92 A CHEMICAL SIGN OF LIFE 
difference between dead and living seeds, namely, the 
matter of metabolic acceleration on injury. The in- 
creased carbon dioxide production on stimulation is the 
chemical sign of life of seeds and tissues generally, as 
well as of nerves. 
Once this interesting similarity between seeds and 
nerves was well established we made further investi- 
gations on other plant tissues, in which conditions 
were somewhat different. It was possible that the 
removal of the heavy coat from seeds in crushing them 
might have something to do with the increased metabolic 
activity, and that this activity, therefore, might not be 
manifested by all tissues. In fact, Crocker has shown 
that the removal of the coat is one of the factors which 
initiates germination in dormant seeds. When we tried 
different leaves, however, they all behaved in the same 
manner as did seeds and nerves. The leaves selected 
for test were necessarily small, with the object in view 
of being able to place the whole leaves in the chamber 
with the least injury. They included such as Japanese 
ivy, common grass, Australian pine, and various others. 
We may add here that the increase of carbon dioxide 
output as a result of some other forms of injury in leaves 
has been recorded by several investigators. 
Some objection might be made against our experi- 
ments, however, on the ground that the injury to the 
stomata may be responsible for the output of more gas 
in the case of leaves. That this is not the sole cause 
of the escape of the gas is shown by our experiments ona 
plant tissue without stomata. Red algae were tested 
at the suggestion of Professor Osterhout and gave 
similar responses. 
