 Brown and Escomlje (2 and 3) came to the conclusion on theoretical 

 grounds that the temperature of a leaf when insolated or exposed to diffuse 

 Jight did not, in the specific cases considered, differ from that of its surroundings 

 by more than + 164° or - 184°. 



In the same year, however, F. F. Blaclunan and G. L. C. Matthaei (1) 

 measured by thermo-electric means the temperatures of shaded and insolated 

 leaves. It was found that a difference of 16° might be established, while a 

 leaf exposed to diffuse light is often l°-3° above its shaded surroundings. 

 The leaves used were of Prunus laurocerasus, and the thenno-junction was 

 embedded in the mid-rib of the leaf. 



Recently I have carried out some similar experiments with leaves of Hedera 

 helix and of Syringa vulgaris. The thermo-couple I used was made of 

 eonstantan and copper. The elements of the couple were in the form of 

 fine silk-covered wires, each end of the copper wire being soldered to a piece 

 of eonstantan wire. In order to reduce the thenno-electric effect so as to 

 give a convenient deflection with a sensitive galvanometer seven metres of 

 No. 42 s.w.g. (diam. = 01 mm.) eonstantan wire were used. The copper element 

 was formed of 50 cm. of No. 36 s.w.g. (diam. = 018 mm.) copper wire. 



