OCR Text |
Show 84 HYPOCOTYLS, EPICO'l'YJ,S, ETC., CHAP. IL were placed in a horizontal position in <lamp air in the dark, and in the course of 4 honrs they again became curved vertically downwards, having passed through 90° in this time. But their sensitiveness to geotropism lasts for only 2 or 3 days; and the terminal part alone, for a length of between ·2 and ·4 inch, is thu · sensitive. Although the petioles of our specimens did not penetrate the ground to a greater depth than about ~ inch, yet they continued for some time to grow rapidly, and finally attained the great length of abont 3 inches. The upper part is apogeotror ic, and therefore grows vertically upwards, excepting a short portion close to the blades, which at an early period bends downwards and becomes arched, and thus breaks through the ground. Afterwards this portion straightens itself, and the cotyledons then free themselves from the seed-coats. Thus we here have in different parts of the same organ widely different kinds of movement and of sensitiveness; for the basal part is geotropic, the upper part apogeotropic, and a portion near the blades temporarily and spontaneously arches itself. The plumule is not developed for some little time; and as it rises between the bases of the parallel and closely approximate petioles of the cotyledons, which in breaking through the ground have formed an almost open passage, it does not require to be arched and is consequently always straight. Whether the plumule remains buried and dormant for a time in its native country, and is thus protected from the cold of winter, we do not know. The radicle, like that of the Megar· rhiza, grows into a tuber-like mass, which ultimately attains a great size. So it is with Ipoma3a pandurata, the germination of which, as Asa Gray informs us, resembles that of I. leptophylla. The following case is interesting in connection with CHAP. II. BREAKING 'l'IIROUGH THE GROUND. 85 the root-like nature of the petioles. The radicle of a seedling was cut off, as it was completely decayed, ~~d the t.wo now separated c?tyledons were planted. Ihey emitted roots from theu bases, and continued green and healthy for two months. The blades of both then withered, and on removing the earth the bases of the petioles (instead of the radicle) were found enlarged into little tubers. Whether these would have had the power of producing two independent plants in the following summer, we do not know. In Quercus virens, according to Dr. Engelmann,* both the cotyledons and their petioles are confluent. The latter grow to a length " of an inch or even more;" and, if we understand rightly, penetrate the ground, so that they must be geotropic. The nutriment within the cotyledons is then quickly transferred to the hypocotyl or radicle, which thus becomes developed into a fusiform tuber. The fact of t~b~rs being formed by the foregoing three widely d1stmct ylants, makes us believe that their protection from ammals at an early age and whilst tender, is one at least of the advantages gained by the remarkable elon~ation ~f the petioles of the cotyledons, t?gether w1th theu power of penetrating the ground hke roots under the guidance of geotropism. The following cases may be here given, as they bear o.n om;. present subject, though not relating to seedhugs. The flower-stem of the parasitic Lathrma squamaria, which is destitute of true leaves, breaks through the ground as an arch ;t so does the flower- S ; ' T~ansa.ct. St. Louis Aco.d. C1ence, vol. iv. p. 190. t 'rhe passage of the flowerstem of the La.thrroa through the ground cannot fail to be greatly facilitated by the extraordinary quantity of water secreted at this period of the year by the subter- |