EPIPHYTES 345 



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. 11 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. 201 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 decline. The diatom flora, however, is an 

 exception to this behaviour, because they increase regularly with 

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

 number of diatomaceous epiphytes. On the other hand, algal 

 zoospores tend to settle on the younger living leaves. There are 

 distinct differences 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, 

 probably because of the greater light intensity. In addition to 

 distribution in relation to increasing age, there is also the relation 

 to the different parts of the phanerogamic substrate. Fig. 201 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 same level. These 

 maxima on the lower leaves are to be associated with the diatom 

 flora. It will also be observed that the number of epiphytes on the 

 internodes remains more or less constant, but rapid growth of the 

 substrate, e.g. the leaf lamina, tends to prevent colonization by 

 epiphytes. The density of epiphytes that are attached to dead 

 organic material is dependent upon the habitat of the substrate, 

 e.g. if it is floating then there are few epiphytes, if it is attached or 

 submerged the epiphytes are numerous, whilst if it is lying on the 



