FRESH WATER ECOLOGY; SOIL ALGAE 373 



in the host with age, e.g. old filaments of the Zygnemaceae lose their 

 mucilage sheath and they then become colonized by many epiphytes. 

 Experimental work and observation show that the greatest 

 growth and number of epiphytes are partly related to conditions of 

 good illumination, a feature which is illustrated by Table 1 3 below. 



Table 13 

 Total number of epiphytes collected on suspended slides 



Sandy bottom Muddy bottom 



Water level 225I 262 (no Eimotia) 



5 cm. 116 > {no Eunotia) loS (102 Eunotia) 



12 cm. ij6j o 



17 cm. 37 (all Eunotid) o 



When considering the effect of illumination it has to be remem- 

 bered that not only are there problems associated with the indivi- 

 dual plants, such as the upper and lower surfaces of leaves, but also 

 that the density of the host plants may be highly significant. Fig. 

 202 shows the distribution of various epiphytes on plants of Equi- 

 setum limosum under different conditions of spacing and the con- 

 trast is exceedingly obvious. Where there is screening of leaves, 

 either on the same plant or by several plants, then the epiphytes 

 develop on the unscreened portion. 



The inter-relations of host and epiphyte are important, and it 

 was noticed that the epiphytes tend to develop in the depressions 

 where the cells of the host adjoined each other. Experiments were 

 then carried out with scratched shdes suspended in the water, and 

 the results obtained from these rendered it clear that depressions 

 in a surface increase the number of epiphytes very considerably 

 (Table 14). 



Table 14 

 Effect of surface upon epiphytes 



Epiphyte No. in scratches No. elsezvhcre 



on slides 



Cocconeis 5i7 297 



Stigeoclonium sp. 665 198 



Chaetopeltis 138 54 



Ulvella IM 200 



Coleochaete scutata 40 1 3 



So far as the attachment organs of the epiphytes are concerned, 

 there is no apparent relation bet\\'een the nature of the substrate 



