go A CHEMICAL SIGN OF LIFE 
(already discussed on p. 34) that carbon dioxide is 
produced by many purely chemical processes. So we 
proceed to inquire whether the output is increased on 
stimulation. 
Increased metabolism in seeds.—The most interesting 
thing ascertained was that the living seed, like any other 
living tissue, can be made to give off more carbon dioxide 
on stimulation. It responds to an injury and is, there- 
fore, irritable. It has already been stated that a nerve 
injured by crushing gives off more carbon dioxide than 
a resting nerve, just as if it had been stimulated by an 
electrical shock. Since there was no way of telling 
what strength of electrical stimulation was required 
in order to arouse the seed, we stimulated it by an 
injury, namely, by crushing it. The seed thus stimu- 
lated showed a marked acceleration of its respiration. 
If two apparently living kernels of wheat are taken and 
one of them is crushed and their carbon dioxide produc- 
tion is compared in the biometer, the crushed one always 
produces more carbon dioxide than the normal one. 
That this is a vital response is shown by the fact that only 
living seeds behave in this way. If one takes two kernels 
of any similar seed, which have been killed in an elec- 
trical oven heated to 60° C., and one of them is crushed, 
there is no difference in the carbon dioxide output of the 
two seeds. The difference in amount of carbon dioxide 
produced by crushing cannot be observed in dead seeds 
or in anesthetized seeds. In this respect a seed and a 
nerve are alike; the chemical signs of irritability are 
identical. Both, as long as they are alive, respond to a 
mechanical stimulation by producing more carbon 
dioxide. : 
