46 THE CELL 



organ. It is worthy of note also that the light produced by the "lightning 

 bug " is deficient at both ends of the spectrum. We have here in other words 

 a source of light which is devoid or almost devoid of the ultra-red and ultra- 

 violet rays (Langley and Very). 



I. FORMATION OF HEAT 



Heat is formed in all dissimilative processes, and since processes of this 

 kind occur everywhere in animate nature, we may say that the generation of 

 heat is universal. This cannot always be demonstrated; for in the isolated 

 elementary organisms the quantity formed is so small that it cannot be meas- 

 ured with our instruments. In plants as a rule heat is formed so slowly 

 that as fast as it is generated, it is radiated to the surrounding medium; 

 consequently the temperature of the plant cannot be elevated perceptibly above 

 the medium. It should be said also that the abundant transpiration occur- 

 ring in plants has much to do with keeping down their temperature. The 

 same is true and for the same reason in most of the so-called cold-blooded or 

 poikilothermos animals, that is, animals in which the body temperature rises 

 and falls with the temperature of the surrounding medium. In dry air, on 

 account of evaporation from the surface of the body, the temperature of a 

 cold-blooded animal is usually lower than that of its medium. In a moist 

 or water-saturated atmosphere the body temperature may rise some tenths of 

 a degree. This is true likewise of cold-blooded animals which live in water. 

 Only in the so-called warm-blooded or, more correctly, homoiothermos animals 

 (birds and mammals) i. e., animals whose body temperature remains con- 

 stant in spite of the variations of the surrounding temperature can the 

 production of heat be demonstrated directly and without difficulty. In these 

 animals the temperature of the body is almost always higher than that of 

 the medium in which they live. 



Under certain circumstances it can be shown very clearly that in plants as 

 well as in cold-blooded animals heat is actually formed. With peas which have 

 been allowed to germinate in a funnel under a bell jar a rise in the temperature 

 of 1.5 C. has been observed. In the spadix of the Aracese (e. g., " skunk- 

 cabbage ") a temperature of 15 C. higher than that of its surroundings has 

 often been witnessed. Likewise in the fermentation of sugar solutions by the 

 yeast plant elevations of temperature of about the same extent may occur. 



With regard to the body temperature of the cold-blooded animals, the fol- 

 lowing data on the excess of the animal's temperature over that of its sur- 

 roundings have been gathered: various invertebrates in water 0.21-0.60 C.; 

 earthworms in a glass vessel 1.4 C.; bees in a beehive 21 C.; moving butter- 

 flies 14 C. 



The animal heat of warm-blooded animals will be more fully discussed in 

 Chapter XIV. 



J. GENERATION OF ELECTRICITY 



The enormous number of investigations on animal electricity begins 

 if we except the electrical fishes with the pregnant observation of Galvani 

 that a frog's thigh contracts when it is touched in two places with the ends of 

 a metallic arc (September 20, 1786). From this observation Galvani thought 



