THE ACTION OF RAYS OF DIFFERENT WAVE-LENGTH 105 



indirect effects are clearly separated l . Although the ultra-violet ' chemical ' 

 rays exert in most cases a greater chemical activity than light-waves 

 of lesser frequency, the latter are able to induce various chemical 

 actions, the difference being merely one of degree and not of kind. 

 Moreover, by means of absorptive or fluorescent sensitizers the region of 

 greatest chemical activity can be considerably displaced, as when the 

 presence of eosin in a photographic film heightens the relative photo- 

 chemical action of the feebly actinic green rays. 



In the red and orange rays green plants assume abnormal shapes 

 owing to the absence of the restraining influence of the more refrangible 

 rays, whereas the latter alone are unable to produce sufficient food for 

 normal development. 



The absence of the ultra-violet rays may act by disturbing the 

 harmonious combination required for normal growth, and it is easily 

 comprehensible why the plant may suffer when illuminated by any single 

 group of rays which in themselves are necessary or useful to it. For if 

 certain functions are stimulated more than others, their harmonious 

 co-operation is disturbed, and in the absence of a sufficient power of 

 adjustment the plant of necessity ultimately falls ill or develops abnor- 

 mally. It is possible that the somewhat problematic injurious action of 

 monochromatic green light may arise in this manner, without its exercising 

 any direct effect upon growth. 



Since it depends upon the specific properties of the substance or 

 organism as to whether light exercises any photo-chemical or physiological 

 action upon it, it is hardly surprising that different plants may respond 

 differently to the same light stimulus, or that a response may either be 

 produced only by a limited region of the spectrum, or in varying degrees 

 by rays of different wave-length. 



Plants are able to perceive, and often to markedly respond to, rays, 

 such as the ultra-red and ultra-violet rays, which make no impression upon 

 the retina. This question in fact needs to be applied to every known form 

 of radiant energy, whether naturally or artificially produced. Electrical 

 radiations, i. e. ether waves of which the shortest measured (-6 cm.) is nearly 

 io 8 times longer than that of the last visible red ray (7 x io~ 9 cm.), are in 

 fact able to produce certain physiological effects. Whether this also applies 

 to the Rontgen rays 2 has yet to be determined, for it is not certain 

 whether the positive results, which only a few workers have been able 



1 Teodoresco (Ann. d. sci. nat, 1899, 8 e ser., T. X, p. 259) quotes a portion of the literature. 



2 In regard to bacteria see Wittlin. Centralbl. f. Bact., 1896, 2. Abth., Bd. II, p. 676; Beck 

 and Schultz, Zeitschr. f. Hygiene, 1897, Bd. xxui, p. 490; Rieder, Beiheft z. Bot. Centralbl., 

 1898-9, Bd. VIII, p. 250. For other actions, including tropistic ones, cf. Schober, Ber. d. Bot. Ges., 

 1896, p. 108; Lopriore. Bot. Centralbl., 1898, Bd. LXXIII, p. 451; Maldiney and Thouvenin, Rev. 

 gen. d. Bot., 1898, T. X, p. 81 ; Atkinson, Beiheft z. Bot. Centralbl., 1898-9, Bd. vm, p. 288; 

 Schaudinn, Pfliiger's Archiv f. Physiol., 1899, Bd. LXXVII, p. 29 (Protozoa). 



