THE MOLECULAR FORCES IN PLANTS. 155 



62. The Temperature of Plants. 



The temperature of a plant structure is dependent on very 

 many factors. Among these are its organisation, its position in 

 the plant, the quantity of water it contains, its specific heat, its 

 rate of transpiration, and its capacity for absorption, conduction, 

 and radiation of heat. It is therefore clear that, in many cases, 

 it is difficult to account exactly for the particular temperature of 

 a plant structure as experimentally determined. Moreover, many 

 conditions here coming under consideration have not yet been 

 investigated at all or not sufficiently. 



Strongly transpiring parts of a plant are often somewhat colder 

 than the air in their neighbourhood, chiefly because a large 

 amount of heat becomes latent in the formation of water vapour. 

 On the other hand, plants which transpire feebly, and are fleshy 

 and succulent in character, often assume a comparatively very 

 high temperature under the influence of direct sunlight. The 

 leaves of a Crassulaceous plant (Sempervivum, Escheveria) 

 which has been exposed to strong sunlight feel quite warm to 

 the touch ; their temperature is far higher than that of the more 

 delicate, thinner leaves of plants growing in their immediate 

 neighbourhood. It is very instructive to determine the tempera- 

 ture of succulent plants accurately by means of therrnometric 

 measurements. 1 I made observations of this kind on a Cactus 

 (Echinopsis multiplex). A hole is bored right to the middle of 

 the Cactus by means of a cork borer. The hole is cleaned with 

 blotting-paper, and then a thermometer with a cylindrical bull) 

 is introduced into it. After taking care to close the opening 

 tightly, e.g. by means of blotting-paper, the plant is placed in 

 the open, in a situation where it will receive during the day 

 direct sunlight. We now observe the temperature of the plant 

 at intervals during the day, and also during the night, and com- 

 pare the readings obtained with those given by a thermometer 

 hung in the shade. Trustworthy determinations of the tempera- 

 ture of the air are, however, by no means easy to make. It is 

 best to place the thermometer in a large zinc box, hanging in 

 front of a window of a room with a north aspect, at a suitable 

 distance from the ground. The box must be so arranged as to 

 permit circulation of air in it ; and further, it must not be placed 

 too near the building. We shall be astonished at the temperature 

 which the Cactus reaches on a warm day in the sunlight. A 



