of diaheliotropism than that offered by many seedlings, the cotyledons of which are extended horizontally.  When they first burst from their seed-coats they are in contact and stand in various positions, often vertically upwards; they soon diverge, and this is effected by epinasty, which, as we have seen, is a modified form of circumnutation.  After they have diverged to their full extent, they retain nearly the same position, though brightly illuminated all day long from above, with their lower surfaces close to the ground and thus much shaded.  There is therefore a great contrast in the degree of illumination of their upper and lower surfaces, and if they were heliotropic they would bend quickly upwards.  It must not, however, be supposed that such cotyledons are immovably fixed in a horizontal position.  When seedlings are exposed before a window, their hypocotyls, which are highly heliotropic, bend quickly towards it, and the upper surfaces of their cotyledons still remain exposed at right angles to the light; but if the hypocotyl is secured so that it cannot bend, the cotyledons themselves change their position.  If the two are placed in the line of the entering light, the one furthest from it rises up and that nearest to it often sinks down; if placed transversely to the light, they twist a little laterally; so that in every case they endeavour to place their upper surfaces at right angles to the light.  So it notoriously is with the leaves on plants nailed against a wall, or grown in front of a window.  A moderate amount of light suffices to induce such movements; all that is necessary is that the light should steadily strike the plants in an oblique direction.  With respect to the above twisting movement of cotyledons, Frank has given many and much more striking instances in the case of the leaves on [page 440] branches which had been fastened in various positions or turned upside down.

In our observations on the cotyledons of seedling plants, we often felt surprise at their persistent horizontal position during the day, and were convinced before we had read Frank’s essay, that some special explanation was necessary.  De Vries has shown* that the more or less horizontal position of leaves is in most cases influenced by epinasty, by their own weight, and by apogeotropism.  A young cotyledon or leaf after bursting free is brought down into its proper position, as already remarked, by epinasty, which, according to De Vries, long continues to act on the midribs and petioles.  Weight can hardly be influential in the case of cotyledons, except in a few cases presently to be mentioned, but must be so with large and thick leaves.  With respect to apogeotropism, De Vries maintains that it generally comes into play, and of this fact we shall presently advance some indirect evidence.  But over these and other constant forces we believe that there is in many cases, but we do not say in all, a preponderant tendency in leaves and cotyledons to place themselves more or less transversely with respect to the light.