so as to stand vertically or almost vertically, having generally moved through an angle of at least 60o.  If we lay on one side the Leguminosae, the cotyledons of which are particularly liable to sleep, 140 genera remain; and out of these, the cotyledons of at least one species in 19 genera slept.  Now if we were to select by hazard 140 genera, excluding the Leguminosae, and observed their leaves at night, assuredly not nearly so many as 19 would be found to include sleeping species.  We here refer exclusively to the plants observed by ourselves. [page 312]

In our entire list of seedlings, there are 30 genera, belonging to 16 Families, the cotyledons of which in some of the species rise or sink in the evening or early night, so as to stand at least 60o above or beneath the horizon.  In a large majority of the genera, namely, 24, the movement is a rising one; so that the same direction prevails in these nyctitropic movements as in the lesser periodic ones described in the second chapter.  The cotyledons move downwards during the early part of the night in only 6 of the genera; and in one of them, Cannabis, the curving down of the tip is probably due to epinasty, as Kraus believes to be the case with the leaves.  The downward movement to the amount of 90o is very decided in Oxalis Valdiviana and sensitiva, and in Geranium rotundifolium.  It is a remarkable fact that with Anoda Wrightii, one species of Gossypium and at least 3 species of Ipomoea, the cotyledons whilst young and light sink at night very little or not at all; although this movement becomes well pronounced as soon as they have grown large and heavy.  Although the downward movement cannot be attributed to the weight of the cotyledons in the several cases which were investigated, namely, in those of the Anoda, Ipomoea purpurea and bona-nox, nor in that of I. coccinea, yet bearing in mind that cotyledons are continually circumnutating, a slight cause might at first have determined whether the great nocturnal movement should be upwards or downwards.  We may therefore suspect that in some aboriginal member of the groups in question, the weight of the cotyledons first determined the downward direction.  The fact of the cotyledons of these species not sinking down much whilst they are young and tender, seems opposed to the belief that the greater movement when they are [page 313] grown older, has been acquired for the sake of protecting them from radiation at night; but then we should remember that there are many plants, the leaves of which sleep, whilst the cotyledons do not; and if in some cases the leaves are protected from cold at night whilst the cotyledons are not protected, so in other cases it may be of more importance to the species that the nearly full-grown cotyledons should be better protected than the young ones.