BROWN PLANT LIFE OF NORTH CAROLINA LAKES. 337 



matter, even though it were rich in mineral substances, was used 

 the plants made no better growth when rooted than when floating 

 free in the water. This was true both when COj was and was not 

 supplied artificially. These experiments seem to show that the C0 3 

 derived from the organic matter of the soil may be an important ele- 

 ment in the growth of submerged plants and may be a factor in the 

 distribution of the submerged plants of Lake Ellis. This is, however, 

 probably not the only factor concerned, as this conclusion can hardly 

 apply to the emergent forms and it is probable that the soil factors 

 which determine the distribution of the emergent forms also affect 

 the submerged ones. 



Why the nature of the soil should determine the distribution of the 

 emergent plants in Lake Ellis is not clear. When we remember that 

 soils have the property of withdrawing salts from solution and that 

 plants can concentrate salts from very dilute solutions it does not 

 seem probable that anywhere in Lake Ellis there are not enough salts 

 to support the plants of the marginal zone. 



The physical nature of the soil, on the other hand, may be a very 

 important factor in determining the zonation. Most of the plants 

 of the two outer zones grow rapidly and have large and rapidly grow- 

 ing roots. The hard-packed condition of the sand would probably 

 offer considerable resistance to the large rhizomes and roots, while 

 the sharp edges of the coarse grains might possibly injure them as 

 they were being pushed through the packed soil. The mud, however, 

 would offer little resistance or injury to the large, soft, and rapidly 

 growing roots and rhizomes. 



PRODUCTION OF PHYTOPLANKTON. 



Kofoid (1903) after making careful quantitative determinations of 

 the plankton in a number of lakes concluded that "The amount of 

 plankton produced by bodies of fresh water is, other things being 

 equal, in some inverse ratio proportional to the amount of its gross 

 aquatic vegetation of the submerged sort." He attributes the scar- 

 city of plankton in lakes containing submerged vegetation to a num- 

 ber of causes, but chiefly to the removal from the water, by the larger 

 aquatics, of a great part of the available food material. 



Pond (1903), as mentioned above, found that a number of aquatics 

 grew better when rooted in a good soil than when anchored over the 

 same soil. From these and other experiments he concluded that 

 these plants " are dependent upon their rooting in the soil for opti- 

 mum growth/' and that "the roots of these plants are organs of 

 absorption as well as of attachment." He says further that these 

 conclusions are probably applicable to all aquatic plants which grow 

 rooted in a soil substratum. 



70272°— vol 13, pt 10—11 3 



