726 
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 \ 
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 2. 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 
climatevique : Mem. de ITnst. vol. XXXV. pp. 460-470.] 
2 Proceedings of the International Horticultural Exhibition and Botanical Congress held in 
London, i865, p. 1 16. 
