LIGHT 17 



light, soon ceases, and only some of the most lowly organized plants 

 reach any considerable depth. 



The intensity of light plays a leading part in determining the distri- 

 bution of species and abundance of individuals of a community, because 

 the optimum is very different for different species. With inadequate 

 illumination plants do not thrive ; they become etiolated and undergo 

 degeneration or die. The distinction between light-plants and shade- 

 plants (for instance, in the forest) is well known. Stebler and Volkart,^ 

 in Switzerland, have made comparative measurements of the intensity 

 of light beneath trees, and investigations concerning the light required 

 by meadow-plants ; these they classify into hght-demanding, light- 

 loving, indifferent, light-avoiding, and light-dreading species. In accord- 

 ance therewith the distribution of species is different in different localities. 

 In arctic countries the nature of the sky (the number of sunny days, the 

 frequency of clouds and mists) certainly causes the contrast mentioned 

 by many travellers between the rich flora and vegetation in the seclusion 

 of the fjords and those of exposed coasts as well as of the islands of the 

 region. - 



6. The development of plants depends not only upon the intensity 

 but also upon the duration of the light to which they are exposed. For 

 instance, in Finland or the north of Norway barley ripens its grain in 

 eighty-nine days from the day of sowing, but in Schonen (in Sweden, 55-7 N. ) 

 it requires one hundred days, despite the higher temperature and the more 

 intense light ; and the explanation of this must in part be that in the 

 former places prolonged illumination promotes anabolism. In the north 

 the periodic vital phenomena of plants set in much more rapidly in 

 summer than in spring, because of the longer duration of the dayhght. 

 Arnell states that, going northw^ard from Schonen, for each degree of 

 latitude anthesis is later by 4-3 days in April, 2-3 days in May, 1-5 days 

 in June, and 0-5 of a day in July. 



7. Direct light promotes the production of leaves and flowers. The 

 side of a tree facing the source of light often acquires foliage before the 

 reverse side : Brazilian Ficus-trees may be seen to be in leaf on their 

 north side, whilst still leafless on their south side ; ^ tufts of Silene acauhs 

 in arctic countries on their south side may be decked with flowers, which 

 also point towards the south, while they are devoid of flowers on their 

 north side.'* 



8. The vegetative shapes of plants are greatly influenced by the 

 intensity and direction of the light. The effects of insu-fficiently intense 

 light are revealed not only in the phenomena of etiolation, which are 

 essentially pathological in nature, but also in connexion with healthy 

 normal individuals. Of this, forest-trees furnish admirable examples. 

 Light, in the first place, determines the shape of the individual tree. 

 The duration of life of the branches depends partially upon the intensity 

 of light. The shade cast by younger branches retards the assimilatory 

 activity of the leaves on older branches, and thus renders impossible the 

 normal development of buds and the ripening of wood. The branches 



' Stebler unci Volkart, 1904. 



' In regard to Spitzbergen and East Greenland respectively, consult Nathorst, 

 18S3, and Hartz, 1895. ' Warming, 1892. 



* Rosenvingc, 1889-90 ; Stefansson, 1894. 



WARMING C 



