476 PLANT RESPONSE 



latent component. In a sub-tonic condition, on the other 

 hand, a greater proportion of the stimulus is temporarily held 

 latent, and expresses itself as the negative after-effect, the 

 direct responses being here correspondingly diminished. 



Above the optimum there is no latent component, and the 

 general receptivity of the organ shows great diminution. 



From the constancy of the sum of the direct and indirect 

 effects it is demonstrated that, with regard to some forms of 

 response at least, response is not disproportionately greater 

 than stimulus. Thus the theory that response must always 

 be due to an explosive chemical change does not hold good. 



The curve showing the relation between stimulus and 

 response is appropriately modified by the tonic condition of 

 the tissue. 



As each stimulus of every form thus finds expression in 

 direct response, direct after-effect, and indirect after-effect, 

 and as there are many forms of stimulus which under natural 

 conditions act on the plant, whose maxima, instead of being 

 coincident, are superposed on each other, in varying differ- 

 ences of phase, highly complex periodicities are induced, and 

 find expression in the various forms of plant response. 



Among such varying factors of stimulation may be 

 mentioned the diurnal alternation of light, temperature, 

 chemical stimulus, and varying turgescence. 



The induced periodicities which result from the conditions 

 described may be seen in the periodic groupings which 

 appear in a continuous record of the autonomous pulsations of 

 Desmodium for example. 



The autonomous response of growth, as the result of the 

 periodically acting stimuli mentioned, exhibits not only a 

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

 a number of sub-waves. But the impression made on the 

 organism by the diurnal period is the deepest of these, and 

 tends in an old plant to subordinate all others in a marked 

 degree. In the responses of a seedling a few days old, how- 

 ever, the minor waves arc very distinct. 



