4 o8 PHYSIOLOGY 



largely to microorganisms, whose active respiration, and especially the 

 fermentation they cause, liberates much heat. 1 If, however, the surface 

 of the seeds is carefully sterilized before germinating, the difference is 

 much less, in many cases with ordinary insulation only 1-1.5 C. 



By using Dewar flasks, which afford very perfect protection against loss of heat 

 by radiation and conduction, differences of 20 C. or more hare lately been found 

 with 80 gm. of peas (weighed dry). 



The opening of flowers crowded into a compact cluster within a bract, 

 as in the calla, causes a decided rise of temperature, differences of 5-10 C. 

 having been noted. This production of heat is continuous, though its 

 rate varies. It is said that a kilogram of seedlings may produce heat 

 enough per minute to warm i gm. of water from o to 50 or even 100 C. 

 Yet under ordinary circumstances this heat is steadily dissipated. 



Comparative activity. It is commonly supposed that at best the 

 aerobic respiration of plants is weak compared with that in animals. 

 This is a mistake. The respiratory rate for active tissues of plants 

 compares well, weight for weight, with that of even warm-blooded ani- 

 mals, and in some cases far exceeds it, if the gaseous changes may be 

 taken as a fair measure of the process. Thus, if a man of 75 kg. pro- 

 duces at light work about 900 gm. CO 2 in 24 hours, the output of CO 2 

 equals 1.2 per cent of his weight. By the buds of lilac the output of 

 CO 2 equals 1.8 per cent of their weight; by those of horse chestnut, 3 

 per cent; by seedlings of poppy, 2 per cent; by molds, 6 per cent. While 

 a man may use in 24 hours i gm. of oxygen for each 100 gm. of his 

 weight, young leaves of wheat use it at the same rate; opening flowers 

 use 4 times as much, and some bacteria 200 times as much. 



The stage of development, the general activity, and the rate of growth influence 

 decidedly the rate of respiration. The younger and more active the tissues or 

 organs, the more rapid, as a rule, is the respiration. 



Life. It has already been indicated that anaerobic respiration begins 

 like aerobic, but that the decompositions cease before they attain the 

 same extent. It may very well be, also, that they pursue a somewhat 

 different course, on account of the lack of oxygen. Growth ordinarily 

 ceases when growing tissues are forced to do without O 2 , though some- 



1 When moist plants or manures are piled up, very high temperatures may be produced 

 in the midst of the mass by the combined activities of many different fungi and bacteria. 

 This "heating" may even suppress or kill off all species except those that flourish at 



c. 



