38 



THE EYE IN EVOLUTION 



Folger. J. Morph., 42, 359 (1926). 



Biol. Bull., 53, 405 (1927). 

 Grindley. The Intelligence of Animals, 



London (1937). 

 Herms. J. e.Tp. Zool., 10, 167 (1911). 

 Hovey. Physiol. Zool., 2, 322 (1929). 

 Hyman. J. exp. Zool., 24, 55 (1917). 

 Jennings. Behavior of Loiver Organisms, 



N.Y. (1906). 

 Luce. Anat. Bee, 32, Suppl., 55 (1926). 

 Mast. J. exp. Zool., 9, 265 (1910) ; 51, 97 

 (1928). 

 Light and the Behavior of Animals, N.Y. 



(1911). 

 J. Morph., 41, 347 (1926). 

 Protoplasma, 8, 344 (1929) ; 14, 321 



(1931). 

 Physiol. Zool., 5, 1 (1932). 

 Mast and Hulpieu. Protoplasma, 11, 412 



(1930). 

 Mast and Pusch. Biol. Bull. 46, 55 (1924). 

 Moore and Cole. J. gen. Physiol., 3, 331 



(1921). 

 Oltmanns. Morph. u. Biol. d. Algen, 

 Jena (1922). 



Pantin. J. marine Biol. Ass., U.K., 13, 

 24 (1924). 

 Brit. J. exp. Biol., 1, 519 (1924) ; 3, 275, 

 297 (1926). 

 Patten. J. exp. Zool., 23, 251 (1917). 

 Pearl. Quart. J. micr. Sci., 46, 509 (1903). 

 Pfeffer. Pflanzenphysiologie, 2, 776 (1904). 

 Pirenne and Marriott. Nature (Lond.), 



175, 642 (1955). 



Rose. La question des tropismes, Paris 



(1929). 

 Stahl. Botan. Z., 42, 146, 162, 187 (1884). 

 Szymanski. Pfliigers Arch. ges. Physiol., 



158, 343 (1914). 

 Ullyott. J. exp. Biol., 13, 253 (1936). 

 Walter. J. e.rjo. Zoo?., 5, 35 (1907). 

 Welsh. Biol. Bull., 65, 168 (1933). 

 Wilber and Franklin. Atiat. Rec, 99, 680 



(1947). 

 Wille. Biologie und Bekdmpfung der 



deutschen Schabe, Berlin (1920). 

 Woodheadand Woodhead. Nature (Lond.), 



176, 349 (1955). 



Young. J. exp. Biol., 12, 229 (1935). 



PHOTOTROPISM 



Used in Strasbiirger's (1878) original sense, the term photo- 

 TROPISM connotes the orientation of sessile organisrns towards or away 

 from light. The phenomenon is a widespread and well-known charac- 

 teristic of plant life and as a rule the stimulus is the sun (heliotropism ; 



TJXios, the sun). Among the higher plants 

 which are fixed in their habitat, heliotropic 

 movements are limited to the component 

 parts ; the aerial vegetative axes usually 

 turn towards the light, thus exhibiting a 

 POSITIVE HELIOTROPISM, the Icaf-bladcs 

 take up a position at right -angles to the 

 rays of light in order to receive as much 

 illumination as possible (transverse or 

 DiA -heliotropism), while tendrils and roots 

 grow from the light (negative helio- 

 tropism) (Fig. 15). Occasionally these 

 movements are remarkably delicate and 

 rapid ; thus the Bengal plant, Hedysarum 

 girans, nods to a passing cloud. Some- 

 times, however, the axes of the plant are 

 photo -negative ; thus several grasses, corn 

 and rice grow erect in darkness and tend to 

 lie prostrate in bright iUumination, becom- 

 ing positively phototropic when shaded 

 (Langham, 1941). 



Fig. 15. — Heliotropism. 



Seedling of Sinapsis alba in 

 water supported on a cork 

 plate. It has been illuminated 

 initially from all sides and 

 then from one side only: the 

 stem turns towards the light, 

 the root away from it, and the 

 leaf-blades at right angles to it 

 (after Strasbvirgt-r). 



