641 



(September and October). The same behavior was shown in the case of 

 the pine, which gave a still more significant percentage of loss. In decidu- 

 ous trees, as is well known, autumn transplantation is preferred. 



Sunburn of Leaves in Nature. 



The death of the tissue, resulting from the action of the sun, is here 

 meant. In such cases, however, light and warmth act together. We do 

 not know how much must be ascribed to each factor in such phenomena of 

 death. The opinion of noted foresters, that all the light in the plant cell 

 passes over into the dynamic force of heat and becomes effective in this 

 form, is not ver\^ probable. My evaporation experiments with a decrease 

 of light, and a simultaneous increase in temperature, indicate rather that 

 at least a part of the light, as such, becomes effective, and influences the 

 process of assimilation. A part without doubt is converted into heat and 

 acts in that way. Upon this hypothesis, it is also probable that a plant 

 would behave differently with the same amount of heat, according to 

 whether it is subjected to this in a dark, or in a lighted place. 



In general, temperatures between 40 to 50 degrees C. are fatal ; yet 

 Askenasy^ has observed, with Crassulae, that they can endure uninjured 

 such amounts of heat. Askenasy was convinced in midsummer that the 

 inner parts of Sempervivum, at an atmospheric temperature of 31 degrees C. 

 in the shade, had undergone a heating up to 48 to 51 degrees C. The 

 warmth within the plants seemed higher in some varieties, lower in others, 

 than on their outer surfaces. The temperature of the outer surface of the 

 leaf, in different days, did not stand in any direct relation to the atmospheric 

 temperature. Sempervivum arenariuni showed, for example, 



at 31.0 degrees C. on the 15th of July, at 3:00 P. M., 48.7 degrees C. 

 " 28.2 " " " " i6th " " " 3 :oo P. M., 46.0 

 "28.1 " " " " i8th " " " 12 :30 P. M., 49.0 



Thin-leaved plants, standing nearby, had a much lower temperature. 



The phenomena of sunburn are observed most frequently in hot-house 

 plants which, in spring, are set out of doors. The leaf is not always killed 

 but often only reddened or browned. In curled leaves only the convexity, 

 on the upper side, becomes colored and, instead of being green, is reddened 

 to a copper color (roses). In the course of a few weeks such a plant can 

 recover even when left in this place. 



I tested experimentally a similar case in spotted specimens of Canna 

 indica, the greatest number of which in cloudy weather were taken from 

 the hot house, in which they had been forced up to the unfolding of the 

 first blossoms, and were set out of doors. Some pots stayed two days 

 longer in the hot house and were then sunk in the earth in the middle of the 

 day beside the specimens set out earlier. In the afternoon the upper leaves 



1 Askenasy, tjber die Temperatur, welche Pflanzen im Sonnenlichte annehmen. 

 Bot. Zeit. 1875, p. 441. 



