Dec. 23, i83o] 



NATURE 



179 



property of leaves : the first is that of Frank (" Die 

 natiirliche wagerechte Richtung von Pflanzentheilen," 

 1870), who ascribes to leaves and to some other organs 

 a specific sensitiveness to light called " transversal- 

 heliotropismus" or diaheliotropism (" Power of Move- 

 ment in Plants," p. 438). Just as an ordinary heliotropic 

 organ has an inherent tendency to become parallel to 

 incident light, so a diahehotropic organ has an inherent 

 tendency to place itself at right angles to the direction 

 of the light. The two classes of organs differ from each 

 other exactly as some creeping rhizomes differ from ordi- 

 nary stems ; the rhizome tends to extend itself horizontally 

 under ground, while the stem above the surface grows 

 vertically upwards (see Elfving, in Sachs' "Arbeiten," 

 1879). 



A different theory has been proposed by de Vnes 

 (Sachs' "Arbeiten," i. 1872), whose views are supported 

 by Sachs ("Arbeiten," ii. 1S79) with additions or modifi- 

 cations. According to these«views it is not necessaiy to 

 assume the existence of any special kind of heliotropism, 

 since the phenomena might result from the ordinary 

 forms of heliotropism and geotropism acting in concert. 

 Thus in the case of the seedling radish illuminated from 

 above, if the cotyledons were apheliotropic (negatively 

 heliotropic) and apogeotropic (negatively geotropic) it is 

 possible that they might be kept in equilibrium by these 

 opposing tendencies. The tendency to move away from 

 a vertical light will make the cotyledons curving down- 

 wards towards the earth, and the apogeotropism or 

 tendency to move away from the centre of the earth may 

 exactly balance the downward tendency, so that the 

 cotyledons remain horizontal. 



Besides the various geotropic and heliotropic tendencies 

 there are other modes of growth which may enter into 

 the combination. In some cases there is a natural pre- 

 ponderance of longitudinal tension or growth along the 

 upper surfaces of the petiole, so that owing to impulses 

 arising within the plant there is a tendency for the leaf to 

 curve downwards, or more accurately in the direction in 

 which the morphologically lower side of the petiole is 

 directed ; this tendency is called longitudinal epinasty, or 

 simply epinasty ; the opposite tendency is called hypo- 

 nasty. According to the theories of de Vries and Sachs 

 epinasty may be opposed by heliotropism, or by apogeo- 

 tropism, while hyponasty will of course be opposed by 

 apheliotropism and geotroprism, and all these opposing 

 forces may combine in producing an equilibrium. The 

 object of the present paper is to test the relative values of 

 the two above described theories : that of Frank, and that 

 of de Vries and Sachs. 



The method employed was to fix the plants under 

 observation to a horizontal spindle, which was kept in 

 slow rotation by clockwork. This instrument (called the 

 klinostat) has been employed by Sachs for the study of 

 ordinary heliotropism ; light is admitted parallel to the 

 axis of rotation, and the plants are thus subjected to a 

 constant lateral illumination, while they arc freed from the 

 disturbing influence of gravitation, for,owing to their being 

 kept in constant slow rotation, there is no reason why 

 they should bend apogeotropically in one direction more 

 than another (see Sachs in his "Arbeiten," Bd. ii. 1S79). 

 On the same principle the behaviour of leaves which 

 place themselves at right angles to the incident liglit has 

 been studied. If a plant with horizontally-extended leaves, 

 which has been illuminated from above, is fi.xed on a 

 slowly-revolving, horizontal spindle, so that the axis of the 

 plant is parallel both to the axis of rotation and to the 

 direction of incident light, we shall have a means of 

 testing the opposing theories above mentioned. 



The plant's leaves will still be illuminated by light 

 striking them at right angles ; therefore if Frank's theory 

 is the right one they ought to remain in this position. But 

 if de Vries and Sachs are correct in their views, the leaves 

 ought jwt to be able to remain at right angles to the 



incident-light, since apogeotropism has disappeared, which 

 was one of the tendencies necessary to keep the leaves in 

 a position of equilibrium. 



A considerable number of experiments were made with 

 the celandine, Raiiunculus ficaria, the results of which 

 are decidedly in favour of Frank's views. The leaves of 

 the celandine are sometimes extremely epinastic, so that 

 they press against the ground, and when a plant is dug 

 up It often happens that, the leaves being released from 

 the resistance of the soil, curve nearly vertically dow-n- 

 wards. If such a plant is fixed on the klinostat in the 

 position above described, the leaves will be pointing away 

 from the light, so that if the leaves were apheliotropic, as 

 might be expected according to de Vries' theory, the leaves 

 would remain pointing away from the window. But this 

 is not the case, they move forwards until they are 

 approximately at right angles to the light, and then come 

 to rest. Again, if a celandine is placed in the dark its 

 leaves rise up so as to be highly inclined above the 

 horizon, if the plant is then placed on the klinostat the 

 leaves (which now of course point towards the light) 

 again accommodate themselves by curving backwards until 

 they are at right angles to the light. Thus the leaves 

 cannot be called heliotropic or apheliotropic ; we are 

 forced to believe that under the stimulus of light they are 

 able to move in either direction, which may be necessary 

 to bring them into the plane at right angles to the light. 

 The other experiments with R.ficaria,<!i\z details of which 

 we omit, lead to the same general result. 



Besides a few observations on Vicia, Cucurbita, and 

 Plantago, a series of experiments were made on seedling- 

 cherries, and these lead to a somewhat different result. 

 A cherry-plant illuminated from above has its leaves 

 approximately horizontal, and when placed on the klino- 

 stat, as above described, the leaves are unable to remain 

 at right angles to the light, but curve backwards so as to 

 become parallel to the stem of the plant. This move- 

 ment can be shown to be due to epinasty, not to apheliotro- 

 pism, and is the result of the loss of balance which follows 

 when apogeotropism is removed. It is clear therefore 

 that the horizontal position of the leaves of seedling- 

 cherries growing normally must largely depend on the 

 balance struck between epinasty and apogeotropism, in 

 accordance with the views of de Vries and Sachs. But 

 since these forces obviously cannot produce the power 

 which the cherry possesses, of altering the position of its 

 leaves in accordance with the direction of the light, we 

 must assume that some kind of heliotropism enters into 

 the combination. The view to which the present research 

 lends most probability is that dia-heliotropism (transverse- 

 hehotropism) is the really important influence at work. 

 In the case of the celandine we have seen that the sensi- 

 tiveness to light is strong enough to determine the 

 position of the leaves— although the jiatural balance is 

 disturbed by the annihilation of apogeotropism. It seems 

 probable that an essentially similar state of things holds 

 good in the case of the cherry. When the plant is grow- 

 ing normally it trusts to epinasty and apogeotropism^ to 

 produce an approximate balance, the final result being 

 determined by the stimulus of light. But when the 

 balance is disturbed by placing the plant on the klinostat, 

 the light-stimulus is not strong enough to produce a 

 condition of equilibrium. 



This view is the same as that given in '■ The Power of 

 ?vIovements in Plants," and is in accordance with the 

 principle there given : that the chief movements in plants 

 are due to modifications of the circumnutating motion. 



II. When a cutting, for instance a piece of a willow- 

 branch, is placed in circumstances favourable for growth, 

 it produces roots at its lower end, while the buds at its 

 upper end grow out into branches. The experiments of 

 Vochting (" Organbildung im Pflanzenreich," Bonn, 187S) 

 on the growth of cuttings were made by suspending 



