From Prof. Wiesner's interesting observations, it is probable that the above movements have been acquired for a special purpose. the chlorophyll in leaves is often injured by too intense a light, and Prof. Wiesner* believes that it is protected by the most diversified means, such as the presence of hairs, colouring matter, etc., and amongst other means by the leaves presenting their edges to the sun, so that the blades then receive much less light. He experimented on the young leaflets of Robinia, by fixing them in such a position that they could not escape being intensely illuminated, whilst others were allowed to place themselves obliquely; and the former began to suffer from the light in the course of two days.
In the cases above given, the leaflets move either upwards
* 'Die Näturlichen Einrichtungen zum Schutze des Chlorophylls,' etc., 1876. Pringsheim has recently observed under the microscope the destruction of chlorophyll in a few minutes by the action of concentrated light from the sun, in the presence of oxygen. See, also, Stahl on the protection of chlorophyll from intense light, in 'Bot. Zeitung,' 1880. [page 447]
or twist laterally, so as to place their edges in the direction of the sun's light; but Cohn long ago observed that the leaflets of Oxalis bend downwards when fully exposed to the sun. We witnessed a striking instance of this movement in the very large leaflets of O. Ortegesii. A similar movement may frequently be observed with the leaflets of Averrhoa bilimbi (a member of the Oxalidae); and a leaf is here represented (Fig. 180) on which the sun had shone. A diagram (Fig. 134) was given in the last chapter, representing the oscillations by which a leaflet rapidly descended under these circumstances; and the movement may be seen closely to resemble that (Fig. 133) by
Fig. 180. Averrhoa bilimbi: leaf with leaflets depressed after exposure to sunshine; but the leaflets are sometimes more depressed than is here shown. Figure much reduced.
which it assumed its nocturnal position. It is an interesting fact in relation to our present subject that, as Prof. Batalin informs us in a letter, dated February, 1879, the leaflets of Oxalis acetosella may be daily exposed to the sun during many weeks, and they do not suffer if they are allowed to depress themselves; but if this be prevented, they lose their colour and wither in two or three days. Yet the duration of a leaf is about two months, when subjected only to diffused light; and in this case the leaflets never sink downwards during the day.]
As the upward movements of the leaflets of Robinia, and the downward movements of those of Oxalis, have been proved to be highly beneficial to these plants when subjected to bright sunshine, it seems probable that they have been acquired for the special purpose of avoiding too intense an illumination. As it would have been very troublesome in all the above cases to [page 448] have watched for a fitting opportunity and to have traced the movement of the leaves whilst they were fully exposed to the sunshine, we did not ascertain whether paraheliotropism always consisted of modified circumnutation; but this certainly was the case with the Averrhoa, and probably with the other species, as their leaves were continually circumnutating. [page 449]
CHAPTER IX.
SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.
Uses of heliotropism--Insectivorous and climbing plants not heliotropic-- Same organ heliotropic at one age and not at another--Extraordinary sensitiveness of some plants to light--The effects of light do not correspond with its intensity--Effects of previous illumination--Time required for the action of light--After-effects of light--Apogeotropism acts as soon as light fails--Accuracy with which plants bend to the light-- This dependent on the illumination of one whole side of the part--Localised sensitiveness to light and its transmitted effects--Cotyledons of Phalaris, manner of bending--Results of the exclusion of light from their tips-- Effects transmitted beneath the surface of the ground--Lateral illumination of the tip determines the direction of the curvature of the base-- Cotyledons of Avena, curvature of basal part due to the illumination of upper part--Similar results with the hypocotyls of Brassica and Beta-- Radicles of Sinapis apheliotropic, due to the sensitiveness of their tips-- Concluding remarks and summary of chapter--Means by which circumnutation has been converted into heliotropism or apheliotropism.