THE LARGER AQUATIC VEGETATION 205 



of these three types of soil the loamy one is capable of supporting 

 the best growth. 



That soil is necessary for good growth has been previously men- 

 tioned and explained. The interesting question arises — why is it 

 that plants artiiicially anchored but not allowed to root in the soil 

 are unable to make a good growth? It surely must be a matter 

 of nutrition, because the soil could hardly be so potent an influence 

 in any other way. When the plants are artificially anchored all 

 other conditions are the same as with plants rooted in the soil 

 except that the roots of the suspended plants are exposed to light 

 and are unable to absorb nourishment from the soil. That light 

 in some measure retards root development and thus diminishes the 

 absorbing capacity of the plant is certain, but this factor is entirely 

 too small to account for the stunted growth of plants denied a soil 

 substratum. 



Chemical analysis has been employed for the purpose of securing 

 if possible some clue to the reason why these plants cannot make 

 normal growth unless rooted in the soil. Vallisneria spiralis, the 

 eel-grass, whose elaborate arrangement for pollination has been 

 described, was selected as a typical water plant of wide distribu- 

 tion. The history of the material to be analyzed must, of course, 

 be known, and in this case a large number of small plants of 

 uniform size were carefully taken from the lake bottom and trans- 

 ferred to submerged boxes which were alike, except that one con- 

 tained soil and the other did not. The roots of the plants arti- 

 ficially anchored in the box without soil were not permitted to 

 touch the box, but dangled in the water, and thus from the water 

 only could nourishment be taken. After a certain time the plants 

 were gathered and carefully washed, only the living specimens, of 

 course, being saved. In order to establish a basis for comparison 

 the volume of each group of plants was obtained by immersion in 

 water and measuring the displacement. The volume of the plants 

 rooted in soil was taken to be just twice the volume of the plants 

 artificially anchored. This material was then dried out and the 

 dry weight taken, which, for the plants rooted in soil, was 52.7 

 grams and for those suspended 37.2 grams. These figures show at 

 once that the suspended plants are relatively much heavier than 



