40 



THE EYE IN EVOLUTION 



plants, and since most plants bend towards the light, it was generally assumed 

 that it had a retarding influence upon growth, a view elaborated in great detail 

 by Blaauw (1909-18). That an explanation so simple could not account for the 

 facts, however, had already been shown in the classical researches of Darwin 

 (1880) on the behaviour of seedlings of grass {Phalaris canariensis) and the oat 

 {Avena sativa) — observations from which all modern views on the mechanism 

 of phototropism have directly descended. Darwin showed that the seedlings 

 only curved towards the light when the tijD of the coleoptile was unilaterally 



Figs. 16-20. — Phototropism in Seedlings. 



(a) (A) (c) 



Fig. 16. 



(a) (b) (c) 



Fig. 17. 



[~3 



^ n 



(a) (b) (c) 



Fig. 18. 





Fig. 20. 



Fig. 16. — Darwin, 1880. The grass coleoptile exposed to lateral illumina- 

 tion (a) bends towards the light (6). When the tip is removed (c) the 

 phototropic response does not occur. 



Fig. 17. — Boysen-Jensen, 1910-11. When a coleoptile tip is excised and 

 replaced with gelatin inserted between it and the stump (a), phototropic 

 curvature results normally ; a diffusible substance therefore jjasses across 

 the plate of gelatin. If, however, a plate of mica is inserted on the shaded 

 side (6), no response occurs. If the mica is inserted on the illuminated 

 side, the response is normal (c). It follows that the diffusible substance 

 passes down the shaded side. 



Figs. 18-20. Went, 1928. 



Fig. 18. When the tip of the coleoptile is removed, growth in length ceases (a). 

 An agar block placed on the stump has no effect (6). An agar block con- 

 taining juice extracted from the excised tip promotes normal growth (c). 



Fig. 19. The coleoptile tip is placed upon an agar block (a), and a piece of the 

 block transferred unilaterally to a decapitated coleoptile (6). Unilateral 

 growth resembling phototropic curvature results due to the diffusion of the 

 hormone from the agar derived from the tip. 



Fig. 20. — When unilateral light falls on an excised tip in contact with two agar 

 blocks separated by a razor blade, the greater part (65%) of the growth- 

 hormone is recovered from the agar on the shaded side. 



illuminated and never when it was shaded by tinfoil even while the rest of the 

 plant was exposed, and that no curvature ever occurred in the stem or the root 

 if the growing tip were removed (Fig. 16). This localization of sensitivity to 

 the growing tip of the seedling was confirmed by subsequent workers. Rothert 

 (1892-96) incised the vascular bundles in various places and proved that the 

 phototropic stimulus travelled from the sensitive tip throughout the plant in 

 the parenchyma, while Fitting (1905-7) observed that the curvature was caused 

 by a difference in the rate of growth of the two sides, in positive phototropism 

 the darkened side growing more rapidly than the illuminated side. The next 



