>j26 GENERAL CONDITIONS OF PLANT-LIFE, 



absorptive power; and that radiation depends not merely on the temperature, but also 

 on the diathermancy of the surrounding medium. 



In the aerial parts of plants, transpiration is an energetic additional cause of loss 

 of temperature ; inasmuch as water in the act of evaporation withdraws from the 

 plant the amount of heat necessary for its vaporisation, and hence makes it colder. 



In investigations of the influence of temperature on the various processes of 

 vegetation, the conditions noticed above must always be carefully considered. It 

 may be assumed in general that the result of their united action is that small water- 

 plants and the underground parts of plants have usually nearly the same temperature 

 as that of the surrounding medium when this temperature is not subject to too great 

 variations ; but that on the other hand leaves and slender stems exposed to air are 

 generally colder than the air ; while the thick stems of woody plants are sometimes 

 warmer, sometimes colder, in consequence of their slow conducting power. How 

 greatly the temperature of parts of plants of considerable superficial extent may be 

 depressed by radiation below that of the air is shown by the fact that a thermometer 

 placed on the grass and exposed to radiation indicates on clear nights a temper- 

 ature several degrees lower than one placed in the air. If the latter is only a few 

 degrees above the freezing-point, the temperature of the leaves of plants may in 

 this manner fall below zero and they will suffer the effects of frost. The formation 

 of dew on summer nights, and of the hoar-frost which is deposited in such large 

 quantities on plants especially in the late autumn, are striking proofs of the effect 

 of radiation in lowering their temperature. The relation of the temperature of 

 plants to that of their surrounding medium is however very complicated when we 

 have to do with solid bodies like trunks of trees, because the different powers of 

 conduction in the longitudinal and transverse directions of the wood, and other 

 causes, then cooperate with the action of radiation and of absorption of heat 

 through the bark. In general, as has been shown by Krutsch's beautiful experi- 

 ments, the trunk is cooler during the day than the surrounding air, but warmer 

 in the evening and night K 



With respect to the changes of volume in masses of tissue and in individual 

 cells as the temperature varies, nothing is known with certainty except as regards 

 dry wood. The numbers given by Caspary as the coefficients of the expansion of 

 wood caused by heat depend on untrustworthy observations and on a complete mis- 

 understanding of the phenomena which take place in the objects observed ^. When 

 leaf-stalks and the branches of trees become curved at temperatures far below the 

 freezing-point, this is obviously not altogether, if at all, caused by the different 

 layers of tissue having different coefficients of heat-expansion; but is mainly 

 a consequence of the fact that the, water of vegetation freezes, while the cell- 

 walls lose water and in consequence contract more or less according to their state 



* [According to Becquerel, trees warm surrounding layers of air during the day and a good 

 part of the night ; they begin to cool them as soon as they have attained the same temperature. 

 The maximum temperature is reached by the air two or three hours after midday ; in the tree 

 it is reached after sunset, in summer towards 9 p. m. See Memoire sur les forets et leur influence 

 climateiique : Mem. de I'lnst. vol. XXXV, pp. 460-470.] 



2 Proceedings of the International Horticultural Exhibition and Botanical Congress held in 

 London, 1866, p. 116. 



