104 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



of flowers 1 , for the reaction is not shown in all cases, and it is well known 

 that the production of flowers is largely dependent upon the vegetative activity. 

 In fact de Candolle found that the growth and increase of dry weight were 

 diminished by the absence of the ultra-violet rays, and it is possible that a 

 similar result might be produced by the partial or complete absence of the 

 violet-blue rays, or by a uniform diminution of all the more refrangible rays. 



It is evidently by no means easy to distinguish between the direct 

 and indirect effects produced by the absence of particular rays, but in 

 general it is the more refrangible rays which exercise the most pro- 

 nounced phototonic and formative action, and which also influence the 

 rapidity of growth to the greatest extent. The same applies to adult ferns 

 and mosses, although germination is excited in the spores of these plants 

 by the less refrangible rays of the spectrum, which is an exception to 

 the general rule. 



The relative activity of the different rays of the spectrum has not 

 been "precisely determined, and hence it is impossible to say whether the 

 curves for the retarding, phototonic, and formative actions exactly coincide 

 or not. In general, however, it may be said that these curves, like that 

 showing the heliotropic effect, attain a maximum in the more refrangible 

 rays, fall nearly or quite to zero in the green or yellow part of the 

 spectrum, and frequently, though not always, again rise to a second 

 smaller maximum in the red end of the spectrum. These rays, however, 

 appear to influence different functions in various degrees in certain cases. 

 Thus in Pilobolus crystallinus the orange rays are able to induce heliotropic 

 curvature but not the formation of sporangia 2 . 



So far as is at present known the action of the different rays of the 

 spectrum gives similar curves in regard to heliotropic and phototactic 

 movements, to protoplasmic streaming and movements of the chloro- 

 plastids, as well as to the photonastic movements produced by growth 

 or by changes of turgor. On the other hand, it is the less refrangible rays 

 which are most active in photosynthesis, and to a less degree in the 

 development of chlorophyll, and probably also in decomposing the organic 

 acids accumulated by succulent plants in darkness. It does not, however, 

 follow that metabolism in general is more affected by the red rays, for 

 growth itself involves metabolic changes, which are therefore directly or 

 indirectly affected most pronouncedly by the more refrangible rays. 

 Which rays are primarily responsible for the action of light in favouring 

 the synthesis of proteids can hardly be determined until the direct and 



1 Cf. also Klebs, Probleme d. Fortpflanzung, 1895, p. 18; Vochting, Jahrb. f. wiss. Bot., 1893, 

 Bd. xxv, p. 201. 



2 Brefeld, Unters. iiber Schimmelpilze, 1881, IV, p. 77 ; Grantz, Einfluss d. Lichtes auf Pilze, 

 1898, p. 19. It is possible that this is due to the unequal sensitivity. 



