OCR Text |
Show 348 MODIFIED CIRCUMNU'rATION. · CHAP. VII. the early spring from shoots on some cut-down plants in the greenhouse, slept in a totally different manner from the normal one; for the throe lea,flets, insten,d of twi ting on their own axes so as to present their lateml edges to tho zenith, turned upwards and stood vertically with their a,pices pointing to the zenith. They thus assumed nearly tho same position as in the allied genus Trifolium; and on the arne principle that embryological characters reveal the lines of d scunt in the animal kingdom, so the movements of the small leaves in tbe above three species of 1\felilotus, perhaps indicate that this genus is clcscende~ from a form which was closely allied to and slept like a Trifolium. Moreover, there is one specie. , M. rnessanmsis, the leaves of which, on full-grown plants between 2 and 3 feet in height, sleep like the foregoing small loaves and like those of a Trifolium. We were so much surprised at this latter case that, until the flowers and tl.-uit were examinec.l, we thought that the seeds of some Trifolium had been sown by mistake instead of those of a Melilotus. It appears therefore probable that J11. messanensis has either retained or recovered a primordial habit. The circumnutation of a leaf of N. ojficinali:s was traced, the stem being loft free; and the apex of the terminal leaflet described three laterally extended elhp. es, between 8 A.M. and 4 P.M.; after the latter hour the nocturnal twisting movement commenced. It was afterwards ascertn.incd that the above movement was compounded of tho circumnutation of the stem on a small scn,le, of the main petiole which moved most, and of the sub-petiole of the terminal lca,flet. Tho main petiole of a leaf having been secured to a stick, close to the base of the subpetiole of the terminal leaflet, the latter described two small ellipses between 10.30 A.M., anc.l 2 P.M. At 7.15 P.M., after this same leaflet (as well as another) had twisted themselves into their vertical nocturnal position, they began to rise slowly, and continued to do so until 10.35 P.M., after which hour they were no longer observed. As JJ!. messanensis sleeps in an anomalous manner, unlike that of any other species in the genus, the circumnutation of a terminal leaflet, with the stem secured, was traced during two days. On each morning the leaflet fell, until about noon,. ~nd then began to rise very slowly; but on the first day tho l'JS~ng movement was interrupted between 1 and 3 P.M. by the formatiOn of a laterally extended ellipse, and on the second day, at the same time, by two smaller ellipses. The rising movement then CHAP. VII. SLEEP OF LEA YES. 349 recommenced, and became mpid late in the evening, when the leaflet was beginning to go to sloop. 'l'he awaking or sinking movement had already commenced by 6.45 A .M. on both mornings. Trifolium (Tribe 3).-The nyctitropic movements of 11 species were observed, and wore found to be clo ely similar. If we select a leaf of 7~ r·C'pens having an upright petiole, and with the three leaflets expandec.l horizontally, tho two Jatcralleaflt:tl'l will be seen in the evening to twist and approach each oth r until their upper surfaces come into contact. At the same tim' they bend downwards in a plano at right angles to that of their former position, until their midribs form an angle of about 4f1o wit~. the upp~r part of. the petiole. 'l'his p culiar change of JJ?Sitwn reqmres .a considerable amount of tor ion in the pttlVInus. The termmn.l leaflet merely ri c.· up without any twi-t- Fig. l.J.l. A. D. Trifolium t·epens: A, leaf during the day; B, leaf asleep at night. ing, and bends over until it rests on and forms a roof over the edg~ of the now vertical and unit d lateral leaflets. Thus the termmal leaflet always passes through an angle of at least 90° g~nerally of 130° or 140°, and not rarely- as was often observed With T. S1tbte1·raneurn-of 180°. In thi latter case the terminal leaflet stands at night horizontally (as in Fig. 141), with its ~ower surface fully exposed to the zenith. Be ides the difference ~n t~e ~~gles, at which the terminal leaflets stftnd at night in 1 he mdlVlduals of the same specie , the degree to which the a~alleafl.ets approach each other often likewi ·e differs. th e h~ve seen that the cotyledonR of some species and not of 0 ~r~. rise up ver~ically at night. The first true leaf is gcnemlly ~ll 0 Iat~ and orbicular; it n.lways rises, and either stands vertith ~at lllght or more ?ommonly hends a little over so as to expose as: ower surface obhqnoly to the zenith, in the same manner t . ~8 the ~erminalleaflet of tho mature leaf. But it docs not WJs Itself like the corresponding first simple leaf of Mclilotus. ,, I |