NYCTITROPIC MOVEMENTS 677 



period of autonomous vibration is much modified, again, by 

 rise or fall of temperature, and other conditions ; whereas the 

 daily period is unaffected by these circumstances. This is 

 due to the fact that the autonomous pulsations are akin to 

 free or natural vibrations, whereas the daily periodic move- 

 ments are, as we shall see, of the nature of forced vibra- 

 tions. 



As the nyctitropic movement in the primary petiole of 

 Mimosa is of a simple character, free from those other 

 complicating considerations which have to be taken into 

 account in the case of the leaflets, this organ has been chosen 

 as the typical specimen by many investigators. We shall 

 therefore consider in detail all its peculiarities in regard to 

 this movement, but I shall at the same time endeavour by 

 the use of the comparative method to trace the evolution of 

 the movement, as first seen in plagiotropic stems, more clearly 

 manifested in certain dorsi-ventral organs like ordinary leaves, 

 and strongly exhibited in pulvinated leaves, such as those of 

 Mimosa. 



Supposed distinction between nyctitropic and other 

 movements of response to stimulus of light. — This nycti- 

 tropic movement has been sharply distinguished from ordinary 

 response to the stimulation of light, by the statement that 

 while the latter depends on the direction, this is determined 

 only by the varying intensity of illumination. As a further 

 distinction, it is also insisted that, whereas the responsive 

 curvature induced by light takes place in all directions, 

 nyctitropic movement always occurs in a single definite plane. 



But whenever we subject the pulvinus of Mimosa to the 

 action of strong light, we obtain response by a greater or 

 smaller depression or fall, which occurs in a definite vertical 

 plane, and is precisely the same in character as that which is 

 exhibited slowly during the course of the whole day, and 

 attains its maximum in the evening. This response to strong 

 light, moreover, is always the same, on whichever side of 

 the pulvinus the stimulus may act — that is to say, it is 

 independent of the direction of light. In analysing this case 



