We have hitherto referred only to those leaves and cotyledons which occupy a permanently horizontal position; but many stand more or less obliquely, and some few upright. the cause of these differences of position is not known; but in accordance with Wiesner’s views, hereafter to be given, it is probable that some leaves and cotyledons would suffer, if they were fully illuminated by standing at right angles to the light.

We have seen in the second and fourth chapters that those cotyledons and leaves which do not alter their positions at night sufficiently to be said to sleep, commonly rise a little in the evening and fall again on the next morning, so that they stand during the night at a rather higher inclination than during the middle of the day.  It is incredible that a rising movement of 2o or 3o, or even of 10o or 20o, can be of [page 443] any service to the plant, so as to have been specially acquired.  It must be the result of some periodical change in the conditions to which they are subjected, and there can hardly be a doubt that this is the daily alternations of light and darkness.  De Vries states in the paper before referred to, that most petioles and midribs are apogeotropic;* and apogeotropism would account for the above rising movement, which is common to so many widely distinct species, if we suppose it to be conquered by diaheliotropism during the middle of the day, as long as it is of importance to the plant that its cotyledons and leaves should be fully exposed to the light.  The exact hour in the afternoon at which they begin to bend slightly upwards, and the extent of the movement, will depend on their degree of sensitiveness to gravitation and on their power of resisting its action during the middle of the day, as well as on the amplitude of their ordinary circumnutating movements; and as these qualities differ much in different species, we might expect that the hour in the afternoon at which they begin to rise would differ much in different species, as is the case.  Some other agency, however, besides apogeotropism, must come into play, either directly or indirectly, in this upward movement.  Thus a young bean (Vicia faba), growing in a small pot, was placed in front of a window in a klinostat; and at night the leaves rose a little, although

* According to Frank (’Die nat.  Wagerechte Richtung von Pflanzentheilen,’ 1870, p. 46) the root-leaves of many plants, kept in darkness, rise up and even become vertical; and so it is in some cases with shoots. (See Rauwenhoff, ‘Archives Néerlandaises,’ tom. xii. p. 32.) These movements indicate apogeotropism; but when organs have been long kept in the dark, the amount of water and of mineral matter which they contain is so much altered, and their regular growth is so much disturbed, that it is perhaps rash to infer from their movements what would occur under normal conditions. (See Godlewski, ‘Bot.  Zeitung,’ Feb. 14th, 1879.) [page 444]