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PLANTS UNDER MECHANICAL STIMMULUS. 299 
of german-silver wire, through which an electric current could be 
sent for heating (see fig. 20). The temperature could be regu- 
lated at will by varying the current. The specimen chosen for 
experiment was the leaf-stalk of celery. It was kept at a given 
temperature for 10 minutes, and record taken. This process 
was repeated. It will be seen how the response undergoes a 
continuous diminution as the temperature is raised. In 
radishes the response disappeared completely at 55° C., but 
with celery, treated in the manner described—by dry heating,— 
I could not obtain its entire abolition at 60° C. or even higher. 
But on taking the specimen out and putting it in water at 55° C. 
for five minutes, the response was entirely abolished (fig. 21). 
This shows that the plant can withstand dry heat much better 
than moist heat. 
Fig. 21. 
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Effect of temperature on response. 
A very curious effect of temperature variation is the marked 
increase of sensitiveness which is often seen as the after-effect. 
This is well exhibited when two series of records are taken, one 
during the rise of temperature, and the second during the fall. 
It is found that the responses are enhanced during cooling, as 
compared with responses given at the same temperature while 
warming. Previous temperature variation has, at least in some 
cases, a stimulating effect (fig. 22). 
Effect of Steam.—The plant was mounted ina chamber into 
which steam could be introduced. I had chosen a specimen 
which gave regular responses. On the introduction of steam. 
With consequent rise of temperature, there was a transitory 
augmentatiou of excitability. But this quickly ‘isappenred and 
Ag 
