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 on a 

 plant tissue without stomata. Red algae were tested 

 at the suggestion of Professor Osterhout and gave 

 similar responses. 



