CONJOINT EFFECTS 121 



reason leaflets folded in sunlight re-expand at first in darkness before they show 

 a nyctinastic response. The leaves of Acacia, Dalbergia, and Robinia. require 

 at least two to three minutes to fold together in sunlight, and may fully expand after 

 being shaded for five to eight minutes. The leaves of Bauhinia, Albizzia, Calli- 

 andra, and Cassia respond still more slowly, the closure requiring five minutes 

 to half an hour, and the re-expansion ten minutes to two hours *.] 



A thermonastic response is also only possible when the change of temperature 

 persists for some time and, since in all cases we are dealing with phenomena of 

 irritability, the extent of the reaction always depends upon the power of perception. 

 It is clear, however, that the time required to produce the maximal movement of 

 a pulvinus will depend upon the rapidity with which a change of turgor follows 

 a change of illumination. Furthermore, in cases where the movement is due to 

 growth, rapidly repeated intense stimulation may produce a certain fatigue effect, 

 such as appears to be shown by the flowers of Crocus after several responses to 

 thermonastic stimulation 2 . 



It is, however, not certain whether the power of photonastic reaction is affected 

 by the movement subsequently induced, for other extraneous demands often influence 

 the power of response. Nevertheless, the increased action of darkening after midday 

 appears to be due merely to the co-operation of the photonastic reaction with the 

 induced periodicity. In any case, however, the summation of dissimilar stimuli 

 involves more complex reactions than that due to the frequent repetition of the same 

 stimulus. 



The latent period of stimulation is shorter in the case of parahelionastic 

 responses than of nyctinastic ones. The minimal difference of illumination required 

 to produce a perceptible response varies in different cases, the leaflets of Mimosa 

 pudica being especially sensitive. Increasing stimulation produces increasing 

 responses within certain limits, but the stimulation needs to increase in geometric 

 proportion to produce equal additional increments of response, quite apart from 

 the reversal of the reaction which ensues under intense illumination. 



Further investigation is needed to determine whether increases of illumination 

 or temperature always produce the same amount of response as decreases, and whether 

 the response is equally rapid in both cases. As regards parahelionastic responses, 

 the closure of the leaflets is always more rapidly produced than the expansion under 

 diminished illumination s . The influence of increases and decreases of temperature 

 and of illumination on growth are not equally pronounced, and exposure to light 

 produces a smaller rise of the leaves of Impatient noli-me-tangere than the subsequent 

 fall on darkening. It is, however, always possible that in such cases the leaf had an 

 inherent tendency to curve to one side, which would minimize the induced curvature 

 in the opposite direction. Especially in the case of transitory stimulation, increases 

 of illumination or temperature may exercise effects which differ quantitatively and 

 qualitatively from those produced by similar decreases. Correns 4 found, in fact, 



1 Ewart, Annals of Botany, Vol. xi, 1897, p. 447 seq. 



a Pfeffer, Physiol. Unters., 1873, p. 182. 



3 Cf. Ewart, Annals of Botany, Vol. XI, 1897, p. 447. * Bot. Ztg., 1896, p. 13. 



