LIGHT AND MOVEMENT 



39 



It is interesting that comparable non-translatory movements of the organs 

 of animals may occur ; thus the hydroid, Eudendrium, and the marine polychsete 

 worm, Spirographis spallanzani, show heliotropic bending movements (Loeb, 

 1890), some shell-fish open and others close their valves, clams retract their 

 siphons (Hecht, 1919-20 ; Light, 1930), snails their tentacles (Grindley, 1937 ; 

 and others) and sea-urchins, such as Diadema antillarum, move their spines if a 

 light is flashed on them (P. and F. Sarasin, 1887 ; v. Uexkiill, 1897 ; Millott, 

 1950), while many sedentary tubicolous polychsete worms, such as Branchiomma, 

 withdraw into their tubes on a decrease in light intensity (Nicol, 1950). 



An interesting variant of this reaction is seen in certain sea-urchins such as 

 the European Strongijlocentrotus (Dubois, 1913) and the Caribbean Lytechinus 

 (Millott, 1957), which normally withdraw their podia when illuminated. When 

 lying in sunlit waters these echinoids gather small stones, the shells of bivalve 

 molluscs, pieces of seaweed or whatever debris may be within reach of their 

 tube-feet, and heap them upon themselves, using them as a parasol to protect 

 themselves from light. 



The mechanism of the phototropic responses of plants is now 

 relatively clear. They are due to the production of growth-regulating 

 phytohormones ^ called auxins, a generic term applied to a number 

 of related chemical substances of wide distribution formed by specialized 

 parts of the plant — the tip of the coleoptile in seedlings and the leaves, 

 particularly the young leaves, of mature plants. There these hormones 

 are formed from precursors on stimulation by light and thence they are 

 transported throughout the tissues of the plant at a rate more rapid 

 than can be accounted for by simple diffusion (about 10 mm. per hour) ; 

 as it travels through the tissues the freely-moving auxin regulates the 

 varying rates of growth that account for such phenomena as photo- 

 and geotropism, while some of it becomes bound in the tissues, there 

 to regulate normal growth. In phototropic curvature the freely avail- 

 able hormone becomes unequally distributed in its passage along the 

 two sides of a laterally illuminated plant, an increase of concentration 

 on the shaded side of the stem leading to a bending of the organ. Its 

 precise mode of action is unknown, but it would seem probable that, 

 in addition to other activities such as the regulation of osmosis, it 

 acts essentially as a co-enzyme in the respiratory activity of the cells, 

 causing them to elongate and sometimes stimulating them to divide. 



In these processes determining the phototropic movements of 

 plants — and also of animals — carotenoid pigments act as sensitizers. 

 These pigments are quite different in chemical structure and absorjDtive 

 properties from the chlorophyll group of pigments which are primarily 

 responsible for the photosynthesis concerned with metabolism in the 

 vegetable kingdom ^ ; they will be more fully described at a later 

 stage ^. 



We have already seen that de Candolle (1832) first, and Sachs (1882-87) at 

 a later date showed that light was responsible for the directional growth of 



1 p. 547. 2 p. 5. 3 p. 118. 



Branchiomma 



