NYCTITROPISM 505 



Desmodium gyrans (Fig. 158, I and //) ; in the case of Phyllanthus, torsion 

 occurs as well as curvature, so that in the night position the leaflets are folded 

 together in pairs with their upper surfaces in contact with each other. Amidst 

 all these variations, which the citation of further examples would only add to 

 (Hansgirg, 1893), one general fact comes out, namely, that in the night position 

 the leaf surfaces are vertical. 



It may be easily shown, say in the case of Phaseolus or Acacia lophantha, 

 that darkening is the cause of the sleep position, and light the cause of the day 

 position, but the reactions, for reasons which we shall learn later on, are not 

 manifested equally well at all hours of the day ; but if the plant be placed in 

 the dark in the afternoon and illuminated in the forenoon we always obtain a 

 very rapid result. Under all conditions, every movement of this kind is followed 

 by a reverse movement later on. 



As is to be expected, sensitivity to changes in temperature is also manifested 

 by the articulations of motile leaves, although it cannot be demonstrated in all 

 cases with equal ease. There is no difficulty in demonstrating movements 

 resulting from changes in temperature in such leaves as those of Mimosa, Acacia 

 lophantha, and Phaseolus multi/lorus, which react well when light is excluded, and 

 it may be easily shown (Jost, 1898) that an increase in temperature operates in 

 the same way as an increase of light, and that cooling has the same effect as 

 darkening. The case, however, is rendered more complicated in consequence of 

 the fact that a rapid rise of temperature, dependent perhaps as well on the abso- 

 lute height of the temperature, also brings about closure. The question now 

 comes to be, whether this closure, due to a rapid rise of temperature, is really 

 identical with the closure due to cooling or darkening, or whether it only resembles 

 it superficially. That the latter must be the correct view would appear to be 

 proved by the behaviour of plants like Rohinia and Phaseolus, whose leaves, when 

 subjected to a considerable rise in temperature, do not reach the sleep position, 

 but go on rising until they succeed in orientating themselves in the same way 

 as they do under the influence of too intense light. At p. 465 we have drawn 

 special attention to this point in the case of Rohinia, and described it as a helio- 

 tropic reaction, but we must not omit to mention here that frequently this 

 ' profile ' position occurring in Rohinia in the open does not exhibit that relation- 

 ship to light which, were it heliotropic, it must have ; hence we conclude that 

 the profile position of Rohinia may perhaps be in many cases not a heliotropic 

 reaction at all ; it must, on the contrary, be quite independent of the direction 

 of light, and may be caused only by too high a temperature or too intense light. 

 The closure of many flowers may also be the result of too intense light, and to 

 Oltmanns (1895) we owe the demonstration of the fact that one and the same 

 flower (e. g. of Lactuca), after being made to open by illumination, may after 

 a time be made to close again by increasing the light intensity as well as by 

 darkening. The condition brought about by too great an intensity of heat or 

 light is also known as ' day-sleep '. 



If we now inquire into the mechanical causes of nyctitropic movements in 

 articulations we discover first of all that the curvature is effected without any 

 growth taking place, since after having performed two opposite motions the 

 articulation has not elongated (Pfeffer, 1875). We have here to deal, therefore, 

 not with nutation movements but with variation movements ; as a matter of 

 fact, the expansive efforts of the convex side must be due to alterations in 

 turgidity. In young pulvini undoubtedly growth also occurs. An elongation 

 in one-half of the articulation owing to osmotic activity might arise just 

 as well by increase of turgor pressure as by a decrease in the rigidity of the 

 cell-wall. Omitting for the moment alterations in the elasticity of the mem- 

 brane as being the less probable cause, curvature of the articulation might take 

 place either (i) when the turgor pressure increases in the convex half of the 



