pins passing through the cotyledons, fixed to the inside of the lids of jars, in which case the hypocotyls 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 Ipomoea (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 Ipomoea 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 [page 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 due, is the result of modified circumnutation, through increased growth along the convex side of the part; such 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 them.  We refer to a movement observed by Sachs in the hypocotyls of the bean and some other Leguminosae, and which is shown in the accompanying figure (Fig. 59), copied from his Essay.* The hypocotyl 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 hereafter often have to recur to this movement, we will, for brevity sake, call it “Sachs’ curvature.”  At first sight it might be thought that the altered position of the radicle in B was wholly due to the outgrowth of the epicotyl (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; but our observations were made chiefly on Phaseolus multiflorus, the cotyledons of which are like-

* ‘Arbeiten des bot.  Instit.  Würzburg,’ vol. i. 1873, p. 403. [page 92]

wise hypogean.  Some seedlings with well-developed radicles were first immersed in a solution of permanganate of potassium; and, judging from the changes of colour (though these were not very clearly defined), the hypocotyl is about .3 inch in length.  Straight, thin, black lines of this length were now drawn from the bases of the short petioles along the hypocotyls