ACTION OF LIGHT ON VEGETATION, 677 



In addition to the large number of the parts of plants which, when illumi- 

 nated unequally bend so as to make the more strongly illuminated side concave, 

 there are a much smaller number which bend in the opposite direction, t. e. become 

 concave on the shaded side. In order to distinguish between them the former are 

 termed positively, the latter negatively heliotropic. 



Both positive and negative heliotropism occur not only in organs containing 

 chlorophyll, but also in those that are colourless ; among the former in the green 

 tendrils of Vitis and Ampelopsis^ ; among the latter in the colourless root-hairs 

 of Marchantia^, the aerial roots of Aroidese, Orchideae, and Chlorophytiim Gayarnwi, 

 and the rootlets of some Dicotyledons, as Brassica Napus and Siftapis alba^. 

 From the fact that' positive heliotropism depends on a retardation of the growth 

 of the organ exposed to the stronger light, it might be inferred that negative 

 heliotropism is occasioned conversely by a more vigorous growth of the side 

 exposed to the stronger light. This conclusion would be confirmed by a superficial 

 examination of the phenomena; but if the attendant circumstances are observed 

 more closely, some considerations arise which I shall examine in detail in Chap. 

 IV. It need only be mentioned here that according to a theory started by Wolkoff, 

 two different explanations are possible : — Very transparent organs, like the apices 

 of the roots of Aroidese and of Chlorophytum refract the light which falls upon 

 them in such a manner, that the shaded side of the organ may actually be more 

 strongly illuminated than the other; and its negative heliotropism is then only 

 a special case of positive heliotropism. But in other cases, as in the ivy and 

 TropcBohim ??iajus, the internodes are positively heliotropic when young, but 

 negatively when old before growth ceases ; and Wolkoff supposes that the curvature 

 which is in these cases convex on the illuminated side is caused by the more 

 vigorous assimilation and consequent longer duration of growth. It depends there- 

 fore upon nutrition which only affects the mechanism of growth in a secondary 

 degree. 



(/) Aciio7i of Light on the tension of the tissue of the coiitractile organs of leaves 

 endowed zvilh motion^. The leaf-blades of Leguminosse, Oxalideae, Marantaceae, 

 Marsileaceae &c., are borne on modified petioles which serve as contractile organs, 

 bending upwards or downwards under various external and internal influences, 

 and thus giving a variety of positions to the leaf-blades. If these plants are 

 placed in permanent darkness, the curvatures due to internal changes alternate 

 upwards and downwards. Light exercises an immediate influence on these peri- 

 odically contractile organs ; any increase of its intensity tends to give the blade an 

 expanded position, such as it occupies in the day-time ; any diminution tends to 

 cause it to assume a closed position upwards or downwards such as it has m the 

 night. This sensitiveness, which I previously termed ' paratonic,' is not the cause 

 of the periodic movements ; but rather counteracts the periodicity caused by the 

 internal forces. In most leaves endowed with periodic movements the paratonic 



1 Knight, Phil. Trans. 181 2, Pt. I, p. 314- 



2 Pfeffer, Arbeiten des hot. Inst, in Wiirzburg 1871, Heft I, Div. 2. 



3 For the literature on this subject see Sachs, Exp.-Phys. p. 41- 



* See Sachs, Ueber vorhergehende Starrezustande, &c., Flora, 1S63. — Further details will 

 be given in Chap. IV. 



