36 



HISTORY OF FARM 



? 



mud at the bottom, that 

 stand erect and emer- 

 gent with their tops 

 above the water. A 



Fig. 15. Floating plants: o, duckweeds; and feW of the more Strik- 

 ing and characteristic 

 are shown m i^'igitre i6. Algae 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. 



6, the floating liverwort (Ricciocarpus natans) 



of these are shown in Figure 



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 p.^. le. Aquatics that rise from standing 



means of which theymay rtt"s^ee?laTw?^r.liSf*l'"^nS: 

 change location. The tafu^Z/toTw^.-or"''"''*""'' ^""^ ''• *^ ''^'" 



