CHAPTER XL. 



LACK OF HEAT. 



A General Survey. 



Life Phenomena at Low Temperatures. 



The plant is much more dependent on the temperature of the air than 

 on the temperature of the soil. Before the soil can follow the fluctuations 

 in the warmth of the air, this has already awakened plant life and at times 

 brought it to considerable development. The individual parts of the plant 

 naturally do not respond to the fluctuations in the temperature equally 

 quickly. While the warmth of leaves and thin stems, in the shortest possible 

 time, increases or decreases, parallel with the temperature of the air, thick 

 trunks will need considerably longer time, more particularly since all plant 

 tissues are poor conductors of heat. From this last circumstance it is 

 evident that thick trunks are sometimes warmer than the surrounding- air, 

 sometimes cooler, and, in fact, are on an average cooler than the air in the 

 daytime and warmer at night. Rut those parts of the plants which extend 

 into the air are also cooler in the daytime. The cooling down of the leaves 

 comes from their radiation of heat. This will be greater the greater the 

 surface of the part in proportion to its bulk. Evaporation should also be 

 taken into consideration as a further cause ; it proceeds at the expense of 

 the warmth of the plant part. These two causes explain the phenomenon 

 that, on bright nights, the thermometer shows a temperature several degrees 

 lower if it stands directly between densely growing plants with thin leaves, 

 such as meadow grass, than is found in the air layer above them. If the 

 temperature of the air itself approaches the freezing point of water, the 

 parts of the plants may be cooled below zero degrees C. by their heat radi- 

 ation and, as a result, die, or, at least, at times some of their functions are 

 arrested. According to Sach's^ observations the chloroplasts of the firebean 

 (Phascohis miiltiflorus) and maise (Zea Mays) cannot turn green if the 

 temperature does not rise to at least 6 degrees C. Rape acts in the same 



1 Lehrbuch III, p. 636. 



