22 



GROWTH OF PLANTS 



pedinatus and Vallisneria spiralis. The former thrives in clear water up to 

 a depth of 8 feet and the latter up to a depth of 12 feet. Recent work ^^ 

 indicates that in clear water the latter and perhaps the former may grow 

 at two or three times the depths mentioned. There were two sources of par- 

 ticles of matter causing the turbidity — organic and inorganic. The water 



Figure 11. Photomicrographs (about 50 X) of Cordylophora lacustris Allman. A, 

 Typical hydranth. B, Stalk of a colony with its gelatinous secretions. C, Embryos. 

 D, Diatoms growing in the gelatinous secretions left by hydroid colonies on a leaf of 

 an aquatic plant. 



flowing down the canal had many sewage particles in suspension. This and 

 plants killed by the hydroid formed a sludge on the bottom that was being 

 continually stirred up by wave motion. Inorganic clay particles were kept 

 in suspension by wave action because of the lack of a bottom-cover of 

 Chara and Nitella. 



McAtee ^ reports that at the time of his study of these waters in 1909 

 there was an almost complete bottom-cover of Chara in Currituck Sound. 

 In 1926 to 1930 there were only patches here and there of Chara and Nitella 

 as bottom-covers. Where they did appear the water Avas clear; also there 



