FUNCTION.] PROPER HEAT OP PLANTS. 215 



state, plants possess the property of producing cold, for they 

 almost always have a lower temperature than that of the 

 surrounding air. If, however, evaporation is prevented, it is 

 easy to observe the proper temperature of plants ; Dutrochet 

 used a thermo-electrical apparatus, and for comparison the 

 experiments were made both with* living and dead plants ; 

 dead plants acquired the temperature of the surrounding 

 medium ; live plants the same, with the addition of that which 

 was lost by evaporation, and which M. Dutrochet reckons at 

 the most to be 4 Cels ; often only -, or even -^ or -fa. 

 The proper heat of young twigs and leaves vanishes in the 

 night, or in general in the dark, and appears again under the 

 influence of light. The higher the external temperature, 

 the greater is the vegetable warmth. That part of the heat 

 of plants which is carried off by the evolution of oxygen 

 cannot be determined quantitatively. 



Proofs that plants possess a peculiar heat, dependent upon 

 their vital forces, was long since published in Germany; 

 and in my "Physiology," says Meyen, I proved that an 

 extrication of heat occurs not only in germinating seeds, and 

 in the fresh fruits of Areca Catechu when lying together, but 

 also in leaves and herbage in general ; " singly they do not 

 exhibit any warmth on account of the evaporation, but they 

 do when brought together in masses. I convinced myself 

 of the truth of this by the thermometer. I have several 

 times experimented with fresh-cut grass and fresh spinach 

 leaves. Dutrochet has added that new researches confirm 

 the former ones. In the stem of Euphorbia Lathyris he 

 remarked the vegetable heat amount to C., but only so 

 long as it was in a verdant state. He also found heat in 

 roots, fruits, and even embryos. Complete exclusion of light 

 totally prevents the rise and fall of temperature, but this does 

 not always take place the first day. M. Dutrochet remarked 

 the change of temperature by night and by day even on the 

 second day of the experiment." 



" MM. Bergsma and Van Beck have clearly proved that 

 perspiration is the cause of the difficulty in measuring the 

 temperature proper to plants. They chose (in January, 

 1839) a hyacinth growing in a glass for their experiments. 



