60 THE FACTORS [Part I 



They also exert a retarding influence on growth, and if very intense 

 decompose chlorophyll and kill the protoplasm. According to Sachs, the 

 ultra-violet rays play a prominent part in the production of flowers. 

 Investigations on this subject have however hitherto been confined to 

 a single plant, Tropaeolum majus. 



Besides absolute optima of illumination which for certain functions 

 coincide with very unfavourable oecological conditions — the optimum light 

 for the growth of axes and certain leaves is zero or darkness — there is also, 

 as is the case with heat, an oecological optimum for light, which corresponds 

 to the normal life of a plant as a whole, and is compounded of the 

 harmonic light-optima of the several functions. A plant strives in various 

 ways to obtain possession of the oecological optimum of light. Many 

 Algae which are capable of movement by means of cilia collect in places 

 where the light is of a definite and generally moderate intensity, and desert 

 places where another, but to them less favourable, degree of light prevails. 

 Fixed plants and plant-parts, that are therefore limited in their power of 

 movement, strive for the same advantage by means of the exposed and 

 changing lie of their foliage-leaves, as well as by means of their heliotropic 

 movements through which, according to the needs of the plant, a stronger 

 or weaker illumination is attained. A similar end is also often achieved 

 by movements of the chlorophyll-corpuscles \ 



In nature these diversified movements would seem to bring the plant 

 usually under the most favourable conditions of illumination ; but this is 

 not always the case. Even here perfection is not attained. Among the 

 various functions demanding as they do unequal supplies of light one often 

 gains the upper hand to the detriment of the others. Such discords are 

 still more frequent under artificial conditions of cultivation, in which species 

 of plants, that in their native habitats may have often received too little 

 light but hardly ever too much, strive after intensities of light corresponding 

 to their absolute optima, and therein act in a manner so highly inimical to 

 their oecology that they have sometimes to pay for it the penalty of death. 



5. SUN AND SHADE. 



Sun and shade, as terms describing the illumination of habitats occupied 

 by plants, had but vague signification until Wiesner defined them in 

 formulae giving the actual photic ration of plants. 



Even plants that are apparently very well illuminated obtain only a 

 fraction of the full amount of daylight. The plants occupying flat deserts 

 or other horizontal surfaces alone receive an almost intact supply of light, 

 and that certainly to their own detriment. Trees growing in dense forests 

 and underwood receive light chiefly from above, lianes and epiphytes on 



1 See Stahl, I and II ; Wiesner, III ; Schimper, III. 



