LECTURE XVII. 



place at the lower ends of the internodes. The same effect 

 is produced even after normal growth in length has ceased, 

 for the heliotropic action of light induces a resumption of 

 growth. 



In connexion with these considerations there naturally 

 arises the question as to the seat of heliotropic irritability in 

 growing organs. It would appear that, in most cases, the 

 whole growing region is irritable, whereas in some cases the 

 irritability is localised. Darwin concludes from his experi- 

 ments with Phalaris canariensis, that the illumination of the 

 upper part of the cotyledons of seedlings of this plant mate- 

 rially affects the capacity for curvature, and the extent of it, 

 in the lower part ; yet some observations seemed to render it 

 probable that the simultaneous stimulation of the lower part 

 by light greatly favours, or is almost necessary for, its well- 

 marked curvature. Experiments made upon cotyledons of 

 Avena sativa (Oat), on hypocotyls of Brassica oleracea (com- 

 mon Cabbage) and of Beta vulgaris, and on roots of seedlings 

 of Sinapis alba, led him to much the same conclusions. 



The experiments were performed by covering the tips of the organs 

 with opaque screens, or by painting them black. In the majority of cases 

 the organs thus treated remained upright when exposed to lateral light, 

 whereas similar organs which had not been so treated became strongly 

 curved. 



With regard to the mode in which heliotropic curvature is 

 effected, it may be stated generally that it is due to a change 

 in the relative length of the two sides of the organ. When 

 the organ is growing rapidly, both sides elongate, but the side 

 which will be convex does so more rapidly than the side 

 which will be concave ; when the organ is growing slowly, 

 the concave side elongates but little ; when curvature takes 

 place in an organ which is not growing, the concave side 

 shortens. 



In illustration of the relation between the elongation of the two sides, 

 the following table of measurements made by Miiller-Thurgau may be 

 given. The numbers give, in millimetres, the elongation of the two 

 sides of a portion, 20 millimetres long, of a growing shoot of Valeriana 

 officinalis. In the column A are the successive increments in length of 



