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. 



