GROWTH AND MOVEMENT 



27 



the light. Roots as a rule react in the opposite sense, turning away 

 from the source of light, when they are exposed to it. This converse 

 reaction of root and shoot may be seen with special clearness in 

 seedlings grown floating on water, and 

 lighted only from one side (Fig. 88). 

 Such reactions are described as pheno- 

 mena of Heliotropism. The parts which 

 curved towards the light are styled 

 positively heliotropic, those which curve 

 away from it negatively heliotropic. But 

 shoots of some plants are negatively 

 heliotropic. The climbing shoots of 

 the Ivy, and especially the tendrils 

 with attachment-discs of the Virginia 

 Creeper, which are in fact shoots speci- 

 alised for climbing, turn from the light. 

 By so doing they approach the tree- 

 trunk or other support. Thus positive 

 heliotropism is not inherent in all 

 shoots. Other parts dispose them- 

 selves so as to offer the flattened 

 surface to the incidence of light. This 

 is so with most leaves. The curvature 



leading to this result is styled transverse heliotropism.. But again, this 

 is not characteristic of all leaves. The mature Gum-Trees of Australia 

 have their leaves twisted so that they are expanded in a vertical 

 plane, though their seedling leaves are disposed as usual (Figs. 3, 4,). 



The heliotropic reaction appears as before in the zone of most rapid growth, 

 and it consists in an inequaHty of the rapidity of growth on the sides of the 

 part towards and away from the source of light. It is less certain here than 

 in geotropism exactly how or where the stimulus is received , It has been 

 suggested that the prevalent convexity of surface of the superficial cells of 

 stem or leaf makes each cell act as a condensing lens, concentrating the light 

 upon the receptive protoplast. There is no doubt that the cells do so act, 

 for photographs of various objects have been made, using epidermal cells as 

 lenses. Moreover, certain modifications of superficial cell-walls are consistent 

 with specialisation of this lens-character, and are held to support an " ocellar " 

 theory. Whether or not this is the right explanation of the receptivity in 

 Heliotropism, the ultimate receptive body is, as in geotropism, the living 

 protoplast. 



A common occurrence in the country is that drains of porous tile 

 are stopped by invading roots of neighbouring trees. It sometimes 



Fig. 88. 



Seedling of Cress, germinated with its 

 root in water, and exposed to oblique light - 

 ing from the right. The shoot shows a 

 curve of positive, and the root of negative 

 heliotropism. (Dr. J. M. Thompson.) 



