1895.] Vegetable Physiology. 1101 
sity of Pennsylvania, finds that the hot sun position assumed by sensi- 
tive plants is not due to the action of solar heat rays, as a number of 
observers have stated,-and as he was himself formerly inclined to be- 
lieve, but to the more refractive rays of the solar spectrum. His 
studies were made upon Cassia nictitans, C. chamecrista, C. tora and 
Oxalis stricta, and some of his conclusions are as follows : 
“Tn all cases it has been found that Sachs’ statement is so far cor- 
rect, viz.: that when sensitive plants are placed under colored screens 
the leaflets fold as in the nyctitropic state, most powerfully under red, 
less so under yellow, only feebly or not at all under green, and that 
under blue screens the leaflets remain open as in ordinary daylight. 
But expansion under the red and yellow screens soon takes place, the 
rapidity of the expansion varying according to the brightness of the 
light and the species experimented on.” “Ifthe light be diffuse, and 
thus of moderate intensity, the flat morning position of the leaves is re- 
tained throughout the entire day, or part of it if the sun ultimately 
shinesout.” “If the light becomes more intense, no alteration, or it may 
be slight deflection in Cassia or inflection in Ovalis, occurs to leaflets 
of plants under the red and yellowscreens. When plants are under a 
green screen and exposed to intense illumination. the leaflets either re- 
main flat or assume a more or less paraheliotropic position, the angu- 
lar change at times amounting to 25°. In all cases under the blue 
screens the leaflets become paraheliotropic more or less powerfully, 
the amount of angular movement being proportioned to the intensity 
of the light. It is impossible at present to say whether the blue or 
violet rays are the more powerful. In all cases, normal nyctitropic move- 
ment is accelerated a half to one and a half hours under a red screen, 
but the movements of the leaves and leaflets then are very peculiar.” 
“Under a yellow screen nyctitropism is not quite so accelerated as 
under red, but the closing movements are nearly or quite regular in 
sequence, and in Cassias are first visible at the leaf extremity. Under 
a green screen the time movement practically coincides with that of 
exposed plants, and is beautifully regular in sequence.” “ Under the 
blue light there is always a distinct retardation of the normal nycti- 
tropic period to the extent of from 4 to 2} hours, the variations seem- 
ing to depend on temperature, on length of exposure to the blue light, 
and on relative intensity of the light for the day.” “ These observations 
seem further to warrant us in concluding that up to 38° C., or even 
43° C. in some species, heat rays either fail to stimulate the tissues, or 
if they do that, their action is interrupted aud antagonized by some 
other form of energy, though this is scarcely likely. The same is true 
