HELIOTROPISM 465 



get the maximum amount of light. In general, however, we may recognize four 

 principal positions which the leaf may occupy with regard to a laterally 

 placed source of light ; it may be illuminated from the front, from behind, or 

 from either flank. The movements exhibited by the leaf have been described 

 by Frank (1870) in the following terms :— ' Leaves facing the light curve 

 until their upper sides are convex, they then bend downwards so as to expose 

 their surfaces to the light. The same relative position is obtained by leaves 

 on the shaded side by the upper surface becoming concave and bending 

 upwards. Lateral leaves obviously cannot attain the optimum light position 

 by bending in one place, and hence they exhibit torsions which place the laminae 

 in a vertical position, one margin pointing upwards, the other pointing down- 

 wards, the surface being orientated at right angles to the light.' These 

 phenomena are illustrated at Fig. 147. If, finally, the source of light be beneath 

 the leaf, a torsion of 180° takes place, so that the normal orientation of upper 

 and under surfaces with reference to light is again attained. This latter 

 case is illustrated by the leaves of 'weeping trees', and it may be induced 

 experimentally in other plants by artificial illumination from the under side, 

 e. g. by aid of a mirror. 



The region of heliotropic movement entirely depends on the structure and 

 distribution of growth in the leaf. Naturally, the basal parts of the leaf are 

 those most concerned, whether the base be in the form of a petiole or merely 

 part of the lamina. Leaves with long petioles are especially worthy of obser- 

 vation, since in such cases the angle which appears between the blade and 

 the petiole shows that the two component parts of the leaf do not react in 

 the same manner to the external stimulus, the petiole curving more into the 

 erect, the lamina more into the horizontal position. We shall find later on, 

 however, that the movement of the petiole is not independent but is initiated 

 by the lamina. If the leaf be branched, heliotropic movements are carried 

 out as a general rule by the individual leaflets, at all events after they reach 

 a certain age. 



Heliotropic curvatures are readily recognized as growth phenomena, but 

 the mechanics of the torsions have not as yet been fully explained. It has 

 long been believed that these torsions were occasioned only by the action 

 of a series of external factors, such as light, gravity, weight of the organ, which 

 individually led to curvatures, but in combination induced torsions, but later 

 investigations have shown that torsions might appear when light only was the 

 functional external factor. Thus Vochting (1888) has clearly demonstrated 

 torsions of purely photic origin in the leaves of the Malvaceae, and Schwen- 

 DENER and Krabbe (1892) have proved that such torsions do not occur in 

 the great majority of leaves when experimentally examined with the aid of 

 the klinostat, although they certainly made their appearance in the peduncles 

 of certain flowers. We cannot enter here into a discussion of the possible reasons 

 which may be advanced to account for the frequent non-occurrence of torsions 

 when plants are subjected to unilateral illumination on the khnostat ; we 

 need only note that, generally speaking, dorsiventral organs, when placed on 

 the klinostat, carry out special movements, such as the remarkable epinastic 

 curvatures of leaves (compare p. 449), and that it has not as yet been shown 

 whether these organs are stimulated geotropically on the klinostat or not 

 (compare p. 438). Although, among external factors, light alone is sufficient to 

 induce torsions, still there is one internal factor which might co-operate 

 (exotropy, Noll, 1885-7), but as to this we are not in a position to give a 

 final decision. If the torsions cannot generally be regarded as due to the 

 combination of two curvatures (Schwendener and Krabbe, 1892) we are 

 completely in the dark as to. the mechanics of their production. One fact only 

 seems to be established, viz. that, in general, growth also plays a part in the 

 production of these torsions. 



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