1913] KNIGHT & CROCKER—TOXICITY OF SMOKE 341 
rather soluble in water, the method certainly gives a very close 
approximation to the constant concentration necessary to give a 
response. The object of the work was to determine the nature 
of the response given by the several gases and vapors and to 
approximate the concentration necessary to produce it. For this 
purpose the method is adequate. 
IIJ. Historical 
I. RESPONSE OF THE PEA EPICOTYL 
a) Horizontal nutation 
As has been stated, the epicotyl of the pea seedling grows pros- 
trate or horizontal in “laboratory” air. NELJUBOW (25) has spoken 
of this response as a horizontal nutation, and mentions ethylene and 
acetylene, both constituents of illuminating gas, as especially 
effective in producing it. He has shown (26), also, that the response 
is not limited to the pea epicotyl, but appears in the etiolated 
epicotyls of Ervum lens, Lathyrus odoratus, Vicia sativa, and 
Tropaeolum. SINGER (42) has observed the same for the potato 
stem. NrELjuBow has shown that the response in the pea epicotyl 
in an atmosphere containing ethylene varies with the concentration 
of ethylene. Beginning with the higher concentrations and passing 
to the lower, he mentions the following grades of response: (1) no 
growth, death; (2) no elongation, a swollen knob; (3) elongation 
slow, swelling, diageotropism; (4) elongation faster, little swelling, 
diageotropism; (5) like (4) except obliquely placed; (6) erect, but 
growth reduced by half 
The horizontal nutation has been a subject of no little 
investigation. With mere mention we can pass over WIESNER’S 
(43) conception of undulating nutation induced by darkness, and 
RIMMER’s (39) autonomic nutation induced by dryness, for at 
that time there were not sufficient data available for a rational 
interpretation of the response. Mo.iscH (20) and KORNICKE (17) 
concluded that impurities of laboratory air affect geotropic and 
heliotropic sensibility in opposite ways, weakening the former 
and strengthening the latter. MotiscH (21, pp. 170, 171) has 
apparently abandoned the idea of increased heliotropic sensibility. 
