EVOLUTION OF HEAT. 207 



in blossoming was first obsei-ved by Lamarck, about seventy years 

 ago, in the European Arum, which, just as the flowers open, " gi-ows 

 hot, as if it were about to burn." It was afterwards shown by Saus- 

 sure in a number of flowers, such as those of the Bignonia, Gourd, 

 and Tuberose, and the heat was shown to be in direct proportion to 

 the consumption of the oxygen of the air, or, in other words, of the 

 carbon of the plant. The increase of temperature, in these cases, 

 was measured by conunon instruments. But now that thermo-elec- 

 tric apparatus affords the means of measuring variations inappre- 

 ciable by the most delicate thermometer, the heat generated by an 

 ordinary cluster of blossoms may be detected. The phenomenon is 

 most striking in the case of some large tropical plants of the Arum 

 family, where an immense number of blossoms are crowded together 

 and muffled by a hooded leaf, or spathe (390), which confines and rever- 

 berates the heat. In some of these, the temperature rises at times 

 to twenty or even fifty degrees (Fahrenheit) above that of the sur- 

 rounding air. This increase of temperature occurs daily, from the 

 time the flowers open until they fade, but is most striking during the 

 shedding of the pollen. At night, the temperature falls nearly to 

 that of the surrounding air ; but in the course of the morning the 

 heat comes on, as it were like a paroxysm of fever, attaining the 

 maximum, day after day, very nearly at the same hour of the after- 

 noon, and gi-adually declining towards evening. In ordinary cases, 

 the heat of floAvering is moi'e than counterbalanced by the vaporiza- 

 tion of the sap and the absorption of solar heat by the foliage ; so 

 that the actual temperature of a leafy plant in summer is lower than 

 that of the atmosphere. 



373. We have remarked that the principal consumption takes 

 place in the flower ; and that a store is laid up in the fruit and seed. 

 But much even of this store is consumed Avhen the seed germinates ; 

 and in germination, as is seen in tlie malting of bai'ley, a large 

 amount of organic matter is decomposed into carbonic acid and 

 water, and a proportionate quantity of heat is evolved. By a not 

 very violent metaphor it may be said, therefore, that the fabled Phoe- 

 nix is realized in the Century-plant (369), which, after living a hun- 

 dred years, consumes itself in producing and giving life to its off- 

 spring, who literally rise from its ashes. 



374. Plants need a Season of Rest. When plants are in luxuriant 

 growth, rapidly pushing forth leafy branches, they are not apt to 

 produce flower-buds. Our fruit-trees, in very moist seasons, or 



