408 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY 



heat in the case of seedlings are probably due to the fact that the respiratory 

 oxidation- is not in all cases complete, and that endothermic changes pre- 

 ponderate in growth and constructive metabolism. 



The whole of the energy transformed into heat is gradually lost by 

 the plant, although it may aid in maintaining transpiration when the 

 temperature of the plant is lower than that of its surroundings. In aquatic 

 plants, however, and in general in the absence of transpiration, the 

 production of heat serves no useful purpose apart from its biological 

 significance in Aroids. Poikilothermic organisms like plants lose much 

 less energy in the form of heat than megatherms like mammals and birds 

 whose body-temperature must be kept approximately constant, but in 

 general the modes of utilization of the liberated chemical energy for 

 mechanical purposes are approximately the same in plants and animals l . 



In plants, however, the chemical energy liberated by metabolism 

 seems to appear almost entirely in the form of heat, and hence very little 

 can be used for mechanical purposes. In some cases, however, a reaction 

 may be performed very economically as, for instance, during the contraction 

 of a muscle or of the stamens of Cynareae, in which less heat is produced 

 relatively to the work done than in a steam-engine or gas-motor 2 . In 

 a muscle performing maximal work one-half of the liberated chemical 

 energy may appear in the form of work, and one-half as heat, whereas it 

 requires a good engine to utilize more than ten per cent, of the energy 

 of the coal consumed ; and during the protoplasmic streaming of an ordinary 

 plant-cell not more than T^TTTT f the energy of respiration is consumed 

 in this form of work 3 . The plant may work more economically under 

 certain conditions than under others. Thus respiration increases con- 

 tinuously up to the lethal temperature, whereas growth and other 

 manifestations of energy are retarded beyond the optimum. Hence in 

 general the plant works more economically at moderate temperatures 

 than at high ones. It must further be remembered that chemical composi- 

 tion is a more important factor in a living organism than in a machine, 

 and that the economic coefficient, that is the ratio between the food 

 absorbed and the increase of body-weight, may vary according to the 

 prevailing conditions. Since the nutritive value of a substance depends 

 upon its chemical constitution, its heat of combustion forms no sure guide 

 as to its nutritive value 4 , although when different materials are consumed 



1 Plants consume their food more completely than animals, which excrete combustible end- 

 products of metabolism. 



2 Cf. textbooks of Physics and Animal Physiology, as well as the Traitd de physique biologique 

 published by d'Arsonval, 1901, T. I, p. 982. 



3 Ewart, Protoplasmic Streaming in Plants, 1903, p. 29. 



4 Cf. Pfeffer, Jahrb. f. wiss. Bot., 1895, Bd. xxvni, p. 258. 



