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Show 78 HYPOCOTYLS, EPICOTYLS, ETO., CHAP. II. breaks through the ground as an arch (Fig. 57). Abronia also has only a single fully developed cotyledon, but in this case it is the hypocotyl which first emerges and is arched. Abronia umbellata, however, presents this peculiarity, that the enfolded blade of the one developed cotyledon (with the enclosed endosperm) Fig. 57. c Cyclamen Persicwn : whilst still beneath the surface has seedling, figure en- · d d 11 1 larged: c, blade of Its apex uptUI·ne a.n para e to cotyledon, not yet the descending leg of the archecl expanded, with arched petiole beginning to hypocotyl ; but it is dragged . straighten itself; It, out of the ground by the conhypocotyl developed intoacorm j r,secoud- tinued growth of the hypocotyl, ary radicles. with the apex pointing downward. With Oycas pectinata the cotyledons are hypogean, Fig. 58. .Acanthus mollis: seedling, with the hypogean cotyledon on the near side removed and the radicles cut off: a, blade of first leaf beginning to expand, with petiole still ' partially arched; b, second and o.pposite leaf, as yet very imper- • fectly developed; c, hypogean coty~edon QJl t.1'c N~''O .. ;f :>'lli!. and a true leaf :first breaks through the ground with its petiole forming an arch. In the genus Acanthus the cotyledons are likewise h ypogean. In A. mollis, a single leaf :first breaks through the ground with its petiole arched, and with the opposite leaf much less developed, short, straight, of a yellowish colour, and with the petiole at first. not half as thick as that of the other. The undeveloped leaf is protected by standing beneath its arched fello\\ ; u.ud it. is an i nstl"lll\- CHAP. II. BREAKING THROUGH THE GROUND. 79 tive fact that it is not arched, as it has not to force for itself a passage through the ground. In the accompanying sketch (Fig. 58) the petiole of the :first leaf has already partially straightened itself, and the blade is beginning to unfold. The small second leaf ultimately grows to an equal size with the :first, but this process is effected at very different rates in different individuals : in one instance the second leaf did not appear fully above the ground until six weeks after the first leaf. As the leaves in the whole family of the Acanthacere stand either opposite one another or in, whorl~, and as these are of equal size, the great in .. . equahty between the :first two leaves is a singular fact~ . We can see how this inequality of development and the ~rchin.g of the petiole could have been gradually acquu~d, If t~ey were beneficial to the seedlings by favourmg theu emergence ; for with A. candelabrum, ~pinosus~ and latifolius there was great variability in the mequahty between the two :first leaves and in the arching of their petioles. In one seedling of A. candelabrum the :first leaf was arched and nine times as long as the second, which latter consisted of a mere little~ yellowish-white, straight, hairy style. In other seedlmgs the difference in length between the two leav~s was as 3 to 2, or as 4 to 3, or as only · 76 to · 62 mch. In these latter cases the first and taller loaf was not properly arched. Lastly, in another seedling there was not the least difference in size between the two. first leaves, and both of them had their petioles stra~ght; their laminre were enfolded and pressed ag~mst each other, forming a lance or wedge, by ~hiCh means they had broken through the ground. Therefore in different individuals of this same species of Acanthus the first pair of leaves breaks through the ground by tw~ widely different methods; and if |