648 



in R. Hartig^ and Nordling-. The latter distinguishes still another "winter 

 sunburn"^ in which the injury to the trunk is found only at its base. The 

 reflection of the sun's rays from the upper surface of the soil is assumed to 

 be the cause. R. Hartig's illustration shows the lower end of the trunk of 

 a red beech sapling with sun cracks*. Since these phenomena, as yet, have 

 only been observed in the late winter and strict experimental proofs are still 

 lacking, we maintain the opinion expressed earlier that the cracks are pro- 

 duced by differences in tension which arise with a sudden sharp change in 

 temperature without the necessity of a warming of the tissue from the sun 

 until it dies, as is the case in sunburned places. Hartig's-^ measurements of 

 a spruce in August show how much the parts of the plants are warmed 

 above the temperature of the air. With an air temperature of 37 degrees 

 C. he found 55 degrees C. in the cambial region of the southwest side ; only 

 45 degrees C. on the south side ; 39 degrees C. on the east side ; 37 degrees 

 C. on the north side. The measurements were made in the afternoon 

 after 4 o'clock. 



Influence of Too Great Soil Heat. 



Sachs" has already furnished abundant material in regard to the deter- 

 mination of the temperature requirements of different plants and especially 

 with respect to the germination of seeds which had been exposed to a high 

 temperature of air and water. In the latter connection it is evident that 

 dry seeds endure a higher temperature without being injured than those 

 already sprouted and that probably all plant tissue (within boundaries 

 required by the species) is in every case the more resistant to heat the less 

 the water content of the cells is proved to be. Corroborative works have 

 been furnished by Haberlandt, Wiesner, Fiedler, Krasan, Just, Nobbe, 

 Iloehnel and recent authors, in regard to which reference must be made 

 to Pfeffer's Physiology. 



Just's'^ experiments show, for example, that unfavorable results may 

 be experienced when, in germinating seed, the temperature is increased 

 above the optimum given for any special variety. He found in these ex- 

 periments, that a prolongation of the germinating time and a slower devel- 

 opment of the seedling is produced by too high temperatures, just as in 

 seeds which are too old. 



Prillieux's^ older work is of importance in regard to the anatomical 

 changes. In bean and pumpkin seeds, sown in pots in which a high soil 

 temperature was maintained by heated wires, the following results were 

 found : the young seedlings grew but little and with difficulty ; however, 



1 Lehrbuch der Baumkrankheiten, 1st ed., p. 188. 



2 Lehrbuch des Forstschutzes, 1884, p. 332. 



3 Baumphysiologische Bedeutung des kalten Winters 1879-80; cit. Illustrierte 

 Gartenzeitung 1881. 



4 Lehrbuch der Pflanzenkrankheiten, 3d ed. 1900, p. 230. 



5 Ibid., p. 228. 



6 Experimental-Physiologie, p. 64 ff. 



7 Cohn's Beitriige zur Biologie der PHanzen. Vol. II, p. 311. 



8 Prillieux, Alterations produites dans les plantes par la culture dans un sol 

 surchauffe. Ann. so. nat. Ser. VI Botanique, t. X, p. 347. 



