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Show 380 l\lODIFIED OIRCUl\INU'r ATION. CHAr. vn. basal leaflets are greatly reduced in size, owing probably to tho want of space for their full dovelopmeDt, so that they may be considered as almost rudimentary. They vary somewhat in size, and both occasionally disappear, or only one. Nevertheless, 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. pttdica tho inner leaflets at the base and between the pinnro are likewise much shortened and obliquely truncated ; this fact was well seen in some seedlings of 111/.. puriica, in which the third leaf abovo tho cotyledons bore only two pinnro, 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 resembl ed pretty closely that of J.lf. albida. In this latter species the main petiole terminates 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 tho leaf is fully developed. There can hardly be a doubt that those little projections are the last and fugacious representrttives 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. rlo of nn inch, whilst the inner one is only 1~ long. Now if the basal pair of leaflets of the existing leaves were to become rudimentary, we should expect that the rudiments would still exhibit some trace of their present great inequality of size. Tho conclusion that the pinnrn of the parent-form of 111. albid1t po sesscd at least throe pairs of leaflets, instead of, as at present, only two, is suppoL'ted by the structure of the first true loaf; for this consists of a simple petiole, often bearing throe pairs of leaflets. This latter fact, as well as tho presence of the rudiments, both lead to the conclusion that 111. 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 close contact with its· fellow. The pinnro 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 tho outside of the united terminal leaflets, and thus all eight leaflets CHAP. VII. SLEEP OF LEAVES. 381 (the rudimentary ones included) form together a single vertical packet. The two pinnre at the same. time that they approach each other sink downwards, and thus mstead of extending horizontally in the same line with the main petiole, as during the day, they depend at night at about 45°, or even at a greater angle, beneath the horizon. The movement of the main petiole seems to be variable; we have seen it in the evening 27° lower than during the day; but sometimes in nearly the same position. Nevertheless, a sinking movement in the evening and a rising one during the night is probably tho normal course, for this was well-marked in the petiole of tho first-formed true leaf. The circumnutation of tho main petiole of a young leaf was traced during 2:1 days, and was considerable in extent, but less complex than that of M. pudica. The movement was much more lateral than is usual with circumnutating leaves, and this was the sole peculiarity which it presented. Tho apex of one of the terminal leaflets was seen under the microscope to travel to of an inch in 3 minutes. Mitrwsrt marginata.-The opposite l~aflot:-> riRo up an1l approach each other at night, but do not come into close contact, except in the case of very young leaflets on vigorous shoots. Full-grown leaflets circumnutate during the day slowly and on a small scale. Schrankia unci?1ata (Tribe 20).-A leaf consists of two or three pairs of pinnm, each bearing many small leaflets. These, when theplantisaslcep, are directed fmwards and become imbricated. T~e angle between the two terminal pinnro was diminished at mght, in one case by 15°; and they sank almost vertically downwards. The hinder pail's of pinnro likewise siuk downwards, but do ~ot eo~ verge, that is, move towards the apex of tho leaf. The ~am petwle does not become depressed, at least during the e~enmg. In t~s latter respect, as well as in the sinking of the pmnro, t~ere IS a marked difference between the nyctitropic movements of the present plant and of 111 inwsa pudica. It ~~o:ld, howe~e:, be added ~hat our spocime~ was not in a very g .rous cond1t10n. The pmnre of &Mankta aculeuta also sink at mght. .Acacia Farnesiana (Tribe 22).-Tb.e different appearance pro~ nted by a bush of this plant when asleep and awake is wonfi erful. The fiiame leaf in the two states is shown in the folio wino· ~e (Fig. 160). 'l'he leaflets move towards the apex of th~ P:~na a~d become imbricated, and the pinnrn then look like bits 0 angling string. The following remarks and measurements |
