IN VEGETABLE BIOLOGY. 245 
set up in an Elodea-leaf plucked off two hours previously, and 
exposed meanwhile to medium diffused light under a cover-slip 
upon a glass slide. At this time the side of a marked cell 
was traversed by the rotating stream in an average of thirteen 
seconds. The leaf was then brought up to direct sunlight, care 
being taken to prevent rise of temperature by constantly irri- 
gating it with cold water at the temperature of the room. After 
ten minutes’ direct insolation, the side of the cell was still traversed 
in thirteen seconds, thus indicating to all appearance that en- 
hanced illumination does not quicken the rate of movement. But 
the facts may be at least as well explained upon the supposition 
that the indicated rate of movement is the maximum possible at 
the given temperature—a supposition backed by the two follow- 
ing results obtained from Elodea. Both were made on days 
favourable for the purpose, chequered by rapidly alternating high 
and low lights. A day with plenty of wind driving along large 
masses of white and neutral-tinted cumulus cloud over tracts 
of sky occasionally revealed should be selected for this expe- 
riment. 
No. I. 
Side of 
Time. Light. Temperature.| marked ce 
i bi P traversed in 
4.25 p.ı.......| Rather poor, some | 64° F. 3)- 3 ecs. 
heavy clouds just 
passed over. 
4.35 p.u....... Light much better. 64? F. 24 seconds. 
4.45 ».u....... Still further im- 64° F. 20 seconds. 
provement in light. 
4.55 ».u....... Somewhat poorer, 64° F. 25 seconds. 
rather dark clouds 
passing over. 
5.5 pw roved, the 8 S.W.| 649.5 F. | 20 seconds. 
am of p ora 
clouds brightly it 
up. 
x 
LINN. JOURN.—BOTANY, VOL. XXIV. 
