INFLUENCE OF TEMPERATURE ON VEGETATION. 64 A 



shown by Krutsch's beautiful experiments, the trunk is cooler during the day than 

 the surrounding air, but warmer in the evening and nights 



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^. When 

 leaf- stalks and the branches of trees become bent at temperatures far below the 

 freezing-point, this is obviously not altogether (even if principally) 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 

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

 place 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 lonjjitudinal direction Proportion. 



0-00000257 25 : I 



o'ooooo37 1 16:1 



0-00000492 12:1 



0*00000385 9 : I 



0-00000368 8 : I 



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 diff'erent 

 directions. ' 



^ rAccording 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 m.dday ; m the tree 

 it is reached after sunset, in summer towards 9 p.m. See Mcmoire sur les f.rets et Icnr mfluence 

 climaterique: Mem. del'Inst. vol. XXXV, pp.4^^o-470.-ED] 



2 Proceedings of the International Horticultural Exhibition and l^otamcal Congress held m 

 London, 1866, p. 116. 



3 Poggendorff's Annalen 1868, vol. 13.^. P- 4' 2. 



