346 FRESH-WATER ECOLOGY 



bottom the epiphytes will be few. The various species to be found 

 are all a residuum from the last living state of the material, and the 

 assertion that dead material bears more epiphytes than living does 

 not appear to be correct in this case and it can only be supposed 

 that it arose in the past through lack of quantitative analysis. In 

 some cases the appearance of epiphytes is due to change in the 

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

 mucilage sheath and they then become colonized by many epi- 

 phytes. 



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 XIX 

 below. 



Table XIX 



Total no. of epiphytes collected on 

 suspended slides 



r 



Level Sandy bottom Muddy bottom 



Water level 225 (no Eunotid) 262 (no Eiinotia) 



5 cm. 176 (no Eunotia) 108 (102 Eiinotia) 



12 cm. 176 (no Eiinotia) o 



17 cm. 37 (all Eunotia) o 



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

 membered that not only are there problems associated with the 

 individual plants, such as the upper and lower surfaces of leaves, 

 but also that the density of the host plants may be highly signi- 

 ficant. Fig. 201 A-D shows the distribution of various epiphytes on 

 plants of Equisetum lifnosiim under different conditions of spacing 

 and the contrast 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 interrelations 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 slides 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. 



So far as the attachment organs of the epiphytes are concerned 

 there is no apparent relation between the nature of the substrate 



