They are somewhat less conclusive than in the case of Phalaris, and this may possibly be accounted for by the sensitive zone varying in extension, in a species so long cultivated and variable as the common Oat. Cotyledons a little under three-quarters of an inch in height were selected for trial: six had their summits protected from light by tin-foil caps, .25 inch in depth, and two others by caps .3 inch in depth. Of these 8 cotyledons, five remained upright during 8 hours of exposure, although their lower parts were fully exposed to the light all the time; two were very slightly, and one considerably, bowed towards it. Caps only .2 or .22 inch in depth were placed over 4 other cotyledons, and now only one remained upright, one was slightly, and two considerably bowed to the light. In this and the following cases all the free seedlings in the same pots became greatly bowed to the light.
Our next trial was made with short lengths of thin and fairly transparent quills; for glass-tubes of sufficient diameter to go over the cotyledons would have been too heavy. Firstly, the summits of 13 cotyledons were enclosed in unpainted quills, and of these 11 became greatly and 2 slightly bowed to the light; so that the mere act of enclosure did not prevent the lower part from becoming bowed. Secondly, the summits of 11 cotyledons were enclosed in quills .3 inch in length, painted so as to be impermeable to light; of these, 7 did not become at all inclined towards the light, but 3 of them were slightly bent more or less transversely with respect to the line of light, and these might perhaps have been altogether excluded; one alone was slightly bowed towards the light. Painted quills, .25 inch in length, were placed over the summits of 4 other cotyledons; of these, one alone remained upright, a second was slightly bowed, and the two others as much bowed to the light as the free seedlings in the same pots. These two latter cases, considering that the caps were .25 in length, are inexplicable.
Lastly, the summits of 8 cotyledons were coated with flexible and highly transparent gold-beaters' skin, and all became as much bowed to the light as the free seedlings. The summits of 9 other cotyledons were similarly coated with gold-beaters' skin, which was then painted to a depth of between .25 and .3 inch, so as to be impermeable to light; of these 5 remained upright, and 4 were well bowed to the light, almost or quite as well as [page 479] the free seedlings. These latter four cases, as well as the two in the last paragraph, offer a strong exception to the rule that the illumination of the upper part determines the curvature of the lower part. Nevertheless, 5 of these 8 cotyledons remained quite upright, although their lower halves were fully illuminated all the time; and it would almost be a prodigy to find five free seedlings standing vertically after an exposure for several hours to a lateral light.
The cotyledons of Avena, like those of Phalaris, when growing in soft, damp, fine sand, leave an open crescentric furrow on the shaded side, after bending to a lateral light; and they become bowed beneath the surface at a depth to which, as we know, light cannot penetrate. The arcs of the chords of the buried bowed portions formed in two cases angles of 20o and 21o with the perpendicular. The open furrows on the shaded side were, in four cases, .008, .016, .024, and .024 of an inch in breadth. Brassica oleracea (Common Red).--It will here be shown that the upper half of the hypocotyl of the cabbage, when illuminated by a lateral light, determines the curvature of the lower half. It is necessary to experimentise on young seedlings about half an inch or rather less in height, for when grown to an inch and upwards the basal part ceases to bend. We first tried painting the hypocotyls with Indian ink, or cutting off their summits for various lengths; but these experiments are not worth giving, though they confirm, as far as they can be trusted, the results of the following ones. These were made by folding gold-beaters' skin once round the upper halves of young hypocotyls, and painting it thickly with Indian ink or with black grease.