314 LECTURE XIV. 



of heat by the plant. Then the nature of the medium by 

 which an organ is surrounded will materially affect its tempe- 

 rature. Thus plants growing submerged in water, or roots 

 buried in the soil, have approximately the same tempera- 

 ture as the water or the soil, but the mean temperature of 

 roots is somewhat lower than that of the soil on account of 

 the current of water passing through them. This is to be 

 accounted for by considering that the temperature of the 

 water or of the soil is not subject to rapid variations, that it is 

 nearly constant for long periods of time; that the organs lose 

 heat neither by radiation nor in connexion with transpiration, 

 but only slowly by conduction. The temperature of subaerial 

 organs, on the other hand, is often different from that of the 

 air, quite apart from the exceptional cases which have come 

 under our notice, in which, in consequence of a period of 

 active destructive metabolism, the temperature of the organ 

 is higher than that of the air for a considerable time.- In the 

 first place, the temperature of the air is liable to rapid varia- 

 tions, and this naturally leads to differences between it and the 

 temperature of the subaerial organs of a plant; again, varia- 

 tions in the temperature of the air have, as we have learned, 

 a considerable effect upon the activity of most of the vital 

 processes of the plant, and in this indirect way they affect its 

 temperature. Finally the form and bulk of a subaerial organ 

 modify its temperature. Small organs, like the mycelium of 

 Fungi or the thallus of Lichens, have approximately the 

 temperature of the air. Dutrochet observed that the tempera- 

 ture of subaerial organs of moderate bulk is in fact always 

 lower than that of the air, and de Saussure noticed in his 

 investigations, that in many cases the temperature of flowers 

 was lower than that of the air. The temperature of leaves 

 is lower than that of the air because of the extent of surface 

 which they offer in proportion to their bulk, and thus radia- 

 tion and transpiration go on actively in them. Maquenne 

 has shewn, on the other hand, that leaves absorb heat; this 

 contributes to prevent their temperature from falling exces- 

 sively in consequence of active transpiration. In the case of 

 fleshy leaves Askenasy has found that when they are exposed 



