314 HENRY B. WARD. 



bodies " are actually gas vacuoles in the protoplasm, present in 

 all the plankton Cyanophyceae, but entirely wanting in the 

 fixed forms. 



On the diatoms of the plankton Zacharias (95h) gives statis- 

 tical records from the enumeration of plankton hauls during 

 the year showing the number found at different seasons and 

 the maxima and minima of various species. Whipple (94) 

 notes the effect of the fall overturning of the water in producing 

 a maximum of diatom development by the distribution of an 

 abundant food supply from the stagnant substratum of the 

 water. In a later paper (96a) are recorded more detailed obser- 

 vations on the effect produced by other causes. The maximum 

 of diatom growth is shown experimentally to be just below the 

 surface of the lake, to be greater in light-colored water and to 

 vary in close correspondence to the variation in the intensity of 

 the light. Apparently the diatoms possess no power to move 

 upward toward the light but are carried upward by convection 

 currents in the water. Such conditions prevail particularly in 

 the fall circulation period. Pero has studied very carefully 

 the distribution of the diatoms in the lakes of a single canton 

 in the Alps. 



The "water bloom" has been studied by Klebahn (96) who 

 finds that thirteen different species may give rise to the phe- 

 nomenon. According to Strodtmann (98) it is only indirectly 

 the cause of actual damage, varying in amount under different 

 conditions, and is of direct value as food, particularly to the 

 Cladocera and Copepoda which are so important as fish food. 

 Here also Richter (94) and Thomas (97). Seligo (97) dis- 

 cusses the damage done by the introduced JElodea canadensis 

 and believes it probably overdrawn. After considering its re- 

 lation to the general biology of the water, the author empha- 

 sizes the small value of the shore plants in the food relations 

 of freshwater and yet on the other hand the known greater 

 abundance of fish where such plants are found. 



For Plon and other lakes in Holstein, Klebahn (95) describes 

 the aquatic vegetation, the regions into which it may be di- 



