316 LECTURE XIV. 



the temperature of the trunks of trees differs from that of the 

 air, for a change in the temperature of the air is only very 

 slowly responded to in the trunk. Some woods conduct better 

 than others, hence some trunks respond more promptly to 

 variations in the external temperature than others ; Krutzsch 

 observed, for instance, that the temperature of the trunk of a 

 Pine changed more rapidly than that of the trunk of a Maple, 

 and he ascertained that the conductivity of the wood of the 

 former is twice as great as that of the wood of the latter. 

 Again, the conductivity of wood is greater in the direction of 

 the length of the fibres than in the transverse direction ; hence, 

 in a trunk, the tendency towards the establishment of uni- 

 formity of temperature between its different parts is greater 

 than that towards the assumption of the temperature of the 

 external air. 



With regard to the relative conductivity of different woods, de la Rive 

 and A. de Candolle find that the denser the wood the better it conducts. 

 They arrange the following trees in the order of the conductivity of their 

 wood : CratcEgus (Pyrus) Aria, Juglans regia, Quercus Robur, Pinus 

 Abies, Populus italic a, Quercus Suber. 



Knoblauch has determined the relative conductivity in the direction 

 of the fibres and across them in the following woods : 



Acacia, Box, Cypress i'25 : I. 



Elder, Lilac, Hawthorn, Walnut, Beech, Elm, Oak i'45 : I. 



Apricot, Brazil-wood ... ... ... ... ... i'6o : I. 



Willow, Chestnut, Lime, Alder, Birch, Pine, Fir ... i'8o : I. 



Sowinsky determined the relation to be, for the Oak 1*15 : i, for the 

 Hornbeam 1*43 : I, for the Lime i'28 : I, and for the Cherry 1*4 : i. He 

 finds, in opposition to de Candolle, that the lighter woods are the 'best 

 conductors : this difference is due to the fact that Sowinsky experimented 

 with fresh woods, whereas de Candolle used dry woods. 



Light. The evolution of light by plants is a phenomenon 

 which has been known from the time of Aristotle and Pliny, 

 and is commonly spoken of as phosphorescence. So far as 

 we know at present, this property is confined to Thallophytes. 

 The so-called phosphorescence of decaying wood is due to the 

 presence of the mycelium of Agaricus melleus (Rhizomorpha), 

 and that of putrefying meat and vegetables to Schizomycetes 

 of the nature of Micrococci (Pfliiger, Lassar). Other luminous 



