CHAP. II. DISTURBED PERIODIC MOVEMENTS. 123 



with seedlings of Trifolium, Lotus, and Oxalis some of 

 the species have a well-developed pulvinus, and others 

 have none, or one in a rudimentary condition. As the 

 movements caused by the alternate turgescence of 

 the cells in the two halves of a pulvinus, must be 

 largely determined by the extensibility and subse- 

 quent contraction of their walls, we can perhaps under- 

 stand why a large number of small cells will be more 

 efficient than a small number of large cells occupying 

 the same space. As a pulvinus is formed by the 

 arrestment of the growth of its cells, movements de- 

 pendent on their action may be long-continued without 

 any increase in length of the part thus provided; 

 and such long-continued movements seem to be one 

 chief end gained by the development of a pulvinus. 

 Long-continued movement would be impossible in any 

 part, without an inordinate increase in its length, if the 

 turgescence of the cells was always followed by growth. 

 Disturbance of the Periodic Movements of Cotyledons ly 

 Light. The hypocotyls and cotyledons of most seed- 

 ling plants are, as is well known, extremely heliotropic ; 

 but cotyledons, besides being heliotropic, are affected 

 paratonically (to use Sachs' expression) by light p that 

 is, their daily periodic movements are greatly and 

 quickly disturbed by changes in its intensity or by 

 its absence. It is not that they cease to circumnutate 

 in darkness, for in all the many cases observed by us 

 they continued to do so; but the normal order of 

 their movements in relation to the alternations of day 

 and night is much disturbed or quite annulled. This 

 holds good with species the cotyledons of which rise 

 or sink so much at night that they may be said to 

 sleep, as well as with others which rise only a little. 

 But different species are affected in very different 

 degrees by changes in the light. 



