being separated by an indifferent zone, where the growth is equal on all sides.  There may be even more than one indifferent zone in the same internode; and the opposite sides of the parts above and below each such zone grow quickest.  This peculiar manner of growth is called by Wiesner “undulatory nutation.”  Circumnutation depends on one side of an organ growing quickest (probably preceded by increased turgescence), and then another side, generally almost the opposite one, growing quickest.  Now if we look at an arch like this [upside down U] and suppose the whole of one side—­we will say the whole convex side of both legs—­to increase in length, this would not cause the arch to bend to either side.  But if the outer side or surface of the left leg were to increase in length the arch would be pushed over to the right, and this would be aided by the inner side of the right leg increasing in length.  If afterwards the process were reversed, the arch would be pushed over to the opposite or left side, and so on alternately,—­that is, it would circumnutate.  As an arched hypo-

* ‘Die undulirende Nutation der Internodien,’ Akad. der Wissench. (Vienna), Jan. 17th, 1878.  Also published separately, see p. 32. [page 100]

cotyl, with the two legs fixed in the ground, certainly circumnutates, and as it consists of a single internode, we may conclude that it grows in the manner described by Wiesner.  It may be added, that the crown of the arch does not grow, or grows very slowly, for it does not increase much in breadth, whilst the arch itself increases greatly in height.

The circumnutating movements of arched hypocotyls and epicotyls can hardly fail to aid them in breaking through the ground, if this be damp and soft; though no doubt their emergence depends mainly on the force exerted by their longitudinal growth.  Although the arch circumnutates only to a slight extent and probably with little force, yet it is able to move the soil near the surface, though it may not be able to do so at a moderate depth.  A pot with seeds of Solanum palinacanthum, the tall arched hypocotyls of which had emerged and were growing rather slowly, was covered with fine argillaceous sand kept damp, and this at first closely surrounded the bases of the arches; but soon a narrow open crack was formed round each of them, which could be accounted for only by their having pushed away the sand on all sides; for no such cracks surrounded some little sticks and pins which had been driven into the sand.  It has already been stated that the cotyledons of Phalaris and Avena, the plumules of Asparagus and the hypocotyls of Brassica, were likewise able to displace the same kind of sand, either whilst simply circumnutating or whilst bending towards a lateral light.