408 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 have 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. 
