INFLUENCE OF TEMPERATURE ON VEGETATION. 725 



soon as they have been gathered ; but that it is necessary to wait for some hours till they 

 have acquired the temperature of the room. (Further details will be given elsewhere.) 

 M^Nab found that a large specimen of Lycoperdon giganteum produced a rise of temper- 

 ature of 1*2° (F. or C..?). Bot. Zeitg. 1873, P- 560. 



CHAPTER HI. 



GENERAL CONDITIONS OF PLANT-LIFE. 



Sect. 7. The Influence of Temperature on Vegetation^ can only be 

 investigated scientifically by observing the influence of definite and different degrees 

 of temperature on the separate vital phenomena of plants, z. e. on the various pro- 

 cesses of assimilation and metastasis, of diffusion, of growth, of the variations in the 

 turgidity of the cells and tension of the tissues, of the movements of protoplasm 

 and irritable organs and of those endowed with periodic motion, &c. 



The determination of the facts which have here to be investigated depends on 

 an accurate determination of the temperature of the plant in any given case, or rather 

 on that of the part of the plant in question on which the experiment is to be made. 

 This is often attended with great difficulties, and is sometimes almost 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 heat 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 ^.^. 1-25 : 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 



* For more detailed proofs see my Handbook of Experimental Physiology, p. 48 et seq. 



