430 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
the sudden change which exerts a marked depressing effect. The 
plant partially regains its sensibility when accustomed to darkness. 
When brought suddenly from darkness to light there is also a transient 
depression followed by enhanced excitability. 
Temperature has also a marked effect on excitability. Up to a 
critical point warmth increases excitability, the recovery being also 
Fic. 7.—Effect of cloud. Dotted up-curve indicates responsive fall, and continuous 
down-line exhibitsslow recovery. First four responses normal; next three show depression 
due to diminution oflight brought on by cloud, the duration of which is indicated by hori- 
zontal line below. Last three records show restoration of excitability brought on by clear- 
ing ofsky. All records read from left to right. 
hastened. Cooling conversely depresses excitability. The motile 
excitability is abolished at about 20° C. 
EFFECT OF AIR, FOOD, AND DRUGS. 
The plant is intensely susceptible to the impurities present in the 
air. The vitiated air of the town has a -very depressing effect. 
According to popular science, what is death to the animal is supposed 
to be life for the plant; for does it not flourish in the deadly 
atmosphere of carbonic acid gas? The record (fig. 8) shows that, 
instead of flourishing, the plant gets suffocated just like a human 
being. Note the gasp of relief when fresh air is introduced. Only 
in the presence of sunlight is the effect modified by photosynthesis. 
In contrast to the effect of carbonic acid, ozone renders the plant 
highly excitable. Sulphuretted hydrogen, even in small quantities, 
is fatal to the plant. Chloroform acts as a strong narcotic, inducing 
a rapid abolition of excitability. The ludicrously unsteady gait of 
the response of plant under alcohol (fig. 9) could be effectively 
