3!»6 MODIFIED CIKCUMNUTATION. Cbap. VU 



p(itioles are effected in two different ways ; firstly, by 

 alternately increased growth on their opposite sides, 

 preceded by increased turgescence of the cells ; and 

 secondly by means of a pulvinus or aggregate of small 

 cells, generally destitute of chlorophyll, which become 

 alternately more turgescent on nearly opposite sides ; 

 and this turgescence is not followed by growth except 

 during the early age of the plant. A pulvinus seems 

 to be formed (as formerly shown) by a group of cells 

 ceasing to grow at a very early age, and therefore does 

 not differ essentially from the surrounding tissues. 

 The cotyledons of some species of Trifolium are pro- 

 vided with a pulvinus, and others are destitute of one, 

 and so it is with the leaves in the genus Sida. We 

 see also in this same genus gradations in the state of 

 the development of the pulvinus ; and in Nicotiana 

 we have what may probably be considered as the 

 commencing development of one. The nature of the 

 movement is closely similar, whether a pulvinus is 

 absent or present, as is evident from many of the 

 diagrams given in this chapter. It deserves notice 

 that when a pulvinus is present, the ascending and 

 descending lines hardly ever coincide, so that ellipses 

 are habitually described by the leaves thus provided, 

 whether they are young or so old as to have quite 

 ceased growing. This fact of ellipses being described, 

 shows that the alternately increased turgescence of 

 the cells does not occur on exactly opposite sides of the 

 pulvinus, any more than the increased growth which 

 causes the movements of leaves not furnished with 

 pulvini. When a pulvinus is present, the nyctitropic 

 movements are continued for a very much longer 

 period than when such do not exist. This has been 

 amply proved in the case of cotyledons, and Pfeffei 

 has given observations to the same effect with respect 



