DETECTION OF LATENT STIMULUS 469 



(3) Chemical Stimulus. — This we see supplied by the salts 

 taken up by the roots, and by the process of photo-synthesis 

 in the leaves. The amount of the former supply is depen- 

 dent not only on the richness of the soil, but also on the 

 suctional activity of the plant ; and the latter on the effective 

 intensity of light. 



(4) Turgidity. — The turgid condition of the plant will 

 depend, firstly, on the supply of water ; secondly, on the 

 suctional activity ; and thirdly, on the relative absence of a 

 loss of water by transpiration. We have seen how watering 

 the roots of the plant will cause an immediate response of 

 enhanced growth, and the seasonal periodicity induced by 

 this cause may be observed in a very striking manner in 

 tropical countries. 



The suctional activity, depending as it does on the 

 internal energy of the plant, will tend to be augmented by 

 an advantageous temperature, and by the after-effect of the 

 absorbed stimulus of light ; but the turgid condition will be 

 reduced by active transpiration, which is relatively greater 

 in the daytime. 



We thus see how numerous are the factors which co- 

 operate to bring about periodic fluctuations in the rate of 

 growth during every twenty-four hours. Of all these factors, 

 the alternation of day and night is the most pronounced in 

 its action. The curve of growth, then, will exhibit not only 

 a large wave of alternation due to the diurnal period, but also 

 a number of sub-waves. And even beyond these, superposed 

 upon them, we may expect, when the magnification is suffi- 

 ciently great, to observe systems of still smaller wavelets, 

 caused by the rhythmicity of growth. 



Continuous photographic record of the pulsations 

 of Desmodium. — Before describing the periodic diurnal 

 variation of that autonomous response which we know as 

 growth, it occurred to me that a continuous record of another 

 form of autonomous response — that is to say, of the rhythmic 

 pulsations of Desmodium — might prove interesting. I was 

 fortunate enough to succeed in obtaining a very good record 



