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.
The foregoing observations are of some value, from having been made when we were not attending to heliotropism; and they led us to experiment on several kinds of seedlings, by exposing them to a dim lateral light, so as to observe the gradations between ordinary circumnutation and heliotropism. Seedlings in pots were placed in front of, and about a yard from, a north-east window; on each side and over the pots black boards were placed; in the rear the pots were open to the diffused light of the room, which had a second north-east and a north-west window. By hanging up one or more blinds before the window where the seedlings stood, it was easy to dim the light, so that very little more entered on this side than on the opposite one, which received the diffused light of the room. Late in the evening the blinds were successively removed, and as the plants had been subjected during the day to a very obscure light, they continued to bend towards the window later in the evening than would otherwise have occurred. Most of the seedlings were selected because they were known to be highly sensitive to light, and some because they were but little sensitive, or had become so from having grown old. The movements were traced in the usual manner on a horizontal glass cover; a fine glass filament with little triangles of paper having been cemented in an upright position to the hypocotyls. Whenever the stem or hypocotyl became much bowed towards the light, the latter part of its course had to be traced on a vertical glass, parallel to the window, and at right angles to the horizontal glass cover. Fig. 170. Apios graveolens: heliotropic movement of hypocotyl (.45 of inch in height) towards a moderately bright lateral light, traced on a horizontal glass from 8.30 A.M. to 11.30 A.M.