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. 
Wood. In radial direction. In longitudinal direction. Proportion. 
Box-wood. . 0-0000614 0-00000257 25:1 
Fir .... 0-0000584 0-00000371 16 : I 
Oak. . . . 0-0000544 0-00000492 12 : I 
Poplar . . . 0-0000365 0-00000385 9 : i 
Maple . . . 0-0000484 0-00000638 8 : i 
Pine. . . . 0-0000341 0-00000511 6 : i 
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 ; i. 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 life 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, 
^ Poggendorff s Annalen, 1868, vol. 133. p. 412. 
^ Sachs, Ueber die obere Temperaturgrenze der Vegetation, Flora, 1864, p. 5; Krasan, Beitr. z. 
Kenntniss des Wachstliums, Silzber. d. Wien. Akad, 1873. 
