12 THE EYE IN EVOLUTION 



The mechanism of these photoperiodic responses is unknown but 

 several facts are now estabhshed. It is significant that a brief exposure 

 (1 sec. in some species) to fight of a very low intensity (of the order of 

 1 ft. candle, that is, of the intensity of moonlight) is sufficient to 

 determine the periodicity. The wave-length of the fight is of import- 

 ance, for the action-spectrum shows a specificity with maxima in the 

 red and blue — a fact which suggests the presence of one or more 

 absorbing pigments ; moreover, there are indications of an antagonism 

 between the action of different spectral regions, while near infra-red 

 radiation takes an active part in the effect (see Wassink and his 

 co-workers, 1950-56). Such a pigment (or pigment -mixture) has not 

 been isolated, but Borthwick and his colleagues (1948-54) speculated 

 that it is an open-chain tetrapyrrol pigment, a distant relative of 

 chlorophyll. With its aid a photosynthetic reaction takes place, the 

 nature of which is unknown ^ with the probable result that one or 

 more plant hormones,^ perhaps both activating and inhibiting in their 

 action, travel down the leaf-stalk and up the shoot to influence 

 flowering ; the substance can travel through living cells and across 

 grafts but not across an inanimate obstacle (Cailahian, 1940). 



The fact that such a substance (or substances) has eluded chemical 

 detection, has stimulated several alternative hypotheses.^ However 

 that may be, and whatever the intimate mechanism, the fact remains 

 that the order of the procession of flowers through the seasons is largely 

 determined by the diurnal periodicity of light and darkness. 



Bonner. Botan. Gaz., 110, 625 (1949). Darwin. The. Power of Movement in 

 Bonner and Thurlow. Botan. Gaz., 110, Plants, London (1880). 



613(1949). Enderle. PZan/a (Berl.), 39, 570, (1951). 



Borthwick, Hendricks and Parker. Botan. Engel and Heimann. Planta (Berl.), 37, 



Gaz., 110, 103 (1948). 437 (1949). 



Borthwick, Hendricks, Toole, E. H., and Engelmann. Pfliigers Arch. ges. Physiol., 



V. K. Botan. Gaz., 115, 205 (1954). 29, 387 (1882). 



Borthwick and Parker. Botan Gaz., 100, Fliigel. Planta (Berl.), 37, 337 (1949). 



374 (1938) ; 101, 806 (1940). Garner and Allard. J . agric. Res., 18, 553 

 Borthwick, Parker and Hendricks. Amer. (1920). 



Naturalist, 84, 117 (1950). Grossenbacher. Amer. J. Botan., 26, 107 

 Biinning. J6. wrss Bo/an., 75, 439 (1931). (1939). 



Ber. dfsch. botan. Ges., 54, 590 (1937). Gunn. J. e.rp. Biol., 17, 267 (1940). 



Biol. Zbl., 64, 161 (1944). Hagan. Plant Physiol., 24, 441 (1949). 



P/ora, 38, 93 (1944). Hamner and Naylor. Botan. Gaz., \{iQ, 

 Naturwissenschaften, 33, 271 (1946). 853 (1939). 



A^oYwr/orsc/)., 3b, 457 (1948). Harder and van Senden. Naturwiss- 

 Plania (Berl.), 38, 521 (1950). enschaften, 36, 348 (1949). 



Ann. Rev. Plant Physiol., 7, 71 (1956). Harder and von Witsch. Gartenhauwiss., 

 Cailahian. C. R. Acad. Sci. U.R.S.S., 12, 15, 226 (1940). 



443 (1936) ; 27, 160, 253, 370 (1940) ; Heimann. Planta (Berl.), 38, 157 (1950) ; 



31, 949 (1941) ; 47, 220 (1945) ; 54, 40, 377 (1952). 



735, 837 (1946) ; 55, 69 (1947). Herms. J. e.r;x Zoo/., 10, 167 (1911). 



1 Review, see Lang, 1952. ^ p. 39. 



3 See Buiming (1937-50), Bonner and Thurlow (1949), Bonner (1949), Harder and 

 van Senden (1949), van Senden (1951). Recent research indicates that the gibberillins are 

 of fundamental importance in the photoperiodism of plants. 



