INFLUENCE OF TEMPERATURE ON VEGETATION. 



727 



of imbibition and of lignification. The phenomenon depends therefore in the first 

 instance on a change in the state of imbibition and turgidity produced by different 

 temperatures. Villari has carefully measured ^ the coefficients of heat-expansion for 

 different dry woods. Like the expansion caused by the absorption of water, that 

 caused by heat is much less in the direction of the fibres than in the radial direc- 

 tion across the fibres ; but with the difference that the coefficients of expansion for 

 absorption are reckoned by hundredths in the radial and thousandths in the longi- 

 tudinal direction, those for heat by hundred-thousandths and millionths ; so that 

 the alterations of the dimensions of dry wood in the two directions caused by 

 changes of temperature are about 1000 times smaller than those caused by the 

 absorption of water. The following table is from Villari for temperatures between 

 2° and 34° C. :— 



Coefficients of heat -expansion for i°C. 



In longitudinal direction. Proportion. 



0-00000257 

 00000037 I 

 0-00000492 

 0-00000385 

 O-OOOCO638 

 0-00000511 



Since these numbers only hold good for dry wood, while wood as a constituent 

 of the living plant can be observed only in the moist state, they cannot be applied 

 directly to the explanation of the physiological phenomena due to changes of 

 temperature; but they are nevertheless of great interest, since they give us an 

 insight into the molecular structure of wood, especially as to its elasticity in different 

 directions. 



Something more is known as to the influence of different degrees of temperature 

 on the vital phenomena of plants. On this subject the important fact must first be 

 noted that the exercise of every function is restricted to certain definite limits of 

 temperature within which alone it can take place ; t. e. all functions are brought into 

 play only when the temperature of the plant, or of the particular part of the plant, 

 rises to a certain height above the freezing-point of the sap, and cease when a 

 definite maximum of temperature is attained, which can apparently never be per- 

 manently higher than 50° C.^ Hence the fife of the plant, i.e. the course of its 

 vital processes, appears to be confined in general within the limits zero and 50° C. 

 It must however be noted that the same functions may have very different limits 

 between 0° and 50° C. in different plants; as is also the case with different functions 

 in the same plant. A few examples will serve to explain this. 



Since the cell-fluids, consisting of aqueous solutions often in a state of high 

 concentration, do not usually freeze at zero, it is always possible for certain pro- 

 cesses of growth to take place when the temperature of the surrounding air is as 

 low as this, although this fact has not yet been sufficiently established. Uloth (Flora, 



^ Poggendorfifs Annalen, 1868, vol. 133. p. 412. 



^ Sachs, Ueber die obere Temperaturgrenze der Vegetation, Flora, 1864, p. 5; Krasan, Beitr. z, 

 Kenntniss des Wachsthums, Sitzber. d, Wien. Akad. 1873. 



