fresh water ecology; soil algae 37i 



Table 12 



Variations in chemical habitat 



NO3 NH3 P,0, Orgatiic CO, 



matter 



{a) Stony and rocky moderately low or low low low 



shores (inorganic) high absent 



{h) Mouths of streams high high high variable high 



(c) Reed swamps low high low high very 

 (organic) high 



Epiphytes 



It is convenient at this point to consider what is known about the 

 distribution of algal epiphytes, and in this connexion a study of two 

 ponds on the outskirts of Epping Forest by Godward (1934) has 

 resulted in considerable advances to our knowledge. Three series 

 of epiphytes were distinguished. 



(i) Winter forms; 16 species approx. 



(2) Summer and autumn forms; 1 1 species approx. 



(3) Forms existing throughout the year; 1 1 species approx. 



An investigation of the effect of the age of the substrate upon the 

 epiphytic flora showed that the nature of the substrate was of great 

 importance. This is illustrated in Fig. 202 e, where it can be seen 

 that, so far as the tips of the leaves are concerned, the total number 

 of epiphytes increases up to the third or fourth leaf from the apex, 

 after which there is a decUne. The diatom flora, however, is an ex- 

 ception to this behaviour, because it increases regularly with the 

 age of the substrate so that the oldest leaves bear the greatest 

 number of diatomaceous epiphytes. On the other hand, algal zoo- 

 spores tend to settle on the younger Hving leaves. There are distinct 

 diff'erences in the epiphytic flora of the upper and lower surfaces of 

 leaves, and it was observed that in the case of the first few leaves 

 below the apex the upper surface was infinitely superior in the 

 number of epiphytes, probably because of the greater light inten- 

 sity. In addition to distribution in relation to increasing age, there 

 is also the relation to the difl'erent parts of the phanerogamic sub- 

 strate. Fig. 202 E illustrates the distribution of epiphytes on the 

 different parts of a phanerogam, and it will be observed that it is 

 only on the leaf tips that the maximum is reached at the third or 

 fourth leaf, whilst the leaf sheaths show a slight maximum at about 

 the tenth leaf with a well-marked maximum for the mid-rib at the 



