NATURAL HISTORY OF THE FARM 
mud atthe bottom, that 
on a0 "| stand erect and emer- 
gent with their tops 
above the water. A 
Pic. 15. Floating plants: 2, duckweeds; few of the more strik- 
b, the floating liverwort (Ricciocarpus natans). ing and characteristic 
of these are shown in Figure 16. Alg# are common 
enough here also. Brown coatings of diatom ooze over- 
spread the submerged stems, and flocculent green mats 
of “blanket algae” lie in sheltered openings, often buoyed to 
the surface on bubbles of oxygen. 
THE ANIMAL LIFE OF THE STREAM 
The animals that live in the rapids are small in size, but 
most interesting in the adaptations by means of which they 
are enabled to withstand the on-rush of the waters. One of 
them at least, the black-fly larva, occurs in such numbers as 
to form conspicuous black patches in most exposed places— 
on the very edge of the stones that form the brink of waterfalls 
and on the sides of obstructions in the current. Individually 
these larvae are small (half an inch long), with bag-shaped 
bodies, swollen toward the rear end, where attached by a 
single sucking disc to the supporting surface. Attached in 
thousands side by side, 
they often thickly cover 
and blacken several 
square feet of surface. 
They sway gently in the 
current as they hang with 
heads down stream. 
These larvae spin at- 
tachment threads by Pic. 16. Aquatics that rise from standing 
means of which theymay the sweet ig (Acorus calamunie. the bar 
; ted (S di the cat. 
change location. The i 4rgeyinm cveerbums ert 
