Fig. 168.  Beta vulgaris:  circumnutation of hypocotyl, deflected by the light being slightly lateral, traced on a horizontal glass from 8.30 A.M. to 5.30 P.M.  Direction of the lighted taper by which it was illuminated shown by a line joining the first and penultimate dots.  Figure reduced to one-third of the original scale.

[For instance two young seedlings of Beta vulgaris were placed in the middle of a room with north-east windows, and were kept covered up, except during each observation which lasted for only a minute or two; but the result was that their hypocotyls bowed themselves to the side, whence some light occasionally entered, in lines which were [page 421] only slightly zigzag.  Although not a single ellipse was even approximately formed, we inferred from the zigzag lines — and, as it proved, correctly—­ that their hypocotyls were circumnutating, for on the following day these same seedlings were placed in a completely darkened room, and were observed each time by the aid of a small wax taper held almost directly above them, and their movements were traced on a horizontal glass above; and now their hypocotyls clearly circumnutated (Fig. 168, and Fig. 39, formerly given, p. 52); yet they moved a short distance towards the side where the taper was held up.  If we look at these diagrams, and suppose that the taper had been held more on one side, and that the hypocotyls, still circumnutating, had bent themselves within the same time much more towards the light, long zigzag lines would obviously have been the result.

Fig. 169.  Avena sativa:  heliotropic movement and circumnutation of sheath-like cotyledon (1 ½ inch in height) traced on horizontal glass from 8 A.M. to 10.25 P.M.  Oct. 16th.

Again, two seedlings of Solanum lycopersicum were illuminated from above, but accidentally a little more light entered on one than on any other side, and their hypocotyls became slightly bowed towards the brighter side; they moved in a zigzag line and described in their course two little triangles, as seen in Fig. 37 (p. 50), and in another tracing not given.  The sheath-like cotyledons of Zea mays behaved, under nearly similar circumstances, in a nearly similar manner as described in our first chapter (p. 64), for they bowed themselves during the whole day towards one side, making, however, in their course some conspicuous flexures.  Before we knew how greatly ordinary circumnutation was modified by a lateral light, some seedling oats, with rather old and therefore not highly sensitive cotyledons, were placed in front of a north-east window, towards which they bent all day in a strongly zigzag course.  On the following day they continued to bend in the same direction (Fig. 169), but zigzagged much less.  The sky, however, became between 12.40 and 2.35 P.M. [page 422] overcast with extraordinarily dark thunder-clouds, and it was interesting to note how plainly the cotyledons circumnutated during this interval.