EARLY PROBLEMS 21 



and favored in its growth by these waters. The profusion of growth of this 

 hydroid on leaves and stems of aquatic plants is shown in Fig. 9. Fig. 10 

 shows the hydroid growing on a leaf of Potamogeton, and Fig. 11 shows other 

 stages in the development of the hydroid and accompanying organisms. This 

 hydroid is not a parasite on the plants but feeds instead on the abundance 



Figure 10. Photomicrograph (about 50 X) of Cordylophora lacustris AUman entwining 

 a leaf of Potamogeton pedinatus L. 



of plankton in the polluted water. The hydroid forms a gelatinous sheath 

 about the stems and leaves of the plants and the main injury occurs after 

 the hydroid dies. This gelatinous sheath is also a good cultural medium 

 for other organisms such as larvae, worms, diatoms, rotifers, fungi, and 

 bacteria, which add to the injury. As a result of the smothering activity of 

 all these organisms the plants were partly killed, and were broken from their 

 moorings, after which they sank to the bottom and added to the pollution 

 and turbidity of the water or were carried to the shore by the wind. 



Bourn indicates that the biggest factor in denuding these waters of plants 

 was the great turbidity of the water. He made many measurements of the 

 light intensity at different depths in these waters during the growing season. 

 He also determined in cultures the light intensity needed for the growth 

 of the various duck food plants. On the basis of these two sets of measure- 

 ments he came to the conclusion that during the growing season in many 

 of these waters there was not enough light penetrating three feet below 

 the surface to support plant growth. Although the minimum light require- 

 ments for the two important bottom-cover plants, Chara and Nitella, were 

 not determined, it is probable that light was deficient in the deeper waters 

 for these also. The lack of light was especially destructive to Potamogeton 



