December 23, 1915] 



NATURE 



469 



Formerly it was thought that the light acts directly 

 ^1 the part that bends, but the researches of C. and 

 E. D^arwin more than thirty years ago proved that 

 young seedlings this is certainly not the case. 

 ley showed quite conclusively by means of a large 

 mber of carefully contrived experiments that the 

 liotropic curvature in the lower part of a seedling 

 determined by the illumination of the upper part. 

 )nsequently no curvature can take place until a 

 tnulus has been transmitted from the upper part, 

 lich behaves as a light-perceiving organ, to the 

 rer part in which the motor response takes place, 

 "^oliage leaves are not usually so sensitive to light 

 the plumules of young seedlings, and do not in 

 my respects so readily admit of experimental in- 

 Kstigations. We know that the leaf-stalk bends 

 'wards and tends to place itself parallel to the rays 

 light, and that the leaf-blade places itself at right 

 angles to the rays of light. We know that when 

 the leaf reaches the position of maximum advantage 

 the movement towards the light ceases, and it then 

 remains fixed, except for slight circumnutating move- 

 ments, until either the direction of the light changes 

 or its intensty is decreased. But we do not yet know 

 — and the problem is not an easy one to solve — by 

 what means the leaf is enabled to adjust its position 

 to the direction of the rays of light, nor just how it 

 perceives that it is or is not in the most advantageous 

 position. 



Fig. \.—Eranthis hieinalis. Le.if stalks curving towards light coming in 

 the direction indicated by the arrow. 



Dutrochet suggested, without any experimental 

 evidence to support it, that the lamina of the leaf 

 exerts an influence on the movement of the leaf-stalk. 

 Hanstein also considered that the lamina was the 

 light-sensitive part of the plant, and even went so far 

 as to compare it with the retina of the eye. C. and 

 F. Darwin were the first to attempt to determine the 

 point experimentally. 



Pieces of blackened paper were gummed to the 

 edges and over the blades of some leaves on young 

 plants of Tropaeoltim majiis and Ranimcuhis ficaria ; 

 these were then placed in a box before a window, 

 and the petioles of the protected leaves became curved 

 towards the light as much as those of the unprotected 

 leaves. 



Rothert repeated Darwin's experiment on 

 Tropaeolum, and found that the leaves reach the 

 right position vihether darkened or not. Krabbe also 

 showed by his ^periments on Phaseolus and Fuchsia 

 that when the leaf-blades were darkened the leaves 

 reach the right position just as readily and as pre- 

 cisely as the undarkened leaves. On the other hand, 

 Vochting came to the conclusion from his exi>eriments 

 on Malva that the curvature of the leaf-stalk only 

 followed when the blade of the leaf was illuminated. 



Haberlandt concluded from his experiments on a 

 variety of leaves that in some cases the lamina is the 

 only percipient organ, that in others both lamina and 



it-stalk are concerned, and that in very few cases 

 the leaf-stalk or pulvinus ^alone responsible. He 

 (insiders that when both laniina and leaf-stalk are 

 NO. 2408, VOL. 96] 



concerned the larger movement is probably brought 

 about by the leaf-stalk and the finer regulating 

 moverrient by the lamina. 



The experiments which I am about to describe are 

 concerned in the first instance with the problem : 

 Does the lamina perceive the light, or is the leaf- 

 stalk the percipient organ, or do both take part in it? 



The observations were carried out by a method 

 suggested by the extremely ingenious and charming 

 device employed by F. Darwin to prove that the geo- 

 tropic sensitiveness of the plumule of a grass seedling 

 is localised at the apex. It consists essentially in 

 keeping the blade of the leaf fixed while the petiole 

 is free to move. Thus if the blade of the leaf is 

 kept in a horizontal position and then exposed to 

 oblique light, what effect will be produced on the 

 petiole? If it is free to move, the petiole ought to 

 curve towards the light; and if the stimulus is 

 localised in the leaf-blade, the curvature ought 

 theoretically to continue so long as the stimulus con- 

 tinues to act and the petiole is capable of growth. 



The experiment was first of all tried with a number 

 of leaves of Eranthis hiemalis. The leaves were care- 

 fully removed from the plant ; the blades were then at- 

 tached to a glass plate, and the stalks were allowed to 

 hang downwards in a glass vessel containing water. 



Fig. ■i.-^Trofiaeolum majus. Leaf exposed to lateral 

 light and then turned round. The curvature of the 

 petiole becomes reversed. 



On exposure to an oblique lateral light the stalks 

 very soon began to curve towards the light, and 

 continued to curve in the same direction for several 

 hours, until in many cases a complete spiral was 

 formed (Fig. i). Similar results were obtained with 

 the leaves of many other plants. If a leaf in the 

 petiole of which this heliotropic curvature has been 

 induced is turned round so that the light impinges 

 upon it on the opposite side, the curvature becomes 

 reversed (Fig. 2). 



The advantages of this method are that the leaves 

 are not submitted to the rough treatment necessary 

 to darken the blades or stalks, and, secondly, there is 

 less interference with the respiratory and assimilatory 

 functions. The disadvantages are that the leaf-stalks, 

 being free to movie, may be stimulated by gravity, and 

 the pronounced curvatures thus induced may, unless 

 proper precautions are taken, be mistaken for photo- 

 tropic curvatures. So long as it is approximately 

 vertical, the leaf-stalk is not influenced, or only 

 slightly, by gravity, but immediately it moves from 

 the vertical in response to the light stimulus, the in- 

 fluence of gravity comes into play, and light in con- 

 junction with the gravitational stimulus takes a share 

 in effecting its curvature. As soon, however, as the 

 leaf-stalk in its upward movement passes beyond the 

 vertical, the gravitational stimulus tends to bring it 

 back to the vertical position, and the light stimulus 

 then, in order to effect any further curvature, has to 

 continue its action against the force of gravity. 



