500 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



usually grow on higher plants — such as aquatic Grasses, Sedges, 

 Rushes, Horsetails, Water-lilies, Pondweeds, and the like. In 

 temperate regions they may often be roughly divided into com- 

 munities living (a) on submerged plants in very shallow waters, 

 these being mostly Green Algae ; {b) on submerged plants at depths 

 of between i and 3 metres, these being mostly other Green Algae 

 such as species of Oedogonium and Coleochaete and, in addition. 

 Diatoms ; and {c) on submerged plants at greater depths. 



This last category extends down to about 6 metres and commonly 

 includes Coleochaete, Diatoms, and Cyanophyceae. Additionally, 

 subdivision of algal epiphytes is often possible in temperate regions 

 into (i) winter annuals, (2) summer and autumn annuals, and (3) 

 perennials. Here the age as well as the nature of the substratum 

 is of great importance ; but whereas there is a general tendency, 

 as might be expected, for older leaves, for example, to be better 

 endowed with epiphytes than younger ones, such a sequence is not 

 always found. There may also be differences in the epiphytic flora 

 of the upper and lower surfaces of leaves, and on different parts 

 of a plant or even organ. Some of these and many other differences 

 appear to be due to differing light-intensities. Moreover, rapid 

 growth, as in the case of most of a leaf (other than its tip), tends 

 to prevent colonization. So far as attachment is concerned, the 

 nature of the ' host ' surface, provided it is large and solid enough, 

 does not appear to matter except to the motile reproductive bodies 

 which tend to come to rest most easily in interstices and depressions. 

 Through germination of such disseminules and subsequent growth, 

 these sheltered situations often become quickly populated with adults 

 of the species concerned. 



Bogs and Saline Waters 



Bogs, which abound particularly in the cooler parts of the northern 

 hemisphere, form a special habitat in which the substratum is com- 

 posed of peat. The peat is usually saturated with water and has lying 

 above it a water-soaked layer of Mosses, particularly of the genus 

 Sphagnum (Bog-mosses). Domes of such Mosses are often sur- 

 rounded by swampy moats ; or species of Sphagnum may extend out 

 into the waters of a lake, often ultimately covering it to the centre. 

 Such features are typically characterized by attendant zones of 

 vegetation, the commonly coniferous tree dominants rapidly decreas- 

 ing in luxuriance and height as they advance into the moat or towards 



