342 BOTANICAL GAZETTE [MAY 
RICHTER (32, 33, 34) speaks of the impurities weakening negative 
geotropism and has continuously maintained that they increase 
heliotropic sensibility. GUTTENBURG (13) has spoken of the 
horizontal nutation as due to weakened negative geotropism and 
vigorously denied increased heliotropic sensibility. He attributes 
the conclusion of other workers on the latter point to inaccuracies in 
experimentation, especially the failure to use the clinostat, thereby 
eliminating the antagonistic action of gravity. 
NELyuBOw (26) has maintained since 1901 that the response 
is induced diageotropism. The evidence set forth in his last paper 
seems conclusive, marked as it is by excellence of experimentation. 
He criticizes the other workers for using “laboratory” air in which 
the quantity of effective impurity must vary from hour to hour, 
and in which the amount of impurity cannot be measured at any 
time. He used mixtures of pure air with 1-3 ppm. of ethylene. 
To maintain constant concentration the mixture was renewed daily. 
In this atmosphere on a horizontal clinostat the etiolated pea 
epicotyls showed great reduction in rate of elongation, also swelling 
but not bending. When grown in the same atmosphere off the 
clinostat, they showed the same characters with the addition of 
horizontal nutation. If in such an atmosphere the epicotyls were 
raised or lowered out of the horizontal position, they again assumed 
it. NELJUBOW maintains that diageotropism is the only assumption 
that can explain this behavior. He emphasizes that there is no 
autonomic nutation determining the direction in which the epicotyl 
turns to assume the horizontal position, but that it is entirely 
determined by its position in relation to the pull of gravity. RicH- 
TER (34), working with seedlings of Vicia sativa, V. villosa, and 
Pisum sativum, shows that such a statement can hold only for 
epicotyls more than 1cm. tall. On a clinostat or in “impure” 
air which “weakens” negative geotropism, the young etiolated 
epicotyls turn “‘backward” (in the direction in which the closed side 
of the curved tip faces) and crowd themselves closely against the 
substratum. This is an autonomic nutation incapable of manifest- 
ing itself off the clinostat in pure air on account of the counter- 
action of negative geotropism. Judging from the work of both 
NeELjyusBow and Ricurer, the horizontal nutation of the etiolated 
