HELIOTROPISM. 



693 



Sunflower, Tournesol, on that account), it is possible to notice how its apical parts 

 follow the course of the sun from morn to eve, always inclining towards it. 



The point is, then, that growing parts of the plant should be illuminated 

 from one side only, or at any rate more strongly there than on the opposite side; hence 

 in most plants cultivated in a room or in a greenhouse very evident heliotropic 

 curvatures of the shoot-axes, petioles, and, in part, the lamina itself may be noticed. 

 Apart from exceptions to be mentioned later, the apical portions of the shoot become 

 directed towards the window and curved convex on the side next the room : petioles 

 behave similarly as a rule, and, in general, by means of the heliotropism of both, 

 the final effect is that the laminae, especially if freely movable on long stalks, 

 assume such a position that the upper side 

 presents itself approximately at right-angles 

 to the strongest incident rays of light. This 

 betrays at the same time the chief purpose 

 of the heliotropic curvatures : in combina- 

 tion with their geotropic properties, the helio- 

 tropism of the shoot-axes and leaves acts in 

 such a way as to place these organs in posi- 

 tions favourable to the well-being of the 

 plant. In this sense it is also to be under- 

 stood that some organs, in accordance with 

 special relations of life of the plant, turn their 

 free ends away from the source of light to- 

 wards the darkest side, and thus behave as 

 negatively heliotropic in contrast to the above 

 described positively heliotropic ones: this is 

 the case, for example, with the aerial roots 

 which serve for climbing in Aroids, &c., as 

 well as the climbing shoot- axes of the 

 common Ivy, which, when cultivated in a 

 room, always tend to curve away from 

 the window. It is very remarkable also that 

 roots, which normally grow beneath the 

 soil, when cultivated in transparent water or 

 in moist air, prove to be heliotropic, some of them positively, others negatively; these 

 roots thus possess in their heliotropism a form of irritability from which, under 

 normal conditions of life, they derive no use whatever, and which therefore certainly 

 cannot have been inherited by them according to Darwinian principles. 



However, these remarks are merely preliminary. In order to study the pheno- 

 mena themselves more closely, we may avail ourselves of a very simple experi- 

 ment. We assume the seedling of the White Mustard {Sinapis Alba), shown 

 in the accompanying figure (Fig. 391), to have its root bde submerged in clear 

 water «w, contained in a transparent glass vessel. We further suppose the plant 

 to be fixed in this position in any convenient manner, and to have been placed 

 hitherto either in the dark or on a horizontal revolving disc in the light, so that it was 

 either not illuminated at all or was equally so on all sides. In both cases the primary 



FIG. 391.— Seedling of White Mustard {Sinafts 

 alba), showing the heliotropic curvature of the shoot- 

 axis and root. 



