204 



FRESH-WATKR BIOLOGY 



soil is very sandy the species may be abundant and likewise where 

 the substratum is distinctively clay or gravel. From field observa- 

 tion alone it would hardly be possible to say that the quality of 

 the soil is the determining factor, because sand and gravel are more 

 common where other factors come in to influence growth and 

 species selection. The loamy soil is most abundant in the coves 

 and bayous where the water is quiet and it is in just such places 

 that plants make good growth and wealth of species abounds. 

 Plants which would perish in exposed situations make excellent 

 growth in the protected coves and we cannot be certain from field 

 observ'ation whether the difference is to be attributed to the soil 

 or to the more favorable locality. By direct experiment, however, 

 it is not difficult to ascertain what quality of substratum favors 

 growth. In such experiments it is very desirable to have condi- 

 tions as nearly Uke those in nature as possible. Three tyipes of 

 soil were selected, clayey, sandy, and loamy. A large rectangular 

 glass aquarium was used to contain a substratum of each one of 

 the given soil t}^es. Then the three aquaria were placed upon a 

 submerged platform so that the aquaria themselves were also sub- 

 merged. This arrangement made the aquaria similar in all condi- 

 tions except as to the quality of soil, and differences in the growth 

 of plants in each aquarium could be very easily referred to the 

 varying quality of soil. Two types of plants were tested in 

 this way — one was Vallisneria, a t^-pical water plant of the phan- 

 erogams, and Chara, one of the higher algae. The latter does not 

 have roots but simple structure called rhizoids which help to secure 

 attachment. With both of these plants the best growth was 

 made in the loamy soil and the poorest in the clay soil, while the 

 sandy soil which of course contained some organic debris supported 

 a medium growth. In the case of Chara an equal number of plants 

 of equal size were planted so that the dry weight of the total 

 growth in each aquarium might be compared. If the poorest 

 growth in clay be taken as one, then that in sand would be about 

 2.06, while that in the loamy soil would be about 3.33. The total 

 dry weight is of course a fair indication of the relative amount of 

 growth made and there can now be no doubt but that the qaahty 

 of soil is much concerned with the plant growth upon it and that 



