LIGHT AND MOVEMENT 33 



To retain a sense of proportion it is well to remember that forms of stimula- 

 tion other than light are operative on living organisms, although none shows the 

 same interest and complexities in the responses elicited. The scientific conception 

 of GEOTROPISM in plants to describe the effects of gravitational influences was 

 introduced by Knight (1806) at a very early date, even before de Candolle (1832) 

 formulated his theory of phototropism. Towards the end of the 19th century 

 however, the study of the responses of organisms to various stimuli rapidly 

 widened. In ec^ually fundamental researches on the action of chemicals on the 

 sperm of ferns and mosses, Pfeffer (1883-88) introduced the term chemotaxis, 

 Stahl (1884) described hydrotropism in fungi, Wortmann (1883) discovered 

 THERMOTROPISM, and Verwom (1889) thigmotropism (contact stimulation ; 

 diyixoL, touch) and galvanotropism. These, however, are not our present 

 concern, and we shall proceed to exemplify shortly the various types of response 

 to light. 



It is also to be remembered that these various responses may be mutually 

 additive ; thus some flat worms are photo -negative and at the same time swim 

 towards a cathode. When the two stimvili are presented together the response 

 depends upon the direction and strength of each. Thus when the light and the 

 cathode are at right angles the worm will swim at an angle bisecting the direction 

 of the stimvili when the density of the current is proportional to the logarithm 

 of the intensity of the illumination. 



Alverdes. Z. wiss. Zoo/., 137, 403 (1930). Claus, Grobben and Kiihn's Lhb. der 



Cailahian. C. R. Acad. ScL, U.R.S.S., 27, Zoologie, Berlin, 246 (1932). 



160, 253, 374 (1940). Mast. Biol. Bev., 13, 186 (1938). 



de Candolle. Phusioloqie vegetale, Paris Pfeffer. Ber. dtsch. botan. Oes., 1, 524 



(1832). (1883). 



Dietrich. Z. mss. ZooZ., 138, 187 (1931). Untersuch. botan. Inst. Tubingen, 1, 362 



^""'iTZn^H^r'' ^"'- '''■ '''"""'' P^a'''l^S^r776 (1904). 



tfU, yo U»m;. ^, ^ . , ,. , Sfahl. Bo/««. Z., 42, 145, 160, 187 (1884). 



Fraenkel and Gunn. TAe Onentatwn of gtrasburger. Jena. Z. Naturw., 12, 551 



Animals, Oxon. (1940). (1878) 



Gunn, Kennedy and Pielou. Nature Verworn. ' Psydiophysiologische Protisten- 



(Lond.), 140, 1064 (1937). studien, Jena (1889). 



Knight. P/n7os. Tra/is. B, 96, 99 (1806). Viaud, Le photoiropisme animal, Paris 



Kiihn. Orientierung der Tiere im Raum, (1948). 



Jena (1919). Went. Rec. Trav. botan. Neerl., 25, 1 



Bethes Hb. norm. path. Physiol., 12 (1), (1928). 



17 (1929). Wortmann. Botan. Z., 41, 457 (1883). 



PHOTOKINESIS 



KINESES [Kivrjois, movement) are the most simple responses of 

 motile organisms to light — they are merely the alteration, either a 

 quickening or a slowmg, of normal random movements witJiout specific 

 directional orientation ; all that is required for their initiation is a 

 mechanism of the simplest type which possesses the ability to react 

 photochemically to variations in the intensity of illumination ; specific 

 photoreceptors (eyes) are in no sense necessary. The phenomenon is 

 essentially the same in character as the alterations in metabolic activity 

 produced by light which we have lately considered. It must be 

 remembered, however, that a motorial response of this type but 

 frequently more dramatic in nature may result from other stimuli 

 such as variations in temperature or moisture. 



1.0.— VOL. /. 3 



