470 



NATURE 



[December 23, 1915 



A striking experiment to show that curvatures may 

 be effected by the phototropic stimulus against the 

 gravitational stimulus was made by placing leaves 

 upside down. Three young leaves of Eranthis hiemalis 

 had their leaf-blades securely fixed between two pieces 

 of black cardboard, the leaf-stalks passing through 

 small holes in one of the pieces of card. The leaves 

 were then placed with the stalks projecting upwards 

 in water in a rectangular glass vessel. Three sides of 

 the glass were darkened, the other side was exposed 

 to a dull lateral light. In the course of the day 

 (Fig. 3) the stalks curved distinctly towards the light 

 against the force of gravity which tends to keep them 

 vertical. 



In all these experiments the light was allowed to 

 act upon the whole of the leaf, both blade and leaf- 

 stalk, but as in many cases the leaf-stalk itself is 

 phototropically sensitive, it was important to deter- 

 mine to what extent either of these organs, submitted 

 separately to the influence of light, might bring about 

 the curvature. Accordingly, experiments were made 

 by which the leaf-blades only were exposed to oblique 

 light. This was done by fitting a light tight cover 

 over an opaque vessel containing water. The stalks 

 of the leaves were passed through small apertures in 

 the cover and allowed to hang down in the water. 

 The blades resting on the surface of the cover 



Fig. 3. — Eranthis hiemalis. Three leaves plsc:d upside down, 

 with their stalks vertical {a) ; on exposure to light they curved 

 towards it {/>). 



were then covered with a piece of clear glass and 

 exposed to the light. After some time the stalks 

 were found to be curved more or less in different 

 directions, no doubt due to the geotropic stimulus, 

 but there was no definite curvature towards the light, 

 although in many experiments the leaves were exposed 

 to the light for a week and even longer. 



When, however, the leaf-stalks are exposed to the 

 light and the blades kept in the dark, the stalks all 

 curve distinctly to the light. A large number of leaves 

 belonging to different families of plants was tested 

 in this way, and the result was always the same. 

 The conclusion therefore seems justified that the per- 

 ception of light is located not in the leaf-blade but in 

 the leaf-stalk. 



The further problem then arises : Does the whole 

 of the leaf-stalk perceive the light or only a portion 

 of it? Have we in the leaf-stalk, as found by C. and 

 F. Darwin in the plumules of seedlings, a percipient 

 region and a motor region separated from one 

 another? To answer this question a simple piece of 

 apparatus was devised, consisting of a shallow box 

 about 10 mm. high, with a thin base and a thin top, 

 leaving a space of about 7 or 8 mm. between them. 

 This was open at one end, and fitted light-tight over 

 an opaque vessel containing water. Through small 

 holes in the top and bottom of the box the leaf-stalks 

 were passed, so that the lower portions were in the 

 dark, the upper 7 or 8 mm. at the apex of the leaf- 

 stalk being exposed to the light. The leaf-blades 

 resting on the upper surface of the box were covered 



NO. 2408, VOL. 96] 



with a piece of black card, and the apparatus was 

 then placed in such a position that light rays entered 

 the box and impinged upon the upper part of the leaf- 

 stalks only. Before the experiment was started, how- 

 ever, in all cases the stalks were allowed to stand 

 for some time in the dark until geotropic curvatures 

 were set up ; the position of the leaves was then so 

 adjusted that the darkened parts of the leaf-stalks 

 were all curved in the opposite direction to that of 

 the light incident upon the upper parts. 



Under these conditions the heliotropic stimulus was 

 acting in opposition to the geotropic stimulus. The 

 results obtained were most striking. The curvature 

 towards the light was very marked, and distinct spiral 

 curvatures were produced (Fig. 4). 



From experiments made in this way on a large 

 number of plants it was found that the apex of the 

 leaf-stalk for a distance of a few millimetres behaves 

 as a percipient region, and is capable of inducing a 

 motor response in the lower part. Experiments were 

 made to determine how much of the apical region 

 it is necessary to expose to the light in order to obtain 

 a response. Leaves of Geranium pratense and 

 Tropaeolum minus were arranged to allow different 



Fig. 4. — Tropaeoliitn tnajus. Upper 6 mm. of stalk only 

 exposed to light. The direction of the light is indi- 

 cated by the arrow. 



lengths of the apex, i mm., 2 mm., 4 mm., 6 mm., 

 8 mm., and 10 mm. respectively, to be exposed to 

 the light. A distinct response was obtained in each 

 case, but the most definite results were obtained with 

 lengths of from 4 to 10 millimetres. 



We have now to consider briefly the mechanics of 

 the movement. The curvature of the stalk is brought 

 about by a more rapid elongation or growth on one 

 side. The tissues of which the stalk is composed are 

 all in a state of strain. The pith and vascular cylinder 

 tend to expand, the cortical tissues to contract. Con- 

 sequently if the stalk is split down the middle the 

 two halves curve outwards, and, if placed in' water, 

 may coil up into a spiral. Now, how does the photo- 

 tropic stimulus affect this state of strain? What will 

 be the effect of splitting a leaf-stalk that has become 

 curved under the influence of light? Will the two 

 halves coil themselves up in opposite directions as 

 before, or will it be found that the tensions have 

 become modified, and the curvatures also modified in 

 consequence? The experiment was tried on a number 

 of different leaves, and it was found that in all cases 

 the posterior half of the leaf-stalk retains the helio- 

 tropic curve, but the end of it tends to coil backwards 

 as before. It is obvious that the light stimulus brings 

 about a permanent change by which the relationships 

 of the tissues to one another as regards their tensions 

 are modified. 



Now what will happen if a stalk is split before the 

 heliotropic stimulus is- applied? Will the stimulus 

 affect the two halves, or will the posterior half remain 



