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Show !90 HYPOCOTYLS, EPICOTYLS, ETC., CHAP. II. had done when the seeds were first buried. But it may be argued in this and other such cases that, as the hypocotyl grows up through the soil, the seed will almost certainly be tilted to one side ; and then from the resistance which it must offer during its further elevation, the upper part of the hypocotyl will be doubled down and thus become arched. 'fhis view seems the more probable, because with Ranunculus jicaria only the petioles of the leaves which forced a passage through the earth were arched; and not those which arose from the summits of the bulbs above the ground. Nevertheless, this explanation does not apply to the Cucurbita, for when germinating seeds were suspended in damp air in various positions by pins passing through the cotyledons, fixed to the inside of the lids of jars, in which case the hypo· cotyls were not subjected to any friction or constraint, yet the upper part became spontaneously arched. This fact, moreover, proves that it is not the weight of the cotyledons which causes the arching. Seeds of Helianthus annuus and of two species of I pomroa (those of I. bona nox being for the genus large and heavy) were pinned in the same manner, and the hypocotyls became spontaneously arched; the radicles, which had been vertically dependent, assumed in consequence a horizontal position. In the case of Ipomooa leptophylla it is the petioles of the cotyledons which become arched whilst rising through the ground; and this occurred spontaneously when the seeds were fixed to the lids of jars. It may, however, be suggested with some degree of probability that the arching was aboriginally caused by mechanical compulsion, owing to the confinement of the parts in question within the seed-coats, or to friction whilst they were being dragged upwards. But CnAP. II. BREAKING 'l'HROUGH THE GROUND. 91 if this is so, we must admit from the cases just given, that a tendency in the upper part of the several specified organs to bend downwards and thus to become arched, has now become with many plants firmly inherited. The arching, to whatever cause it may be ~ue, is the result of modified circumnutation, through mcreased growth along the convex side of the part; sue~ growth .being only temporary, for the part always straightens Itself subsequently by increased growth along the concave side, as will hereafter be described. It is a curious fact that the hypocotyls of some plants, which are but little developed and which never raise their cotyledons above the ground, nevertheless inherit a slight tendency to arch themselves, although this movement is not of the least use to ~hem. We refer to a movement observed by Sachs m the hypocotyls of the bean and some other Leo-umino~ re, and which is shown in the accompanying flo-ure (Fig. 5~), copied from his Essay.* The hypoc~tyl and radicle at first grow perpendicularly downwards, as at A, and then bend, often in the course of 24 hours into the position shown at B. As we shall here~ after often have to recur to this movement we will for ~rev~ty sake, call it "Sachs' curvature." At first sight It ~ngh~ be thought that the altered position of the ra~wle m B was wholly due to the outgrowth of the ?PI.cotyl (e), the petiole (p) serving as a hinge ; and It Is probable that this is partly the cause; but the hypocotyl and upper part of the radicle themselves become slightly curved. The above movement in the bean was repeatedly seen by us; b~t our observations were made chiefly on Phaseolus multijlorus, the cotyledons of which are like- * 'Arbeiten des bot. In:otit. Wiirzburg,' vol. i. 1873, p. 403. |