head.  But we may recall the behaviour of Mimosa in the North, where the sun does not set, and the complete inversion of the daily movements by artificial light and darkness.  It has also been shown by us, that although leaves subjected to darkness for a moderately long time continue to circumnutate, yet the periodicity of their movements is soon greatly disturbed, or quite annulled.  The presence of light or its absence cannot be supposed to be the direct cause of the movements, for these are wonderfully diversified even with the leaflets of the same leaf, although all have of course been similarly exposed.  The movements depend on innate causes, and are of an adaptive nature.  The alternations of light and darkness merely give notice to the leaves that the period has arrived for them to move in a certain manner.  We may infer from the fact of several plants (Tropaeolum, Lupinus, etc.) not sleeping unless they have been well illuminated during the day, that it is not the actual decrease of light in the evening, but the contrast between the amount at this hour and during the early part of the day, which excites the leaves to modify their ordinary mode of circumnutation.

As the leaves of most plants assume their proper diurnal position in the morning, although light be excluded, and as the leaves of some plants continue to move in the normal manner in darkness during at least a whole day, we may conclude that the periodicity of their movements is to a certain extent inherited.* The strength of such inheritance differs

* Pfeffer denies such inheritance; he attributes (’Die Period.  Bewegungen,’ pp. 30-56) the periodicity when prolonged for a day or two in darkness, to “Nachwirkung,” or the after-effects of light and darkness.  But we are unable to follow his train of reasoning.  There does not seem to be any more reason for [[page 408]] attributing such movements to this cause than, for instance, the inherited habit of winter and summer wheat to grow best at different seasons; for this habit is lost after a few years, like the movements of leaves in darkness after a few days.  No doubt some effect must be produced on the seeds by the long-continued cultivation of the parent-plants under different climates, but no one probably would call this the “Nachwirkung” of the climates. [page 408] much in different species, and seems never to be rigid; for plants have been introduced from all parts of the world into our gardens and greenhouses; and if their movements had been at all strictly fixed in relation to the alternations of day and night, they would have slept in this country at very different hours, which is not the case.  Moreover, it has been observed that sleeping plants in their native homes change their times of sleep with the changing seasons.*