n6 MOVEMENTS OF CURVATURE 



of the thermonastic response such as is shown by photonastic organs 

 exposed to increasing illumination. A reversal of this kind does actually 

 appear to occur in the flowers of Crocus l , and possibly also in the leaves of 

 Oxalis when the temperature rises above the optimum. It is, in fact, not 

 impossible that thermonastic reactions of this kind may play a more or less 

 prominent part in the assumption of the midday sleep-positions of the 

 leaves of so many tropical plants. 



SECTION 25. Hydronastic Movements. 



As is well known, changes in the percentage of water in plants com- 

 monly cause disturbances of equilibrium leading to movement. Thus the 

 diminished rigidity due to a decrease of turgidity leads to the flaccid 

 drooping of shoots and leaves, and in the case of dorsiventral organs 

 changes of turgidity which affect the antagonistic tissues unequally may 

 cause curvature. In general, the percentage and supply of water form 

 physiological conditions whose modification affects the activity of growth 

 and the power of response in much the same way that changes of tem- 

 perature do. Granted an appropriate structure, changes in the hydric 

 relationships may even induce hydronastic curvatures, as physiological 

 stimulatory reactions which are widely distinct in character from the purely 

 physical movements mentioned above. It is of course always possible that 

 the same loss of turgidity which primarily produces a drooping movement 

 may also act as a stimulus to a physiological curvature of like or of unlike 

 kind. Furthermore, this curvature may either result from a modification of 

 growth or may be a variation movement due to appropriate changes of 

 turgor induced in response to stimulation. 



The use of the term hydronasty to denote curvatures produced by 

 changes in the hydric relationships says nothing as to the nature of this 

 form of irritability or as to the mode of perception of stimuli. The hydric 

 relationships may, however, exercise various stimulatory actions on growth, 

 so that hydronastic responses may be of varied origin. Hitherto the 

 researches have mainly been confined to determining the existence of such 

 reactions, and frequently no proper discrimination has been made between 

 the physical and physiological responses. Hence only a general account of 

 the phenomena observed and their distribution is possible at present. 

 Indeed, it will always be difficult to determine whether, in a given case, the 

 actual excitation is due to a fall of turgor, to a movement of water with or 

 without transpiration, to changes of consistency, or to other factors 2 . 



1 Cf. Pfeffer, Physiol. Unters., 1873, p. 190; Jost, Jahrb. f. wiss. Bot, 1898, Bd. XXXI, 



PP- 35 1 . 35 8 - 



3 Since these movements are not solely produced in response to changes of turgor, the general 



