On Diurnal Variation of Moto-excitability in Mimosa. 
BY 
J. C. BOSE, M.A., D.Sc., C.S.I. 
Professor , Presidency College , Calcutta. 
With seventeen Figures in the Text. 
S EVERAL phenomena of daily periodicity are known, but the relations 
between the recurrent external changes and the resulting periodic 
variations are more or less obscure. As an example of this may be cited the 
periodic variation of growth. Here the daily periodicity exhibited by a 
plant is not only different in varying seasons, but it also differs in diverse 
species of plants. The complexity of the problem is very great, for not 
only are the direct effects of the changing environment to be taken into 
consideration, but also their unknown after-effects. Even in the case of 
direct effect, different factors, such as light, temperature, turgor, and so on, 
are undergoing independent variations ; it may thus happen that their 
reactions may sometimes be concordant and at other times discordant. 
The nyctitropic movement of plants affords another example of daily 
periodicity. The fanciful name of ‘ sleep 5 is often given to the closure of 
the leaflets of certain plants at night. The question whether plants sleep or 
not may be put in the form of the definite inquiry : Is the plant equally 
excitable throughout day and night ? If not, is there any definite period at 
which it practically loses 1 its excitability ? Is there, again, another period 
at which the plant wakes up, as it were, to a condition of maximum 
excitability ? 
In the course of my investigations on the irritability of Mimosa pudica, 
I became aware of the existence of such a daily periodicity ; that is to say, 
the moto-excitability was found to be markedly diminished or even completely 
abolished at a certain definite period of the day ; at another equally definite 
period, the excitability was observed to have attained its climax. The 
observations on the periodic variation of excitability appeared at first to be 
extremely puzzling. It might be thought, for example, that light would 
prove to be favourable for moto-excitability ; in actual experiment the 
results apparently contradicted such a supposition : for the excitability of 
the plant was found much higher in the evening than in the morning. 
Favourable temperature, again, might be regarded as an important factor 
[Annals of Botany, Vol. XXVII. No. CVIII. October, 1913.] 
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