336 Aquatic Societies 



(b). These free-floating aquatics; species of duck- 

 weed (Lemna, Spirodela), water fernworts (Azolla, 

 Salvinia), liverworts (Riccia), etc. 



3. An outer zone of submerged plants will include 

 such forms as pondweeds (Potamogeton) , hornwort 

 (Ceratophyllum) , crow-foot (Ranunculus), naiad 

 (Najas), eel-grass (Zostera), stonewort (Chara), etc. 



These grow lustily and produce great quantities of 

 aquatic stuff which serves in part while living, but prob- 

 ably in a larger part when dead, for food of the animal 

 population, and the ultimate residue of which slowly 

 fills up the pond. These plants contribute largely to 

 the richness and variety of the life in the pond, by 

 offering solid support to hosts of sessile organisms, both 

 plants and animals. Their stems are generally quite 

 encased with sessile and slime-coat algae, rotifers, 

 bryozoans, sponges, egg masses of snails and insects 

 and dwelling tubes of midges (fig. 196). Especially do 

 floating leaves seem to attract a great many insects to 

 lay their eggs on the under surface. This is doubtless 

 a shaded and cleanly place, so elevated as to be favor- 

 able for the distribution of the young on hatching. 



The alga of ponds are various beyond all enumerating. 

 It is they, rather than the more conspicuous seed-plants, 

 that furnish the basic supply of fresh food for the animal 

 population. Small as they are individually, their rapid 

 rate of increase permits mass accumulation which 

 often become evident enough. Such are: 



(i). The masses of filamentous algae, (Spirogyra and 

 its allies; Ulothrix, Conferva, etc.) collectively called 

 "blanket algae" that lie half-floating in the water, or are 

 buoyed to the surface by accumulated oxygen bubbles. 



(2). The beautiful fringes of branching sessile algae 

 (Chaetophora, fig. 198, Cladophora, etc.) that envelop 

 every submerged stem as with a drapery of green. 



