194 FRESH-WATER BIOLOGY 



are so buoyant that the seedling rises to the water surface often 

 carrying with it the remains of the old seed. 



By vertical distribution is understood that which exists in a 

 plane more or less perpendicular to the earth's surface and may be 

 illustrated by the distribution one may observe in passing from 

 valley to mountain-top or by comparison of species found at vari- 

 ous depths in lake or ocean. Horizontal distribution is, of course, 

 in a plane more or less conformable to the earth's surface and is 

 such as one notices in passing from east to west or north to south, 

 etc. Now the factors which determine the horizontal distribu- 

 tion of water plants are: first, the chemical composition of the 

 water, a factor which gives the two large divisions of fresh and salt 

 water plants; second, temperature which gives zones of plant life 

 such as arctic, temperate, tropical, etc. ; third, competition among 

 the plants themselves, a factor which is likewise influential in 

 vertical distribution though perhaps to a less degree; and fourth, 

 the nature of the substratum, which is, of course, most influential in 

 the distribution of species which grow rooted to the bottom. 



To what extent chemical composition of the water is a factor in 

 the distribution of fresh-water plants cannot at present be stated. 

 Sulphur springs and waters having unusual composition are not, of 

 course, fresh water. By the latter term is understood such as 

 occurs in the rivers and lakes and such as may be used as drink by 

 the animals, so far as chemical composition is concerned. Such 

 waters differ, of course, in the quantity and quality of constituents; 

 but whether such differences are in themselves independent of 

 other factors, sufficient to determine distribution, cannot at present 

 be stated. 



Suppose we should find that the water of some lake in Wisconsin 

 is considerably different in chemical composition from that of a 

 lake in New York and a species of Potamogeton, for example, which 

 is abundant in the Wisconsin lake but unknown to the waters of 

 the New York lake, be taken to the New York lake and planted 

 there. If this plant grows well in the New York lake we would 

 say that, other conditions being equal, the difference in chemical 

 composition of the water in the two lakes is not a determining 

 factor in the horizontal distribution and that the absence of the 



