Juty 23, 1914] 
ing the best mode of applying quantitative stimulus 
to the plant an interesting discovery was made about 
the extreme sensitiveness of certain plants to the 
stimulus of electric current. The most sensitive organ 
by which an electric current can be detected is our 
= 
SHARAN 
MUAH 
ae py 
SSS 
OQ’ 
we 
SSS 
SS 
WS 
RAAAQVW 
Fic, 2.—Upper part of resonant recorder (from a photograph), Thread from 
clock (not shown) passes over pulley P, letting down recording plate. 
S’, screw for adjustment of distance of writing-point from recording 
plate. S, screw for vertical adjustment. T, tangent screw fur exact 
adjustment of plane of movement of recorder, parallel to writing-surface. 
V, Axis of writer supported perpendicularly at. centre of circular end of 
magnet. C,coercer. M, micrometer screw for adjustment of length of 
coercer. 
tongue. An average European, according to Laser- 
stein, can perceive by his tongue a current as feeble 
as 6-4 microamperes—a microampere being  one- 
millionth part of the unit of current. This value 
might be subject to certain variation, depending on 
racial characteristics. One might expect that the 
tongue of the Celt would be far more excitable than 
Ure wir’ 
Fic. 3.—Hourly record for twenty-four hours, exhibiting diurnal variation of excitability 
(spring specimen). 
that of the stolid Anglo-Saxon. In any case, the 
superiority of man has to be established on founda- 
tions more secure than sensibility; for the plant 
Biophytum, I find, is eight times more sensitive to 
an electrical current than a human being. With re- 
gard to the stimulus of induction shock, Mimosa is 
ten times as sensitive. 
NO. 2334, VOL. 93| 
NATURE 
The Sleep of Plants. 
In studying the effect of a given change in the 
external condition, an assumption has to be made that 
during the time of experiment there has been no 
spontaneous variation of excitability. Is the plant 
equally excitable throughout day and night? If not, 
is there any particular period at which the excitability 
remains uniform? Is there again a different time 
during which the plant loses its sensibility—going, as 
it were, to sleep? 
formation has been available. 
On these points no definite in- 
The fanciful name of 
Fic. 4.—Effect of carbonic acid gas. 
sleep is often given to the closure of leaflets of certain 
plants during darkness. These movements are brought 
about by variation of turgor, and have nothing what- 
ever to do with true sleep; for similar closure of 
leaflets takes place under the precisely opposite con- 
dition of strong light. 
In order to find out whether Mimosa exhibits diurnal 
variations of sensibility, I made it record its answer 
to uniform questioning shocks, repeated every hour 
of the day or night. The amplitude of the answering 
" twitch gave a measure of the ‘‘ wakeful- 
ness’? of the plant during twenty-four 
hours. The results obtained were quite un- 
expected. The plant is found to keep up 
very late, and fall asleep only at the early 
hours of the morning. It makes up for its 
late hours by gradually waking up by noon 
(Fig. 3). It then remains in a condition 
of uniform sensibility all the afterncon. 
This period of uniformity is chosen for in- 
vestigations on the effect of changed ex- 
ternal conditions on excitability. 
Effect of Air, Food, and Drugs. 
The plant is intensely susceptible to the 
impurities present in the air. The vitiated 
air of the town has a very depressing effect. 
According to popular science, what is death 
to the animal is supposed to be life for the 
plant; for does it not flourish in 
the deadly atmosphere of carbonic 
acid The record (Fig. 4) shows that, 
instead of flourishing, the plant gets suffocated 
just like a human being. Note the gasp of relief 
when fresh air is introduced. Only in the presence 
of sunlight is this effect modified by photosynthesis. 
In contrast to the effect of carbonic acid, ozone renders 
the plant highly excitable. Sulphuretted hydrogen, 
gas? 
