Preliminary sketch of the sleep or nyctitropic movements of leaves—­ Presence of pulvini—­The lessening of radiation the final cause of nyctitropic movements—­Manner of trying experiments on leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea and on the cotyledons of Mimosa—­Concluding remarks on radiation from leaves—­Small differences in the conditions make a great difference in the result — Description of the nyctitropic position and movements of the cotyledons of various plants—­ List of species—­Concluding remarks—­Independence of the nyctitropic movements of the leaves and cotyledons of the same species—­Reasons for believing that the movements have been acquired for a special purpose.

The so-called sleep of leaves is so conspicuous a phenomenon that it was observed as early as the time of Pliny;* and since Linnaeus published his famous Essay, ‘Somnus Plantarum,’ it has been the subject of several memoirs.  Many flowers close at night, and these are likewise said to sleep; but we are not here concerned with their movements, for although effected by the same mechanism as in the case of young leaves, namely, unequal growth on the opposite sides (as first proved by Pfeffer), yet they differ essentially in being excited chiefly by changes of temperature instead of light; and in being effected, as far as we can judge, for a different purpose.  Hardly any one supposes that there is any real analogy

* Pfeffer has given a clear and interesting sketch of the history of this subject in his ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, P. 163. [page 281]

between the sleep of animals and that of plants,* whether of leaves or flowers.  It seems therefore, advisable to give a distinct name to the so-called sleep-movements of plants.  These have also generally been confounded, under the term “periodic,” with the slight daily rise and fall of leaves, as described in the fourth chapter; and this makes it all the more desirable to give some distinct name to sleep-movements.  Nyctitropism and nyctitropic, i.e. night-turning, may be applied both to leaves and flowers, and will be occasionally used by us; but it would be best to confine the term to leaves.  The leaves of some few plants move either upwards or downwards when the sun shines intensely on them, and this movement has sometimes been called diurnal sleep; but we believe it to be of an essentially different nature from the nocturnal movement, and it will be briefly considered in a future chapter.

The sleep or nyctitropism of leaves is a large subject, and we think that the most convenient plan will be first to give a brief account of the position which leaves assume at night, and of the advantages apparently thus gained.  Afterwards the more remarkable cases will be described in detail, with respect to cotyledons in the present chapter, and to leaves in the next chapter.  Finally, it will be shown that these movements result from circumnutation, much modified and regulated by the alternations of day and night, or light and darkness; but that they are also to a certain extent inherited.