THE METABOLISM OF PLANTS. 315 



to the sun their temperature may be much higher than that 

 of the air: thus the leaves of Sempervivum alpinum had a 

 temperature of 52 G, whilst that of the air was 28-1 C. When 

 the organ is bulky, the trunk of a tree for example, the tem- 

 perature is never the same as that of the air, and it is different 

 in different parts. With regard to the temperature of trees 

 Krutzsch states that in general the temperature of the trunk 

 and main branches is lower than that of the air during the 

 day and higher during the night, and that, during the day, 

 the temperature of the trunk is lower than that of the branches; 

 but neither the trunk nor the larger branches attain within 

 the twenty-four hours either the maximum or the minimum 

 temperature attained by the air: the temperature of the 

 smaller branches is approximately that of the air. Becquerel 

 finds that the maximum temperature of the trunk is attained 

 about sunset, and the minimum about sunrise'. The mean 

 temperature of the trunk is higher than that of the air in 

 autumn and in winter, and lower in spring and in summer : 

 but the mean annual temperature of the trunk is, as Becquerel 

 points out, the same as that of the air. We see, then, that 

 the temperature of trunks and branches of a certain bulk 

 depends entirely upon that of the external air. In these 

 organs the proportion of actively living cells to dead cells is 

 so small that the heat evolved in the metabolism of the 

 former is insufficient to affect perceptibly the temperature of 

 the whole organ. In the course of his observations upon the 

 evolution of heat in plants, Dutrochet found that the tem- 

 perature of stems and branches is highest near the growing 

 point, and that the temperature of the pith when its cells are 

 living is higher than when its cells are dry and dead : these 

 differences of temperature are of course due to the greater 

 metabolic activity in the growing-point and in the young pith 

 than in older parts. 



The reason why the differences of temperature, which arise 

 in the different parts of a plant in obedience to internal or to 

 external causes, are sufficiently permanent to be detected is 

 due to the fact that the tissues of plants are bad conductors 

 of heat ; and this also explains to some extent why it is that 



