114 PULVINI OF COTYLEDONS. CHAP. II. 



without such aid, is reduced to the expansion of the 

 cells not being followed by growth in the first case, 

 and being so followed in the second case. 



Dots were made with Indian ink along the midrib 

 of both pulvinated cotyledons of a rather old seedling 

 of Oxalis VdLdiviana ; their distances were repeatedly 

 measured with an eye-piece micrometer during 8| days, 

 and they did not exhibit the least trace of increase. 

 It is therefore almost certain that the pulvinus itself 

 was not then growing. Nevertheless, during this 

 whole time and for ten days afterwards, these coty- 

 ledons rose vertically every night. In the case of 

 some seedlings raised from seeds purchased under the 

 name of Oxalis floribunda, the cotyledons continued 

 for a long time to move vertically down at night, and 

 the movement apparently depended exclusively on 

 the pulvini, for their petioles were of nearly the same 

 length in young, and in old seedlings which had pro- 

 duced true leaves. With some species of Cassia, on 

 the other hand, it was obvious without any measure- 

 ment that the pulvinated cotyledons continued to 

 increase greatly in length during some weeks ; so that 

 here the expansion of the cells of the pulvini and the 

 growth of the petiole were probably combined in 

 causing their prolonged periodic movements. It was 

 equally evident that the cotyledons of many plants, 

 not provided with pulvini, increased rapidly in length ; 

 and their periodic movements no doubt were exclu- 

 sively due to growth. 



In accordance with the view that the periodic 

 movements of all cotyledons depend primarily on the 

 expansion of the cells, whether or not followed by 

 growth, we can understand the fact that there is but 

 little difference in the kind or form of movement 

 in the two sets of cases. This may be seen by com- 



