TEMPERATURE OF PLANT UNDER NORMAL CONDITIONS 381 



frost when a thermometer in air does not sink below zero. This fall of 

 temperature induces the deposition of dew upon the plant, the latent heat 

 of the condensed vapour exercising a pronounced warming action l . 



The water ascending the trunk of a tree usually exercises a more or 

 less pronounced cooling effect, according to the rate of ascent and the 

 coldness of the water. Rameaux 2 found that the centre of a transpiring 

 tree-trunk was ioC. colder than that of a non-transpiring dead tree when 

 both were exposed to the sun, but when the branches were cut off the living 

 tree the temperature in both trunks became approximately the same. 



Convection currents within the cells will aid in transferring heat 

 upwards in elongated cells, but not downwards, and it is owing to the 

 conduction through the elongated wood-elements that heat is able to pass 

 more rapidly longitudinally than transversely through wood 3 . During winter 

 the centre of a stem is usually warmer than the surrounding air, owing 

 to the upward conduction of heat from the warmer water in the soil, 

 coupled with the action of the cork jacket in retaining heat, and the absence 

 of transpiration. In the higher portions of the stem this heating effect 

 is slight owing to the poor conduction of heat by the wood, while in the 

 smaller branches it is negligible. It is partly owing to the slow inward 

 conduction of heat that Hartig 4 observed that the maximal temperature 

 was reached 4 cm. deep in an oak stem at 6 p.m., and 20 cms. deep not 

 until towards midnight after a day's insolation. 



The thinner plant-organs are subjected to greater extremes of tempera- 

 ture than the centre of a thick stem, which responds but slowly to changes of 

 temperature ; but the latter is subject to greater daily and yearly variations 

 of temperature than the root-system, owing to the more constant tempera- 

 ture of the soil 5 . Hartig found, for instance, that the interior of a tree- 

 trunk sank to I3C. during a winter when the air was frequently at 

 I5C. to 22 C., in spite of the upward flow of heat from the warmer 

 roots. 



1 Cf. Jamin, Naturforscher, 1879, p. 140; Wollny, Forschung. a. d. Gebiete d. Agricnltur- 

 physik, 1892, Bd. xv. 



4 Rameaux, I.e., p. 23. Hartig observed (Bot. Jahresb., 1874, p. 760) that the temperature in 

 the interior of a stem sinks when the buds unfold and transpiration becomes active. 



3 Researches on the conductivity of wood to heat were carried out by de Candolle, Ann. d. 

 Physik u. Chemie, 1828, Bd. XIV, p. 590; Knoblauch (ibid., 1858, Bd. cv, p. 623); \Viesner, Die 

 Rohstoffe des Pflanzenreichs, 1873, p. 292 ; Sowinsky, Bot. Jahresb., 1875, p. 773. Sowinsky found 

 the ratios between the transverse and longitudinal conductivities of wood to be as i : 1-15 (Quercus 

 robur) and I : 1.43 (Carflinus betulus}. Sowinsky found that some woods conducted better when 

 dry, others when moist. 



* Hnrtig, Bot. Jahvesb., 1873, p. 508. See also Goppert, Die Wanneentwickelung i.d. Pflanzen, 

 1830, p. 160. Cf. also Miiller-Thurgau, I.e.; Ihne, I.e. 



5 On the temperature of subterranean tubers see Seignette, Rev. ge"n. de Bot., 1889, T. I, p. 573. 



