496 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



zone is the marked decline in green types of Algae in favour of Diatoms 

 which here show especially dark-brown coloration. Nevertheless 

 the chlorophycean Cladophor a profunda and Dichotomosiphon tuberosus 

 may still be plentiful. Stones at 10-20 metres' depth are also liable 

 to have a blackish or reddish to violet covering of particular cyano- 

 phycean and other forms not found at shallower depths. Here 

 several types of Rhodophyceae such as species of Hildenbrandia 

 (' Hildbrandtia ', etc.) and Chantransia may occur, the coloration 

 of which, like the brown of Diatoms, enables them to use for assimil- 

 ation the short-waved green and allied light-rays that penetrate most 

 deeply. The occasional green plants that persist here and below are 

 often strikingly dark in colour, and their assimilation appears to be 

 favoured either by increases in chlorophyll content or by changes in 

 the proportions of the component pigments in such a way as to aid 

 absorption. In some clear lakes the dysphotic region apparently 

 extends to very considerable depths, fairly frequent plants occurring 

 (sometimes in fair abundance) to 90 metres, and a very few Diatoms, 

 particularly, having been dredged from over 160 metres. It is, 

 however, by no means certain that these forms were actually living 

 and reproducing at such depths — that they had not merely sunk from 

 upper levels. 



The depth to which the deeper plant communities extend of course 

 depends on the transparency of the water; but even in the profundal, 

 where photosynthesis is no longer possible, or at all events where a 

 positive assimilation-balance is no longer found, heterotrophic, etc., 

 organisms occur on suitable substrata. In addition to animals and 

 their parasites, the heterotrophs include saprophytic Fungi and 

 Bacteria living, for example, on rotting wood and leaves. There are 

 also some autotrophs in the form of chemosynthetic Bacteria liv- 

 ing under particular and often narrowly circumscribed conditions. 

 Quite apart from this. Bacteria and Fungi appear to play much the 

 same role of disease-producing parasites in water as on land. Bacteria 

 more generally attacking animals and Fungi parasitizing many plants 

 as well as animals. 



Except on outcrops along steep shores, lake-basins are usually 

 covered by sedimentary ooze which may attain a thickness of many 

 metres. The ooze normally consists of an intricate mixture of 

 organic and inorganic matter that is either autochthonous (formed in 

 the lake itself by vital or physico-chemical processes) or allochthonous 

 (introduced from outside by inflowing water, falling of dust, etc.). 

 The amount and composition of allochthonous materials will depend 



