oo 



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 tc 

 a single plant, Tropaeolum majus. 



Besides absolute optima of illumination which for certain function! 

 coincide with very unfavourable oecological conditions — the optimum ligh 

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

 as is the case with heat, an oecological optinnim for light, which correspond; 

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

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

 ways to obtain possession of the oecological optimum of light. Man; 

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

 where the light is of a definite and generallj' moderate intensity, and deser 

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

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

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

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

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

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

 by movements of the chlorophyll-corpuscles '. 



In nature these diversified movements would seem to bring the plan 

 usually under the most favourable conditions of illumination : but this i 

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

 various functions demanding as they do unequal supplies of light one ofte^ 

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

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

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

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

 to their absolute optima, and therein act in a manner so highlj' inimical t 

 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 occupie 

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

 formulae giving the actual photic ration of plants. 



Even plants that are apparently very well illuminated obtain only 

 fraction of the full amount of daylight. The plants occupying flat deser' 

 or other horizontal surfaces alone receive an almost intact supply of ligli 

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

 ar.d underwood receive light chiefly from above, lianes and epiphytes c 



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



