Comparative Physiology/. 337 



injurious eiFect ; and the dormant condition of their functions in 

 winter also tends to preserve the vitality of the system. Heat 

 also injures seeds, probably by a physical change. Grains of corn 

 will germinate after exposure to a cold that would freeze mer- 

 cury, but their vitality is destroyed by exposure to vapour of 

 167°, probably from rupturing the vesicles of starch. A curious 

 effect of heat is noticed by Mr. Knight in melon and cucumber 

 23lants proditcing all the flowers staminifei'ous in excessive heat, 

 and all female or pistilliferous in cold." 



On the Evolution of Heat in vegetables. Chap. XII., he 

 remarks that " much dispute has occurred, whether plants 

 have a proper heat or not. Although the excretion of carbonic 

 acid is constantly going on, there is also a process of evapo- 

 ration ; the surface is so exjaosed, and the circulation so slow, 

 that a general maintenance of vital warmth can scarcely be anti- 

 cipated. In small j)lants, the temperature is found to vary with 

 that of the atmosphere, and the trunk in large plants to be 

 warmer than the atmosphere in winter, and colder in summer ; 

 probably from the slow nature of the transverse conducting 

 power of the wood, and the communication with the deep roots 

 in the soil. Dutrochet has ascertained by recent experiments, 

 that plants do possess the power of generating heat in the parts 

 in which the most active changes are taking place. In the 

 leaves and young shoots he found an increase of temperature 

 of almost one degree, diifering with the species, the energy of 

 vegetation, and the time of the day ; highest about noon. In 

 germination and flowering, the evolution of heat is most ap- 

 parent, from the quantity of carbonic acid furnished. In malting 

 of seeds in a heap, the thermometer has been seen to rise to 1 10°. 

 The flower of a cistus, notwithstanding the conducting power 

 of the atmosphere to carry ofl" the heat as generated, has been 

 found 79° whilst the air was 76° ; and that of a geranium 87° 

 when the air was 81°. In the centre of twelve spadixes of the 

 viVum cordifolium, the heat has been found at 121° while the 

 air was only 66°. Brongniart found the spathe of Colocasia 

 odora, at opening, 8*1° above the surrounding air; next day it 

 was 18° above it; and during the three days of the emission of 

 pollen it increased to 20° ; after which it diminished with the 

 fading of the flower. Vi'olik and Vriese also found the tem- 

 perature increase when the spadix was placed in oxygen, and 

 no increase at all of temperature to take place when the 

 spadix was placed in nitrogen, nor any carbonic acid evolved." 



Heat, light, and electricity are so intimately connected, the 

 one being so generally accompanied with the others, that it is 

 difficult to separate their actions. Vegetation is incomparably 

 most rapid and powerful between the tropics, and there is a far 

 greater preponderance of heat and electricity there than light, 



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