58 THE FACTORS [Part I 



sooner than the sexual organs. The injurious influence of too feeble illumina- 

 tion was more apparent in sun-plants such as Malva vulgaris under light of 

 higher intensity than it was in shade-plants such as Impatiens parvifiora. 



Among the cJicmical processes affected by light, the formation of chlorophyll 

 and also that of the pigments of the Brown and the Red Algae are the most 

 accommodating ; they attain their optimum under light of very moderate 

 intensity. The minimum light required for the reduction of carbon dioxide 

 is considerably higher than that for the manufacture of the above colouring 

 matter, and the intensity of the process rises proportionally with that of the 

 light. There exists no optimum beyond which the curve for assimilation 

 would descend ; the latter appears rather to ascend uniformly till the 

 decomposition of the pigments, by intense light, puts an end to it. 



Very intense light acts fatally on protoplasm, and this action is quite 

 independent of the accompanying heat phenomena. Under natural con- 

 ditions, however, only a few vegetable organisms are sufficiently sensitive 

 to be exposed to danger of death from excessive light. Among these 

 are many bacteria and some of the larger aquatic plants, especially Algae, 

 which are attuned to light of feeble intensity and die as soon as their 

 habitat' is more strongly illuminated, as for instance by the advancing 

 season. Most usually death from excess of light seems in the first place 

 to be indirect, owing to the decomposition of pigments associated with 

 assimilation ; for Algae that are quite decolorized, after exhausting their 

 reserve material, die from want of nourishment. Terrestrial plants under 

 normal conditions show a much greater power of resistance ; in their case, 

 the death of entire plants, or even of only separate organs of plants, 

 owing to excessive illumination, apparently does not occur. Nevertheless, 

 they frequently suffer from a considerable disintegration of their chloro- 

 phyll. The vegetation of very sunny spots is never pure green, but always 

 exhibits an admixture of yellow and brown tints due to the products of 

 decomposition of chlorophyll. It will be proved, further on, that intense 

 tropical light may even completely bleach the foliage. 



The need for the protection of plants, and especially of their chromato- 

 phores, against excessively intense light, is shown in many devices which 

 may reach great perfection, especially in the case of aquatic plants that 

 are very sensitive to light 1 . Long and thick coatings of hair cover many 

 marine Algae like an overshadowing cloud ; other Algae produce in their 

 cells special light-absorbing plates which, like window-shutters, cover the 

 peripheral walls during bright illumination, but as the light decreases are 

 forced on to lateral walls (Fig. 38). Finally, the whole form of growth of 

 many Algae is governed by the need for protection against light. All these 

 contrivances are of course better developed in the strongly illuminated seas 

 of low latitudes than in those of higher latitudes, where the light, in itself 



1 Berthold, I. 



