192 



NATURAL HISTORY OF THE FARM 



and sparkling, of a low and constant temperature, with a low 

 oxygen content, and, owing to prolonged contact with the 

 soil, with a high mineral content that varies much according 

 to the character of the soil traversed. Deposits of sulphur 

 and of iron are often formed about the mouths of mineral 

 springs. But where the ordinary spring bubbles up, one 

 usually sees only miniature deltas of clean-washed sand at the 

 bottom of a limpid pool, which clears itself quickly after 

 r\ roiling. 



Spring water has a population of its own. 

 Man and bird and beast are transient 

 visitors who only quaff its waters; but 

 there are other creatures, that permanently 

 dwell in them. They are things that cannot 

 endure too great heat in summer or freezing 

 in winter: things that like low equable 

 temperature and partial shade. The most 

 characteristic plant that grows in spring 

 water is water cress (fig. 74) ; it was used 

 by the pioneer to garnish his meat platter, 

 and it is still so used. There are water- 

 mosses, also suited to such a habitat, and 

 many lesser algas of various kinds, both 

 green and brown. 



There are animals, also, that live in 

 spring water; such are the salamander shown in figure 75, 

 and the brook trout, which does its best in water not warmer 

 than 6o° F., and many other lesser creatures. Most of 

 the great groups of animals are represented there, if 

 by only a few forms: crustaceans; by the scuds, clamb- 

 ering over and feeding upon the water-cress, and by 

 asellus, wallowing in the soft bottom of the pools 

 (fig. 20); molluscs, by little white clams (half an inch 

 long, more or less), of the genus Sphcerium, furrowing the 



Fig. 74. A leaf of 

 watercress. 



