^M^ GENERAL CONDITIONS OF PLANT-LIFE. 



This is often attended with great difficuhies, and is sometimes ahnost impossible. 

 Independently of the changes of temperature, usually inconsiderable, caused by 

 respiration in the interior of the plant, the temperature of each cell depends on 

 its position in the mass of tissue and on the variations of the surrounding tem- 

 perature. A constant interchange of temperature is going on between the plant and 

 its surrounding medium by conduction and radiation which essentially determines 

 the temperature of any part of a plant at any particular time. 



In reference to the conduction of heat, it must be mentioned in the first place 

 that all parts of plants are bad conductors ; the differences of temperature between 

 them and the air, earth, or water that is in contact with them become only very 

 slowly adjusted in this way. The conductivity for heat is probably also always 

 different in different directions ; that in the longitudinal direction in dry wood bears 

 the proportion to that in the transverse direction of e.g. i"25: i in the acacia, box, 

 and cypress, of i '8 : i in the lime, alder, and pine. 



The radiation of heat is on the other hand a very frequent and rapid cause of 

 changes of temperature in most parts of plants ; the chief effect of these changes 

 being to bring about differences between the temperature of the surrounding medium 

 and that of the plant, especially when the parts of the plant are of small size but 

 have a large hairy surface, as is the case with many leaves and internodes. It must 

 be noted in this connection that the radiating power of a body is equal to its ab- 

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

 on the diathermacy 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 phenomena noticed above must always be carefully considered It 

 may be assumed in general that the result of their united agency 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 foliage of plants may in this manner fall below- 

 zero and 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 



