^S 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 parviflora. 

 AAmong the chemical 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 

 lirrht. 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 theii 

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

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

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

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

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

 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 alway 

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

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



trnpicaniVht may evpn rn mpletely bleach th efoliage. 



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

 phores, against excessively intense light, is shown in many devices whic! 

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

 are very sensitive to light ^ Long and thick coatings of hair cover man; 

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

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

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

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

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

 contrivances are of course better developed in the strongly illuminated seal 

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



' Berthold, I. 



