PLANT-AUTOGRAPHS—BOSE. 429 
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 sensi- 
bility—going, as it were, to sleep? On these points no definite 
information has been available. The fanciful name of sleep is often 
given to the closure of leaflets of certain plants during darkness. 
These movements are brought about by variation of turgor, and 
Fig. 6.—Record for twenty-four hours, exhibiting diurnal variation of excitability 
(spring specimen). The displacements of base line are due to nyctitropic move- 
ments. 
have nothing whatever to do with true sleep; for similar closure of 
leaflets takes place under the precisely opposite condition 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 and night. The amplitude of 
the answering twitch gave a measure of the ‘‘wakefulness” of the 
plant during 24 hours. The results obtained were quite unexpected. 
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 grad- 
ually waking up by noon. (Fig. 6.) It then remains in a condition 
of uniform sensibility all the afternoon. This period of uniformity 
is chosen for investigations on the effect of changed external condi- 
tions on excitability. 
EFFECT OF LIGHT AND TEMPERATURE. 
Does the plant feel the depressing effect of darkness? The fol- 
lowing record shows the effect of a passing cloud. (Fig. 7.) It is 
