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 –2° 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., –4° 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 –4° 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. After another interval, the plant with 9 leaves pinned out, was exposed for 35–40 m. to a clear sky and to nearly the same, or perhaps a rather lower, temperature (for the thermometer by an accident had been left on a sun-dial close by), and 8 of these leaves were killed. Of the free leaves (those on the trailing branches not being considered), a good many were killed, but their number, compared with the uninjured ones, was small. Finally, taking the three trials together, 24 leaves, extended horizontally, were exposed to the zenith and to unobstructed radiation, and of these 20 were killed and 1 injured; whilst a relatively very small proportion of the leaves, which had been allowed to go to sleep with their leaflets vertically dependent, were killed or injured.