In the cases above alluded to of seedlings exposed to a lateral light with their hypocotyls secured, it is impossible that epinasty, weight and apogeotropism, either in opposition or combined, can be the cause of the rising of one cotyledon, and of the sinking of the other, since the forces in question act equally on both; and since epinasty, weight and apogeotropism all act in a vertical plane, they cannot cause the twisting of the petioles, which occurs in seedlings under the

* ‘Arbeiten des Bot.  Instituts in Würzburg,’ Heft. ii. 1872, pp. 223-277. [page 441]

above conditions of illumination.  All these movements evidently depend in some manner on the obliquity of the light, but cannot be called heliotropic, as this implies bending towards the light; whereas the cotyledon nearest to the light bends in an opposed direction or downwards, and both place themselves as nearly as possible at right angles to the light.  The movement, therefore, deserves a distinct name.  As cotyledons and leaves are continually oscillating up and down, and yet retain all day long their proper position with their upper surfaces directed transversely to the light, and if displaced reassume this position, diaheliotropism must be considered as a modified form of circumnutation.  This was often evident when the movements of cotyledons standing in front of a window were traced.  We see something analogous in the case of sleeping leaves or cotyledons, which after oscillating up and down during the whole day, rise into a vertical position late in the evening, and on the following morning sink down again into their horizontal or diaheliotropic position, in direct opposition to heliotropism.  This return into their diurnal position, which often requires an angular movement of 90o, is analogous to the movement of leaves on displaced branches, which recover their former positions.  It deserves notice that any force such as apogeotropism, will act with different degrees of power* in the different positions of those leaves or cotyledons which oscillate largely up and down during the day; and yet they recover their horizontal or diaheliotropic position.

We may therefore conclude that diaheliotropic movements cannot be fully explained by the direct action of light, gravitation, weight, etc., any more

* See former note, in reference to Sachs’ remarks on this subject. [page 442]

than can the nyctitropic movements of cotyledons and leaves.  In the latter case they place themselves so that their upper surfaces may radiate at night as little as possible into open space, with the upper surfaces of the opposite leaflets often in contact.  These movements, which are sometimes extremely complex, are regulated, though not directly caused, by the alternations of light and darkness.  In the case of diaheliotropism, cotyledons and leaves place themselves so that their upper surfaces may be exposed to the light, and this movement is regulated, though not directly caused, by the direction whence the light proceeds.  In both cases the movement consists of circumnutation modified by innate or constitutional causes, in the same manner as with climbing plants, the circumnutation of which is increased in amplitude and rendered more circular, or again with very young cotyledons and leaves which are thus brought down into a horizontal position by epinasty.