8(54 MODIFIED CIECUMNUTATION CHAP. VII 



sequent appearance, may be attributed to reversion to more 01 

 less distant progenitors.* 



. No one supposes that the rapid movements of the lateral 

 leaflets of D. yyrans are of any use to the plant; and why 

 they should behave in this 1 manner is quite unknown. Wo 

 imagined that their power of movement might stand in somo 

 relation with their rudimentary condition, and therefore ob- 

 served the almost rudimentary leaflets of Mimosa albida vel 

 scnsitiva (of which a drawing will hereafter be given, Fig. 159); 

 but they exhibited no extraordinary movements, and at night 

 they went to sleep like the full-sized leaflets. There is, how- 

 ever, this remarkable difference in the two cases ; in Dcsmo- 

 dium the pulvinus of the rudimentary leaflets has not been 

 reduced in length, in correspondence with the reduction of the 

 blade, to the same extent as has occurred in the Mimosa ; and it 

 is on the length and degree of curvature of the pulvinus that the 

 amount of movement of the blade depends. Thus, the average 

 length of the pulvinus in the large terminal leaflets of Desmo- 

 diuni is 3 mm., whilst that of the rudimentary leaflets is 2 - 86 mm. ; 

 BO that they differ only a little in length. But in diameter they 

 differ much, that of the pulvinus of the little leaflets being only 

 0'3 mm. to 0'4 mm.; whilst fhat of the terminal leaflets is 

 I "33 mm. If we now turn to the Mimosa, we iind that the 

 average length of the pulvinus of the almost rudimentary 

 leaflets is only 0'466 mm., or rather more than a quarter of the 

 length of the pulvinus of the fall-sized leaflets, namely, 1 66 mm. 

 In this small reduction in length of the pulvinus of the rudi- 

 mentary leaflets of Desmodium, we apparently have the proxi- 

 mate cause of their great and rapid circumnutating movement, 

 in contrast with that of the almost rudimentary leaflets of the 

 Mimosa. The small size and weight of the blade, and the little 

 resistance opposed by the air to its movement, no doubt also come 

 into play ; for we have seen that these leaflets if immersed in 

 water, when the resistance would be much greater, were pre- 

 vented from jerking forwards. Why, during the reduction of 

 the. lateral leaflets of Desmodium, or during their reappearance 

 T-if they owe their origin to reversion the pulvinus should 

 have been so much less affected than the blade, whilst with the 



* Desmodium rfizpertilionis is rudimentary lateral leaflets. Du- 

 closely allied to D. (ji/rant>, and chart re, ' Kle'mentsde Botanique, 

 it seems only occasionally to bear 1SG7, p. 3oo. 



