124 MOVEMENTS OF CURVATURE 



Although the dependence of the sinking of the primary petiole upon the 

 movement of the secondary petioles has been empirically determined, it does not 

 follow that the fall is directly due to their increased statical moment, which may 

 sometimes increase by as much as one-half. A suddenly increased load does 

 actually cause a perceptible fall of the primary petiole, but there can be no doubt 

 that we are here dealing with a complex physiological reaction. Indeed, in their 

 normal habitat, the primary pulvini of Mimosa are capable of response to mechanical 

 excitation, even when the petiole has reached its lowest nyctinastic position, and 

 when the plant is highly turgid the pulvinus may curve to such an extent as to 

 temporarily bend the leaf back across the stem in a partially inverted position, so that 

 the mechanical moment is considerably increased *. Pfeffer inclines to regard 

 the gradual increase of the mechanical moment as being the stimulus responsible 

 for the fall of the primary petiole, but it might also occur in indirect correlation with 

 the other movements without being directly due to them. In addition, this fall does 

 not always appear to occur 2 , while Schilling has shown that, during the daytime, 

 a load causing an enforced curvature of the main pulvinus of Mimosa excites a 

 reaction tending to the restoration of the original position of equilibrium 8 . 



If leaves capable of sleep- movements are exposed during the day to light coming 

 from one side only, so that the plane of the leaf remains oblique during the day, next 

 morning they may again assume a similar position, even when in darkness 4 . This 

 after-effect may, however, be different in character to those resulting from realized 

 sleep-movements, which, even when mechanically prevented, may lead to after-effects 

 if the plant strives to produce them. 



Photonastic and thermonastic curvatures are not only possible when 

 the required physiological dorsiventrality is of internal origin, but also 

 when it is due to the stable or labile induction of external factors. In 

 the last case, the power of aitionastic curvature is naturally only retained 

 as long as the induction persists, and the reaction is, therefore, rapidly 

 modified when an alteration in the external condition modifies the tone 

 of the organ. Certain negatively geotropic pulvini afford good instances 

 of the modification of the photonastic reaction by labile geotropic in- 



1 Ewart, Annals of Botany, Vol. XI, 1898, p. 453. [The mechanical moment is less in the fully 

 drooping position than in any other. In addition, the mechanical moment may increase during the 

 assumption of the parahelionastic position by as much as it does at the commencement of the nycti- 

 nastic movement without producing any distinct fall of the primary petiole.] 



3 Cunningham, Annals of the Royal Botanical Garden of Calcutta, 1895, Vol. vi, p. 135. [In 

 some cases the fall may take place without any movement of the secondary petioles, and the 

 temperature appears to have some effect. Plants of Mimosa pudica appear rarely to be capable 

 of the same rapidity of response in European hothouses as in their natural habitat, and in addition 

 readily fall into an irresponsive condition although the leaflets may remain green, normal, and capable 

 of photosynthesis.] The mechanical considerations put forward by Schwendener (Ges. bot. Mittheil., 

 1897, Bd. n, p. 238) do not alter the facts in the least. 



3 Schilling, Der Einfluss der Bewegungshemmungen auf die Arbeitsleistungen d. Blattgelenke 

 von Mimosa pudica, 1895. 



* Darwin and Pertz, Proc. of the Phil. Soc., Cambridge, 1900, Vol. x, p. 259; Annals of Botany, 

 1903, Vol. xvu, p. 93. 



