It was not possible to decide whether the leaves with their upper or those with their lower surfaces turned to the zenith had suffered most.
Melilotus suaveolens.--Some plants with 8 leaves pinned out were exposed to a clear sky during 2 h., the temperature on the surrounding grass being -2o C. Next morning 6 out of these 8 leaves were in a flaccid condition. There were about 150 free leaves on the plant, and none of these were injured, except 2 or 3 very young ones. But after two days, the plants having been brought back into the greenhouse, the 6 pinned-out leaves all recovered.
Melilotus Taurica.--Several plants were exposed for 5 h. during two nights to a clear sky and slight frost, accompanied by some wind; and 5 leaves which had been pinned out suffered more than those both above and below on the same branches which had gone to sleep. Another pot, which had likewise been kept in the greenhouse, was exposed for 35 - 40 m. to a clear sky, the temperature of the surrounding grass being between -3o and -4o C. Nine leaves had been pinned out, and all of these were killed. On the same plants there were 210 free leaves, which had been allowed to go to sleep, and of these about 80 were killed, i.e. only 38 per cent.
Melilotus Petitpierreana.--The plants were exposed to a clear sky for 35 - 40 m.: temperature on surrounding grass -3o to -4o C. Six leaves had been pinned out so as to stand about ½ inch above the cork, and four had been pinned close to it. These 10 leaves were all killed, but the closely pinned ones suffered most, as 4 of the 6 which stood above the cork still retained small patches of a green colour. A considerable number, but not nearly all, of the free leaves, were killed or much injured, whereas all the pinned out ones were killed. [page 292]
Melilotus macrorrhiza.--The plants were exposed in the same manner as in the last case. Six leaves had been pinned out horizontally, and five of them were killed, that is, 83 percent. We estimated that there were 200 free leaves on the plants, and of these about 50 were killed and 20 badly injured, so that about 35 per cent of the free leaves were killed or injured.
Lotus aristata.--Six plants were exposed for nearly 5 h. to a clear sky; temperature on surrounding grass -1.5o C. Four leaves had been pinned out horizontally, and 2 of these suffered more than those above or below on the same branches, which had been allowed to go to sleep. It is rather a remarkable fact that some plants of Lotus Jacoboeus, an inhabitant of so hot a country as the Cape Verde Islands, were exposed one night to a clear sky, with the temperature of the surrounding grass -2o C., and on a second night for 30 m. with the temperature of the grass between -3o and -4o C., and not a single leaf, either the pinned-out or free ones, was in the least injured.
Marsilea quadrifoliata.--A large plant of this species--the only Cryptogamic plant known to sleep--with some leaves pinned open, was exposed for 1 h. 35 m. to a clear sky, the temperature on the surrounding ground being -2o C., and not a single leaf was injured. After an interval of some days the plant was again exposed for 1 h. to a clear sky, with the temperature on the surrounding ground lower, viz., -4o C. Six leaves had been pinned out horizontally, and all of them were utterly killed. The plant had emitted long trailing stems, and these had been wrapped round with a blanket, so as to protect them from the frozen ground and from radiation; but a very large number of leaves were left freely exposed, which had gone to sleep, and of these only 12 were killed. After another interval, the plant, with 9 leaves pinned out, was again exposed for 1 h., the temperature on the ground being again -4o C. Six of the leaves were killed, and one which did not at first appear injured afterwards became streaked with brown. The trailing branches, which rested on the frozen ground, had one-half or three-quarters of their leaves killed, but of the many other leaves on the plant, which alone could be fairly compared with the pinned-out ones, none appeared at first sight to have been killed, but on careful search 12 were found in this state.