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1102 The American Naturalist. [December 
of the less refrangible light rays, and of these the orange-yellow, yellow 
and yellow-green seem to give the most uniform results, for so long as 
plants were exposed to intense light the leaflets remained either quite 
flat or became slightly reflexed. Under the green screen the leaflets 
of Cassia nictitans and O. chamecrista, when strongly illuminated re- 
mained flat or became inflexed in.some cases to 25°, but those of C. 
Tora under equal illumination inflexed through an angle of 15° ; those 
of Oxalis stricta remained flat. The paraheliotropic movement 
thus started under the green screen in some species became greatly 
more pronounced under the blue in all, and during intense illumina- 
tion in Ozxalis almost amounted to the nyctitropic position. Grouping 
the above facts, the conclusion is reached that the heat rays, the less 
refrangible rays, and the more refrangible rays are all trophic up to a 
certain point. When that point is crossed the heat rays and less re- 
frangible rays continue to be trophic up to a much higher point, but 
the more refrangible rays (from green-blue to violet) act as a stimu- 
lant or irritant.’ “It may be worth emphasizing here that sensitive 
movements are most pronounced in tropical plants, are less so in sub- 
tropical and warm-temperate species, and are rare or feebly expressed 
in temperate and sub-arctic plants. But, as is well known, leaves that 
are exposed to an intense light show more rapid metabolic changes than 
those that are shaded. Any change, therefore, in the tissues of a plant 
which would insure protection of the lamina from the intense blue-vio. 
let rays, and its exposure again when these rays become subdued, 
would have every likelihood of perpetuation in sub-tropical and tropi- 
cal regions, and such is the state of matters as we find them. We do not 
» know accurately, as yet, the mechanism involved in a sensitive pulvi- 
nus, or the changes effected on stimulation of it, but anyone can readily 
prove that every gradation from non-sensitive to highly sensitive 
leaves is met with in such groups as the Oxalidee and Leguminose, 
and that, broadly speaking, the sensitiveness increases as we pass from 
regions where the sun’s rays are of minor intensity to others where the 
rays are of increased intensity. The writer, therefore, regards the ac- 
tion of the more refrangible rays, when of a definite intensity, as one of 
stimulus, because (1) the angular inflection of leaflets is proportionate 
to the intensity of the stimulating rays; (2) the movement is not due 
to indirect action from the green laminar substance to the pulvinus 
cells, but is wholly centered in the latter; (3) ifthe inflection movement 
is considerable, the white cushion of the pulvinus shows a visible change 
from white to a dull leaden green:color ; (4) when the more refrangi- 
ble rays are cut off by a color screen the stimulus is removed, and then 
