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Show 118 rUL VINI OF COTYLEDONS. CHAP. II. the species seen by us are pulvinated.; so it is with the cotyledons of T. subterraneum and strictum, which stand verticn.lly at night; whereas those of T. 1·esupinatum exhibit not a trace of a pulvinus, nor of any nocturnal movement. This was ascertained by measuring the distance between the tips of the cotyledons of four seedlings at mid-day and at night. In this r-pecics, however, as in the others, the first-formed leaf, which is simple or not trifoliate, rises up and sleeps like the terminal leaflet on a mature plant. In another natural genus, Oxalis, the cotyledons of 0. Valdiviana, rosea, jloribunda, articulata, and sensitivct are pulvinated, and all move at night into an upward or downward vertical position. In these several species the pulvinus is seated close to the blade of the cotyledon, as is the usual rule with most plants. Oxalis corniculata (var. Atro-purpurea) differs in several respects; the cotyledons rise at night to a very variable amount, rarely more than 45°; and in one lot of seedlings (purchased under tho name of 0. tropmoloides, but certainly belonging to the above variety) they rose only from 5° to H? above the horizon. The pulvinns is developed imperfectly and to an extremely variable degree, so that apparently it is tending to·wards abortion. No such case has hitherto, we believe, Leon described. It is coloured green from its cells containing chlorophyll; and it is seated nearly in the midule of the petiole, instead of at the upper end as in all the other species. The nocturnal movement is effected partly by its aid, and partly by the growth of the upper part of the petiole as in the case of plants destitute of a pulvinus. From these several rensons aud from our having partially traced the develop· rnent of the pulvinus from an early age, the case seems worth describing in some detail. CnAP. II. PUL VINI OF COTYLEDONS. 119 When the cotyledons of 0. corm'culatr.t, were diRscctcd out of a Reed from which tlwy wonld soon have naturally emerged, no trace of a pulvinus could be detected; and all tho cells forming the short petiole, 7 in number in a longitudinal row, were of nearly equal size. In seedlings 0110 or two days old, tho pulvinus was so indistinct that we thought at first that it did not exist; but in the middle of the petiole an ill-defined tmnsvorso zone of cells could be seen, which wore much Rhorter than thoso both above and below, although of tho same 1)roadth with them. Tbcy presented tho appearance of having been just formed by the transverse division of longer cells; and there can be little doubt that this had occurred, for the cells in the petiole which had Fig. G4. A. B. Oxrclis corniculatrt.: A and B the alm0st rurlimcntm·y pulvini of the cotyled<. ns of two rather old seedlings, viewed as transparent objects. Magnific<.l 50 times. been dissected out of tho seed averaged in length 7 divisions of the micrometer (each division equalling ·003 mm.), and were a little longer than those forming a well-developed pulvinus, which varied between 4 and 6 of these same divisions. After a few additional days the ill-defined zone of cells becomes distinct, and although it does not extend across the wbolo width of the petiole, and although the cells arc of a green colour from containing chlorophyll, yet they certainly constitute a pulvinus, which, ns we shall pro~ently see, acts as one. These small cells were arranged in longitudinal rows, and varied from 4 to 7 in number; and the cells themselves varied in length in different parts of the |