Dixon — Variations in the Permecibilitij of Leaf- Celts. 



355 



The extreme ends of the elements, stripped of their silk covering, were twisted 

 together and soldered. The soldered junction so formed was reduced to about 

 0-25 mm. in length by an oblique cut of a sharp scissors. By this means the 

 actual junction was very close to the point formed, so that when the double 

 wire was pushed through even a thin leaf, the junction was still embedded in 

 the tissue, as the point was just emerging. The other ends of the constantan 

 wires were soldered to thicker copper wires, and these latter junctions, together 

 with the support carrying the copper terminals, were immei'sed in a small- 

 vessel of petroleum. By this means, and by means of a connexion already 

 described, thermo-electric errors were avoided (4, 5, 8). 



The thermo-couple thus arranged gave a deflection of 26 mm. per 1*^ 

 difference of temperature of the junctions. 



100 



Fig 7. 



In cool weather with intermittent sunshine early in May an insolated patch 

 on a leaf of Hedera helix was found to be 4-7°-6-2 above an adjacent patch 

 which was shaded by an overhanging leaf. Under similar circumstances a 

 temperature difference of 6-3°-6-6° was found in leaves of Syringa vulgaris. 

 In each case the periods of insolation were less than 5 mins., and the leaves 

 were not normal to the direction of illumination. The leaves were exposed 

 to a brisk breeze and were well supplied with water. Rapid temperature 

 changes were observed, fluctuations of l°-6° within a minute being not 

 uncommon. Records of four sixty-second observations are shown in fig. 7. 

 Bright diffuse light caused a rise of 1° or 2° above a shaded portion. In 

 more favourable weather, differences as large as those foiuid by Blackman and 

 Matthaei would probaljly have been obtained. 



From these experiments it is evident that during sunshine there are 

 temperature-differences between shaded and insolated leaf -areas of 10° and more. 



The results connecting resistance and temperature show that these ternpera- 

 ture-differences will lead to large fluctuations in permeability, the cells in the 

 heated areas becoming more peimealjle. It has already been pointed out that 

 such differences in penueability acting in concert with the hydrostatic tension 

 throughout the plant furnish a mechanism for the distribution of dissolved 

 substances through the plant body. 



