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Show 88 HYPOCO'l'YLS, EPICO'rYLS, E'l'C., CHAP. II. or epicotyl, being thus saved from abrasion and pressure whilst brea~\:ing throng h the grou_nd. But we think that some Importance may be attnbuted to the increased force gained by the hypocotyl, epicotyl, or other organ by being at first arched ; for both legs of the arch increase in length, and both have points of resistance as long as the tip remains enclosed within the seed-coats· and thus the crown of the arch is ' ' pushed up through the earth with twice as much force as that which a straight hypocotyl, &c., could exert. As soon, however, as the upper end has freed itself, all the work has to be done by the basal leg. In the case of the epicoty 1 of the common bean, the basal leg (the apex having freed itself from the seedcoats) grew upwards with a force sufficient to lift a thin plate of zinc, loaded with 12 ounces. Two more ounces were added, and the 14 ounces were lifted 11p to a very little height, and then the epicotyl yielded and bent to one side. With respect to the primary cause of the arching process, we long thought in the case of many s.eedli1~gs that this might be attributed to the manner m whiCh the hypocotyl or epicotyl was packed and curved within the seed-coats; and that the arched shape thus acquired was merely retained until the parts in question reached the surface of the ground. But it is doubtful whether this is the whole of the truth in any case. For instance, with the common bean, the epicotyl or plumule is bowed into an arch whilst breaking through the seed-coats, as shown in Fig. 59 (p. 92). The plu:qmle first protrudes as a solid knob (e in A), which after twenty-four hours' growth is seen ( e in B) to be the crown of an arch. Nevertheless, with several beans which germinated in damp air, and had ot~er· wise been treated in an unnatural manner, httle 0IIAP. II. BREAKING THROUGH THE GROUND. 89 plumules were developed in the axils of the petioles of both cotyledons, and these were as perfect! y arched as the normal plumule; yet they had not been sul>jected to any confinement or pressure, for the seedcoats were completely ruptured, and they grew in the open air. This proves that the plumule has an innate or spontaneous tendency to arch itself. In some other cases the hypocotyl or epicotyl protrudes from the seed at first only slightly bowed; but the bowing afterwards increases independently of any constraint. The arch is thus made narrow, with the two legs, which are sometimes much elongated, parallel and close together, and thus it becomes well fitted for breaking through the ground. With many kinds of plants, the radicle, whilst still enclosed within the seed and likewise after its first protrusion, lies in a straight line with the future hypocotyl and with the longitudinal axis of the cotyledons. This is the case with Oucurbita ovifera; nevertheless, in whatever position the seeds were buried, the hypocotyl always came up arched in one particular direction. Seeds were planted in friable peat at a depth of about an inch in a vertical position, with the end from which the radicle protrudes downwards. Therefore all the parts occupied the same relative positions which they would ultimately hold after the seedlings had risen clear above the surface. Notwithstanding this fact, the hypocotyl arched itself; and as the arch grew upwards through the peat, the buried seeds were tu~ned either upside down, or were laid horizontally, bemg afterwards dragged above the ground. Ultimately the hypocotyl straightened itself in the usual manner; and now after all these movements the several parts occupied the same position relatively to one another and to the centre of the earth, which they |