THE REACTION OF A PHOTOTOXIC PLAXT 95 



geotropic organs, and also on organs in which the stimulus of light is 

 necessary to awaken growth, that is, to produce the proper condition 

 of phototonus l . In the latter case, exposure to light at first causes 

 an acceleration of growth, owing to the removal of the partial or com- 

 plete dark-rigor. As the conjoint result of this phototonic action, and 

 the retarding effect of intense light, it follows that the growth, and 

 hence the resulting adult size of the leaf, are greatest at a certain 

 optimal intensity of light, and diminish above and below it. If no photo- 

 tonic action is necessary for growth, this will then, in general, be most active 

 in darkness. Hence the excessive elongation of etiolated stems in darkness, 

 which is partly aided by the longer duration of growth. 



Growth, however, is in all cases the result of a variety of co-operating 

 factors, which may at once, or after a time, be unequally affected by 

 a change in the illumination. Hence the curve showing the dependence 

 of growth upon assimilation need not in all cases be the same, and may 

 in fact show secondary maxima and minima 2 when phototonic or photo- 

 synthetic (nutritive) effects come into play. Growth is, moreover, not the 

 sole factor in vital activity, and hence, from a general biological standpoint, 

 there is a certain optimum intensity of illumination for every plant requiring 

 light, which in general is not that of intense sunlight, but of bright diffuse 

 daylight, or, in the case of shade-loving plants, comparatively feeble diffuse 

 light 3 . The optimal intensity of illumination for the interior of a cell, 

 especially if chlorophyllous, will naturally be considerably lower, and does 

 not necessarily coincide with the optimal intensity of external illumination 4 , 

 of which only a fraction is ever actually utilized by the plant. The various 

 adaptations and structural relationships of plants to light have been closely 

 studied by Wiesner (1. c.). 



So long as the plant remains in a condition of phototonus, its growth 

 is much less affected by changes of illumination than by changes of 

 temperature. Thus even the change from bright light to complete darkness, 

 or vice versa, usually affects the rate of growth by from 5 to 30 per cent., 

 and only rarely by as much as 50 per cent., while small changes of 

 illumination produce no perceptible effect. A rise or fall of temperature 

 almost immediately affects the rate of growth, whereas a pronounced 

 change in the intensity of the illumination does not become perceptible. 



1 On leaves cf. Prantl, Arb. d. Bot. Inst. in Wiirzburg, 1873, Ed. I, p. 371 ; G. Kraus, Jahrb. 

 f. wiss. Bot., 1869-70, Bd. vil, p. 228. 



2 Wiesner (I.e., 1880, II, p. 15) obtained stich curves, but possibly other factors besides the 

 light-rays entered into play. 



3 Wiesner, Sitzungsb. d. Wien. Akad., 1893, Bd. Cll, p. 291 ; 1895, Bd. CIV, I, p. 605 ; 1900, 

 Bd. CIX, I, p. 436; Ber. d. Bot. Ges., 1894, p. 78; Denksch. d. Wien. Akad., 1896, Bd. LXIV, 

 p. 73; 1898, Bd. LXVII, p. I ; Schimper, Pflanzengeographie, 1896. p. 61. 



4 Ewart, Annals of Botany, Vol. Xii, 1898, p. 383 seq. Here, and in Wiesner's experiments, the 

 photo-chemical intensity of the light was determined by means of sensitized paper. 



