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Show 86 HYPOCOTYLS, EPICOTYLS, ETC., CHAP. II. stem of the parasitic and leafless Monotropa hypopitys. With Helleborus niger, the flower-stems, whic~ rise up independently of the leaves, likewise break through the ground as arches. '11his is also the case with the greatly elongated flower-stems, as well as with the petioles of Epimedium pinnatum. So it is with the petioles of Ranunculus ficaria, when they have to break through the ground, but when they arise from the summit of the bulb above ground, they are from the first quite straight; and this is a fact which deserves notice. The rachis of the bracken fern ( Pleris aquilina), and of some, probably many, other ferns, likewise rises above ground under the form of an arch. No doubt other analogous instances could be found by careful search. In all ordinary cases of bulbs, rhizomes, ranean scale-like leaves; not that there is any reason to suppose that the secretion is a special adaptation for this purpose : it probably follows from the great quantity of sap absorbed in the early spring by the parasitic roots. After a long period without any rain, the earth had become lightcoloured and very dry, but it was dark-coloured and damp, even in parts quite wet, for a distance of at least six inches all round each flower-stem. The water is secreted by glands (described by Cohn, 'Bericht. Bot. Sect. der Schlesischen Gesell.,' 1876, p. 113) which line the longitudinal channels running through each scale-like leaf. A large plant was dug up, washed so as to remove the earth, loft for some time to drain, and then placed in the evening on a dry glass-plate, covered with a hell-glass, and by next morning it har'l secreted a large pool of water. The plote was wiped dry, and in the course of the succeeding 7 or 8 hours another little pool was secreted, and nfter 16 additional hours several large drops. A Fmaller plant was washed and placed in a large jar, which was left inclined for an hour, by w hi cit time no more water drained off. 'l'hc jnr . was then placed upright and closed: after 20 hours twodrachms of water were (•ollectcd from tho bottom, and a little more ofter 2j additional hours. 'fhe flower· stems wero now cut off, for they do not secrete, and the subter· ranean pn,rt of the phmt was found to weigh 106 · 8 grams (1611 gmins), and the water se~rfted during the 48 hours wmgh~l 11·9 grams (183 grains),.-~bat 1s,, one-ninth of the whole we1ght ot the plant, excluding the flower· stems. We should remembt·r thnt plants in a state of nature would probably secrete in 48 hours much more than the above }urge amount, for their roots would continue all the time absorbing sap from the plant on which they were para· sitic. CHAP. II. BREAKING THROUGH THE GROUND. 87 root-stocks, &c., buried beneath the ground, the surface is broken by a cone formed by the young imbricated leaves, the combined growth of which gives them force sufficient for the purpose. With germinating monocotyledonous seeds, of which, however, we did not observe a large number, the plumules, for instance, those of Asparagus and Canna, are straight whilst breaking through the ground. With the Graminere, the sheath-like cotyledons are likewise straight; they, however, terminate in a sharp crest, which is white and somewhat indurated; and this structure obviously facilitates their emergence from the soil: the first true leaves escape from the sheath through a slit beneath the chisel-like apex and at right angles to it. In the case of the onion (Alli~tm cepa) we again meet with an arch; the leaf-like cotyledon being abruptly bowed, when it breaks through the ground, with the apex still enclosed within the seed-coats. The crown of the arch, as previously described, is developed into a white conical protuberance, which we may safely believe to be a special adaptation for this office. The fact of so m~ny organs of different kindshypocoty ls and epicoty ls, the petioles of some cotyledons and of some first leaves, the cotyledons of the onion, the rachis of some ferns, and some flowerstems- being all arched whilst they break through the ground, shows how just are Dr. Haberlandt's * remarks on the importance of the arch to seedling plants. He attributes its chief importance to the upper, young, and more tender parts of the hypo<?otyl "' 'Die Schutzeinrichtun~en in der Eutwickelung dE:r Keimptlanze,' 1877. We have learned much from this interesting essay, tltough our observations lead us to differ on some points from tn~:~ author. ., .. ;J!, ,t:,· IW' |
