* The cotyledons, though bright green, resemble to a certain extent hypogean ones; see the interesting discussion by Haberlandt (’Die Schutzeinrichtungen,’ etc., 1877, p. 95), on the gradations in the Leguminosae between subaërial and subterranean cotyledons. [page 111]

time they are expanded almost horizontally.  The circumnutating movement is thus at least partially periodic, no doubt in connection, as we shall hereafter see, with the daily alternations of light and darkness.  The cotyledons of several plants move up so much at night as to stand nearly or quite vertically; and in this latter case they come into close contact with one another.  On the other hand, the cotyledons of a few plants sink almost or quite vertically down at night; and in this latter case they clasp the upper part of the hypocotyl.  In the same genus Oxalis the cotyledons of certain species stand vertically up, and those of other species vertically down, at night.  In all such cases the cotyledons may be said to sleep, for they act in the same manner as do the leaves of many sleeping plants.  This is a movement for a special purpose, and will therefore be considered in a future chapter devoted to this subject.

In order to gain some rude notion of the proportional number of cases in which the cotyledons of dicotyledonous plants (hypogean ones being of course excluded) changed their position in a conspicuous manner at night, one or more species in several genera were cursorily observed, besides those described in the last chapter.  Altogether 153 genera, included in as many families as could be procured, were thus observed by us.  The cotyledons were looked at in the middle of the day and again at night; and those were noted as sleeping which stood either vertically or at an angle of at least 60o above or beneath the horizon.  Of such genera there were 26; and in 21 of them the cotyledons of some of the species rose, and in only 6 sank at night; and some of these latter cases are rather doubtful from causes to be explained in the chapter on the sleep of cotyledons.  When [page 112] cotyledons which at noon were nearly horizontal, stood at night at more than 20o and less than 60o above the horizon, they were recorded as “plainly raised;” and of such genera there were 38.  We did not meet with any distinct instances of cotyledons periodically sinking only a few degrees at night, although no doubt such occur.  We have now accounted for 64 genera out of the 153, and there remain 89 in which the cotyledons did not change their position at night by as much as 20o—­that is, in a conspicuous manner which could easily be detected by the unaided eye and by memory; but it must not be inferred from this statement that these cotyledons did not move at all, for in several cases a rise of a few degrees was recorded, when they were carefully observed.  The number 89 might have been a little increased, for the cotyledons remained almost horizontal at night in some species