336 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



THEOEETICAL DISCUSSION. 

 ZONATION. 



The relation of soils to aquatic vegetation has been very little 

 studied. It is generally believed that the value of a soil depends 

 largely on its ability to retain moisture. This, however, can hardly 

 be true for submerged soil winch is always saturated with water. 

 Pieters (1901), in his work on Lake Erie, has shown that there is 

 probably a relation between the soil and aquatic vegetation. He 

 says that his soil samples were not numerous enough to make gen- 

 eral deductions possible, but he indicates the direction in which they 

 seem to point. To quote: "As a rule the soils on which plants 

 occurred in abundance were composed largely of firm sand and con- 

 tained relatively little silt, fine silt, and clay, while the soils on which 

 few or no plants occurred, although the depth of water and other 

 physical conditions were favorable, were composed largely of silt, 

 fine silt, and clay, and were poor in fine sand and very fine sand." 

 And again: "The water in sandy soils is undoubtedly better aerated 

 than that in clay soils, though both are under water, because in the 

 former case the water passes through the soil more rapidly than it 

 does in the latter, and it would seem that even the roots of aquatics 

 are unable to thrive in a soil so poor in oxygen as the saturated heavy 

 clays." This explanation can not account for the zonation in Lake 

 Ellis, for here we have no clay and the poorest soil is a coarse sand. 



Pond (1903) as the result of experiments concluded that Philotria, 

 Myriophyllum, and a number of other plants grew better when 

 rooted in a good soil than when anchored over a soil or rooted in 

 clean washed sand. The writer repeated these experiments with 

 Philotria canadensis in the greenhouse of the Johns Hopkins Uni- 

 versity during the winter and spring of 1910. In these experiments, 

 which will be described more fully in a later paper, as in those per- 

 formed by Pond, the plants rooted in a good soil grew better than 

 those anchored over the same soil. In the latter case, however, the 

 soil soon became covered by a dense growth of algae. When the 

 same algao were transferred to a jar with rooted plants the algae 

 failed to make any appreciable growth. This suggested that the 

 slight growth of the anchored plants of Philotria was due to the use 

 by the algae of some substance which came from the soil and which, 

 in the case of the rooted plants, could be used by them, since they 

 were nearer the soil than the anchored ones. Further experiments 

 seemed to show that this substance was C0 2 , derived from the organic 

 matter in the soil. Plants were grown under a number of different 

 conditions but in no case did they make a normal growth unless 

 some C0 2 other than that obtained by the water from the air was 

 supplied them. When a soil which contained little or no organic 



