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Show I 1 41G STRUC'l'URE OF CHAP. VII. as an embryological habit, probably the result of Melilotus being descended from some form which slept like a Trifolium. This view is partially supported by the leaves on old and young branches of another species, M. Messanensis (not included in the above 15 species), always sleeping like those of a Trifolium. The first true leaf of Mimosa albida consists of a simple petiole, often bearing three pairs of leaflets, all of w hieh are of nearly equal size and of tho same shape: the second leaf differs widely from the first, and resembles that on a mature plant (sec Fig. 159, p. 379), for it consists of two pinnre, each of which bears two pairs of leaflets, of which the inner basal one is very small. But at the base of each pinna there is a pair of minute points, evidently rudiments of leaflets, for they are of unequal sizes, like the two succeeding leaflets. These rudiments arc in one sense embryological, for they exist only during the youth of the leaf, falling off and disappearing as soon as it is ft1lly grown. vVith Desmodium gyrans the two latontl leaflets are very much smaller than tho corresponfling leaflets in most of the species in this largo genus ; they vary also in position and size ; one or both are sometimes absent; and they do not sloop like tho fully-developed leaflets. They rna y therefore be considered as nlm?st rudimentary; and in accordance with the genentllmn· ciples of embryology, they ought to be more constautly and fully developed on very young than on old plants. But this is not the case, for they wore qnite abse~t on some young seedlings, and did not app,em: unti! from 10 to 20 leaves had been formed. rJus fac leads to the suspicion that D. gyrans is clesconcleu through a unifoliate form (of which some exist) from a trifoliate species · and that the little lateral leaflets reappear through r' eversion. However thi·s may be ' CHAP. VII. FIRST-FORMED LEAVES. 417 the interesting fact of the pulvini or organs of movement of these little leaflets, not having been reduced nearly so much as their blades-taking the lar0'p terminal leaflet as t~e standard of comparison-giv~s us probably the proximate cause of their extraordinary power of gyration. 0 E |