The oscillatory and jerking movements of the leaves of Dionaea, which resemble those of the hypocotyl of the cabbage, are highly remarkable, as seen under the microscope.  They continue night and day for some months, and are displayed by young unexpanded leaves, and by old ones which have lost their sensibility to a touch, but which, after absorbing animal matter, close their lobes.  We shall hereafter meet with the same kind of movement in the joints of certain Gramineae, and it is probably common to many plants while circumnutating.  It is, therefore, a strange fact that no such movement could be detected in the tentacles of Drosera rotundifolia, though a member of the same family with Dionaea; yet the tentacle which was observed was so sensitive, that it began to curl inwards in 23 seconds after being touched by a bit of raw meat.

One of the most interesting facts with respect to the circumnutation of leaves is the periodicity of their movements; for they often, or even generally, rise a little in the evening and early part of the night, and sink again on the following morning.  Exactly the same phenomenon was observed in the case of cotyledons.  The leaves in 16 genera out of the 33 which were observed behaved in this manner, as did probably 2 others.  Nor must it be supposed that in the remaining 15 genera there was no periodicity in their movements; for 6 of them were observed during too short a period for any judgment to be formed on this head, [page 262] and 3 were so young that their epinastic growth, which serves to bring them down into a horizontal position, overpowered every other kind of movement.  In only one genus, Cannabis, did the leaves sink in the evening, and Kraus attributes this movement to the prepotency of their epinastic growth.  That the periodicity is determined by the daily alternations of light and darkness there can hardly be a doubt, as will hereafter be shown.  Insectivorous plants are very little affected, as far as their movements are concerned, by light; and hence probably it is that their leaves, at least in the cases of Sarracenia, Drosera, and Dionaea, do not move periodically.  The upward movement in the evening is at first slow, and with different plants begins at very different hours;—­with Glaucium as early as 11 A.M., commonly between 3 and 5 P.M., but sometimes as late as 7 P.M.  It should be observed that none of the leaves described in this chapter (except, as we believe, those of Lupinus speciosus) possess a pulvinus; for the periodical movements of leaves thus provided have generally been amplified into so-called sleep-movements, with which we are not here concerned.  The fact of leaves and cotyledons frequently, or even generally, rising a little in the evening and sinking in the morning, is of interest as giving the foundation from which the specialised sleep-movements of many leaves and cotyledons, not provided with a pulvinus, have been developed. the above periodicity should be kept in mind, by any one considering the problem of the horizontal position of leaves and cotyledons during the day, whilst illuminated from above. [page 263]