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Show 94 RUDIMENTARY COTYLEDONS. CHAP. II. this is exactly the kind of curvature which actually occurs in these two plants, though to a much less degree. Therefore we can hardly doubt that their short hypocotyls have retained by inheritance a tendency to curve themselves in the same manner as they did at a former period, when this movement was highly important to them for breaking through the ground, though now rendered useless by the cotyledons being hypogean. Rudimentary structures are in most cases highly variable, and we might expect that rudimentary or obsolete actions would be equally so; and Sachs' curvature varies extremely in amount, and sometimes altogether fails. This is the sole instance known to us of the inheritance, though in a feeble degree, of movements which have become superfluous from changes which the species has undergone. Rudimentary Cotyledons.-A few remarks on this subject may be here interpolated. It is well known r .\. Fig. 60. c c B. c that some dicotyle-donous plants produce only a single cotyledon ; for instance, certain species of Rannnculus, Corydalis, Chmroph y II urn ; and we will here endeavour to show that the loss of one or both cotyledons is apparently due to a store of nutriment being laid up in some other part, as in Cit,·us au,·antiztm: two young seedlings: c, larger cotyledon; c', smaller cotyledon ; 11, thickened hypocotyl ; r, radicle. In A the epicotyl is still arched, in B it has become erect. the hypocotyl or one of the two cotyledons, or- one of the secondary radicles. CHAP. II RUDIMEN'l'AR Y COTYLEDONS. 95' With the orange (Citrus aurantium) the cotyledons are hypogean, and one is larger than the other, ns may be seen in A (Fig. 60). In· B the inequality is rather greater, and the stem has grown between the points of insertion of the two petioles, so that they do not stand opposite to one another; in another case the separation amounted to one-fifth of an inch. The smaller cotyledon of one seedling was extremely thin, and not half the length of the larger one, so that it was clearly becoming rudimentary.* In all these seedlings the hypocoty I was enlarged or swollen. With Abronia urnbellata one of the cotyledons is quite rudimentary, as may be seen (c') in Fig. 61. In this specimen it consisted of a little green flap, 8 '-:rth inch in length, destitute of a petiole and covered with glands like those on the fully developed cotyledon (c). .Fig. Gl. At first it stood opposite to the Ab1·onia umbellata: seedlarger cotyledon ; but as the petiole ling twice natural size: f h c, cotyledon; c', rudi- O t e latter increased in length meutat·y cotyledon; h, and grew in the same line with enlarged hypocotyl, h with a heel or projec-t e hypocotyl (h), the rudiment tion(l/)at the lower appeared in older seedlings as if end ; r, radicle. seated .some way down the hypocotyl. With Abronia arenana there is a similar rudiment, which in one ~ In Pachira aquatica, ns descnbed Ly Mr. R. I. T .ynch (' ~~mrnal Linn. Soc. Bot.' vol. xvu. 1878, p. 147), one of the hypogean cotyledons is of immense size ; the othe1• hi small and soon falls off; the pair do not always stand opposite. In another and very different water-plant, Trapa nttlans, one of the cotyledons, filled with farinaceous matter, is much larger than the other, which is scarcely visible, as is stated by Aug. de CHndolle, 'Physiologie \' eg.' tom. ii. p. 834, 1832. |
