388 TEMPERATUEB OF PLANTS. 



produced locally by applying the vapour only to certain parts of the 

 plant. For remarks on the effects of gases on living plants, see pp. 

 159-161. 



2. — Temperature of Plants. 



The heat developed during the expansion of flowers and the pre- 

 paration of the pollen, especially in the case of Aroidese, and also at the 

 period of germination, has been already considered (pp. 259 and 260). 

 These phenomena appear to be strictly of a chemical nature, and may 

 be traced to the absorption of oxygen, and its combination with the 

 carbon of the starch, the latter being converted into dextrine and 

 grape sugar. It is now proposed to consider the observations which 

 have been made relative to the general temperature of plants. 



Great diiferences of opinion have prevailed as to the existence of 

 a proper heat in plants. Hunter examined the heat of the internal 

 parts of the trunks of trees by boring holes of different depths in 

 them, and inserting thermometers ; and similar experiments were made 

 by Sohubler at Tubingen. The results of these experiments were, 

 1st, That the temperature of trees is higher than that of the air in 

 winter, and lower in summer ; 2d, That the temperature corresponds 

 to the depth in the soil to which the roots penetrate ; and 3d, That 

 it depends on the temperature of the fluid matters taken up by the 

 roots, as well as the bad conducting power of the wood of the trees. 

 Dr. Hooker found that the temperature of the juices of plants in 

 India depended materially on that of the soil at their spongioles. 

 Dutrochet made experiments to determine the temperature of the 

 growing parts of plants. He found, by means of a thermo-electric 

 apparatus, that this varied from two or three tenths of a degree to 

 one degree above that of the air. This generation of heat only takes 

 place when the plant is active and vigorous, and seems to be connected 

 with processes going on in the interior of the cells. When the cells 

 cease to be actively engaged in the processes of vegetable life, they 

 cease to manifest this vital heat. It reaches a daily maximum, the 

 period of which varies in different plants, according to their vigour. 

 Rameaux has confirmed Dutrochet's observations. He observed that 

 during the day the temperature of the various layers diminished from 

 the surface to the centre, while during the night the reverse took place, 

 and that both. of these processes were materially influenced by the 

 nature of the surrounding temperature and the diameter of the tree. 

 There appear, therefore, to be two sources of heat in plants, one de- 

 pending on organic actions carried on in the growing parts, and the 

 other on meteorological influences, which either act directly through 

 the air, or indirectly through the fluid matters brought up from a 

 certain depth in the earth. 



