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Show 120 PUL VINI OF COTYLEDONS. CUAP. II. same pulvinus and in different individuals. In the accompanying figures, A and B (Fig. 64), we have views of the epidermis* in the middle part of the petioles of two seedlings, in which the pulvinus was for this species well developed. They offer a striking contrast with the pulvinus of 0. r·osea (soc former Fig. 63), or of 0. Vu.ldiviana. With the seedlings, falsely called 0. tropceoloides, the ·cotyledons of which rise very little at night, the small cells were still fewer in num bcr and in parts formed a single transverse row, and in other parts short longitudinal rows of only two or throe. Nevertheless tboy sufficed to attract the eye, when the whole petiole was viowecl as a transparent object beneath the microscope. In those seedlings thoro could hardly be a doubt that the pulvinus was becoming rudimentary and tending to disappear; and this accounts for its great variability in structure and function. In the following Table some measurements of the cells in fairly well-developed pulvini of 0. cor·niculatu aro given:- Seedling 1 day old, with cotyledon 2·3 mm. in length. Average length of cells of pulvinus .. Length of longest cell below the pulvinus Length of longest cell above the pulvim1s Divisions of Micrometer.t 6 to 7 13 20 Seedl-ing 5 du.ys old, cotyledon 3·1 mm. in lengtl~, with the pulvinus quite distinct. Average length of cells of pulviuus .. 6 Length of longest cell below the })tllvinus 22 Length of longest cell above the pul vinus 40 Seedling 8 clays old, cotyledon 5 mm. in length, witli a trt'6leaf formed but not yet expanded. Average length of cells of pulvinus .• Length of longest cell below the pulvinus Length of longest cell above the pulvinus 9 44 70 Seedling 13 days old, cotyledon 4· 5 mm. in length, with ct small true leaf f ully developed. Average length of cells of pulvinus .• Length of longest cell below the pulvinus Length of longest cell above the pulvinus 7 30 00 * Longitudinal sections show pulvinus. that the forms of the epidermic t Each division equalled ·003 cells may be taken as a fair repre- ' mm. sentatiou of those constituting the CnAP. II. PUL VINI OF COTYLEDONS. 121 We here see that the cells of the pulvinus increase but little in length with advancing age, in comparison with those of tho petiole both above and below it ; but they continuo to grow in width, and keep equal in this respect with the other cells of the petiole. The rate of growth, however, varies in all parts of the cotyledons, as may be observed in the measurements of the 8-days' old seedling. The cotyledons of seedlings only a day old rise at night considerably, sometimes as much as afterwards; but there was much variation in this respect. As tho pulvinus is so indistinct at first, the movement probably does not then depend on the expansion of its cells, but on periodically unequal growth in the petiole. Dy the comparison of seedlings of different known ages, it was evident that the chief seat of growth of tho petiole was in the upper part between the pulvinus and the blade; and this agrees with the fact (shown in the measurements above given) that the cells grow to a greater length in the upper than in the lower part. With a seedling 11 days old, the nocturnal rise was found to depend largely on the action of the pulvinus, for tho petiole at night was curved upwards at this point; and during the day, whilst the petiole was horizontal, the lower surface of the pulvinus was w1·inkled with tho upper surface tense. Although the cotyledons at an advanced age do not rise at night to a higher inclination than whilst young, yet they have to pass through a larger angle (in one instance amounting to 63°) to gain their nocturnal position, as they arc generally deflected beneath the horizon during tho day. Even with tho 11-days' old seedling the movement dicl not depend exclusively on the pulvinus, for tho blade whore joined to the petiole was curved upwards, and tbis must be attributed to unequal growth. 'rherefore the periodic movements of the cotyledons of U. cornir culata ~epend on two distinct but conjoint actions, namely, the expansiOn of the cells of the pul vinus and on the growth of the upper part of the petiole, including the base of the blade. Lotus Jacobreus.-The seedlings of this plant present a case po.rallel to that of Uxctlis corniculata in some respects, and in others unique, as far as we have seen. The cotyledons during the first 4 or 5 days of their life do not exhibit any plain nocturnal movement; but afterwards they stand verticalJy or al~ostvertically up at night. There is, however some degree of var1 ab 1Tt · ' 1 Y m this respect, apparently dependent on the season and on the degree to which they have been illuminated during |
