5/0 LECTURE XXI. 



taking place simultaneously in the cells of its two opposed 

 halves. 



The nyctitropic movement of the leaflets of Mimosa has 

 been described (p. 541) as a folding upwards and forwards in 

 pairs at night. The state of the pulvinus of the leaflet in the 

 nocturnal position differs from that of the pulvinus of the 

 primary petiole, in that the former has lost its power of 

 movement on stimulation whereas the latter retains it. The 

 cells of the upper irritable half of the pulvinus of the leaflet 

 pass at night into just the same condition as that which is 

 induced by a stimulus ; they are flaccid. Whether or not 

 this flaccidity of the cells of the upper half of the pulvinus is 

 accompanied by an increase in the turgidity of the cells of 

 the lower half, we do not know ; but in any case the me- 

 chanism of the movement is this, that, in the absence of any 

 opposition from the upper half of the pulvinus, the leaflet is 

 raised by the upward pressure of the turgid cells of the lower 

 half. A further peculiarity in the behaviour of the pulvinus 

 of the leaflet is this, that this relative condition of the two 

 opposed halves is maintained until morning, when the tur- 

 gidity of the cells of the upper half is restored. 



The mechanism of the daily periodic movements of motile 

 organs may be briefly stated as variations in the relative 

 degree of turgidity of the two opposed halves of the pulvinus, 

 variations which, in accordance with the conclusions pre- 

 viously arrived at, we ascribe to variations in the molecular 

 structure of the protoplasm of the cells. These variations, 

 moreover, are rhythmical, and are induced by the normal 

 alternation of day and night. In endeavouring to explain 

 the influence of the alternation of day and night in inducing 

 these variations, we must be careful not to regard the effect 

 as immediate. For instance, we must not attempt to explain 

 the movements of the leaflets of Mimosa by assuming that 

 the nocturnal flaccidity of the cells of the upper half of the 

 pulvinus is the direct effect of darkness, and that the diurnal 

 turgidity of these cells is the direct effect of light, on their 

 protoplasm. It is the leaflet as a whole which is affected by 

 the variations in the intensity of light to which it is highly 



