380 MODIFIED CIECUMNUTATION. CHAP. VII. 



basal leaflets are greatly reduced in size, owing probably to the 

 want of space for their full development, so that they may be 

 considered as almost rudimentary. They vary somewhat in 

 size, and both occasionally disappear, or only one. Neverthe- 

 less, they are not in. the least rudimentary in function, for they 

 are sensitive, extremely heliotropic, circumnutate at nearly the 

 same rate as the fully developed leaflets, and assume when 

 asleep exactly the same position. With M. pudica the inner 

 leaflets at the base and between the pinnae are likewise much 

 shortened and obliquely truncated ; this fact was well seen in 

 some seedlings of M. pudica, in which the third leaf above the 

 cotyledons bore only two pinnse, each with only 3 or 4 pairs of 

 leaflets, of which the inner basal one was less than half as long 

 as its fellow; so that the whole leaf resembled pretty closely 

 that of M. albida. In this latter species the main petiole termi- 

 nates in a little point, and on each side of this there is a pair 

 of minute, flattened, lancet-shaped projections, hairy on their 

 margins, which drop off and disappear soon after the leaf is 

 fully developed. There can hardly be a doubt that these littlo 

 projections are the last and fugacious representatives of an 

 additional pair of leaflets to each pinna; for the outer one is 

 twice as broad as the inner one, and a little longer, viz. y^ of an 

 inch, whilst the inner one is only f ^- long. Now if the basal 

 pair of leaflets of the existing leaves were to become rudimen- 

 tary, we should expect that the rudiments would still exhibit 

 some trace of their present great inequality of size. The con- 

 clusion that the pinnse of the parent-form of M. albida possessed 

 at least three pairs of leaflets, instead of, as at present, only two, 

 - is supported by the structure of the first true leaf; for this 

 consists of a simple petiole, often bearing three pairs of leaflets. 

 This latter fact, as well as the presence of the rudiments, both 

 lead to the conclusion that M. albida is descended from a form 

 the leaves of which bore more than two pairs of leaflets. The 

 second leaf above the cotyledons resembles in all respects the 

 leaves on fully developed plants. 



When the leaves go to sleep, each leaflet twists half round, 

 so as to present its edge to the zenith, and comes into dose 

 contact with its fellow. The pinnae also approach each other 

 closely, so that the four terminal leaflets come together. The 

 large basal leaflets (with the little rudimentary ones in contact 

 with them) move inwards and forwards, so as to embrace the 

 outside of the united terminal leaflets, and thus all eight leaflets 



