I50 HYDROPHYTES sect, iv 



aquatic plants in summer, at which season the temperature is higher and 

 the Hght more intense. 



Light in water. We must assume tliat every aquatic plant has its 

 own minimum, optimum, and maximum intensity of hght. Illumination 

 is of profound importance in relation to the distribution of algae, also to 

 the abundance of their species at different seasons ^but on this latter point 

 very little is known. The farther apart lie the maximum and minimum 

 the more extensive will be the area of distribution of the species concerned. 



Light plays in assimilation the same part as in the case of land-plants ; 

 yet there are some peculiar relationships to be considered. Light is 

 weakened, partly by reflection from water, partly by absorption in water, 

 and partly by floating particles ; the weakening is therefore more con- 

 siderable the dirtier the water. Submerged plants, for this reason and 

 also because there is no transpiration, acquire as a whole the pattern 

 of a shade-leaf ; they become lanky, like etiolated plants, thin, and their 

 assimilatory tissue shows little differentiation. 



Light penetrates downwards only to a certain distance, and con- 

 stantly weakens with increasing depth, so that the assimilatory energy 

 varies greatly with the depth ^ ; consequently, except in the case of 

 bacteria, plants cannot be active at great depths. A ' regional ' distribu- 

 tion of the vegetation results from the different powers possessed by 

 plants of living in different intensities of light. Distinctions have been 

 drawn between : 



1. Euphotic vegetation, which receives an abundance of light. 



2. Dysphotic vegetation, which lives in weakened light. 



3. Aphotic vegetation, which lives in very weak light or darkness. 

 Spermophyta descend at most to thirty metres (Zostera, for instance, 



to twelve or fourteen metres in Denmark) ; algae to forty metres, but 

 living algae have been found 120-150 metres below the surface (in clear 

 alpine lakes in Switzerland Characeae descend to 25-30 metres, but in 

 Baltic lakes only to 6-8 metres) ; in Lake Geneva, according to Forel, 

 a moss, Thamnium alopecurum var. Lemani, has been found at a depth 

 of 60 metres ; the extreme depth to which light apparently penetrates 

 is 400-500 metres. The presence of the protococcaceous Halosphaera 

 viridis at 2,200 metres below the surface of the sea is certainly to be 

 explained as a result of sea-currents or as a periodic sinking. 



As the rays of different colours are unequally absorbed plants descend 

 to different depths. Red rays are absorbed in the upper layers of water ; 

 the green, blue, and ultra-violet not before the lower layers. Ultra- 

 violet rays can be detected by means of photographic plates at a depth 

 of 400 metres. Correlated with these facts is the ' regional ' distribution 

 of Algae, according to depth? Green Algae assimilate best in red light, 

 Brown Algae in yellow light, but Red Algae are most active in green and 

 blue light ; consequently, the first named occur only in the upper layers 

 of water, while the last are especially in the deeper layers. Against this 

 theory maintained by Engelmann the objection is urged by Oltmanns 

 that with algae it is only a question of the intensity of light, and that 

 the colour of sea-water merely acts as a screen. Recently, Gaidukow^ 

 has shown that when Oscillatorieae are cultivated in coloured light they 



' Proved by B. Jonsson, 1903. ^ Borgesen, 1905. ' Gaidukow, 1903. 



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