59 



it would seem from the intergradation with the normal condition 

 that it is a phenomenon of physiological import rather than of 

 specific significance. It would seem desirable that experimental 

 breeding of diatoms should be employed as a test before specific 

 diagnoses utilize this character. 



Synedra capitata Ehrbg. is occasionally adventitious in the plank- 

 ton in spring months. 



Synedra ulna (Nitzsch) Ehrbg.* Average number, 302,308 

 (silk, 34,510). This appears somewhat irregularly in the plankton, 

 with a vernal pulse on May 17 of 5,400,000 and an autumnal one 

 November 15 of 1,800,000. It is abundant on the ooze of exposed 

 springy shores after rapid decline of the river, and is probably 

 adventitious in the plankton to some extent from this region. 



Tabellaria fene strata Kiitz., which is exceedingly abundant in the 

 plankton of European lakes and in our own Great Lakes, was found 

 but a single time in the w r aters of the Illinois. It can hardly be lack 

 of food elements which prevents its development, and there are 

 times when favorable thermal conditions would seem to be offered 

 in spring and autumn, when the river temperatures do not exceed 

 the summer temperatures of our Great Lakes. It may be that the 

 chemical conditions attending sewage contamination exert a dele- 

 terious influence upon this species and others of the genus, such as 

 T. flocculosa, which abound in purer lake waters. 



CONJUGATE. 



This group of algae is represented in the plankton only by a few 

 desmids, which neither in number, or quantity, play any important 

 part in the ecology of the plankton. The filamentous algae are 

 abundantly represented in spring in the backwaters of the Illinois 

 River, where they form extensive littoral fringes of "blanket moss," 

 which load down the emerging littoral flora. This fringe is fre- 

 quently stranded by the retreat of flood waters. In some localities, 

 as in Phelps Lake, it plays a very important part in the food cycle, 

 since by its decay, as temperatures approach the summer maximum, 

 it contributes immediately its store of organic nitrogen to the sup- 

 port of the small algre and flagellates which develop in great num- 

 bers on those waters at that season. Some species of Spirogyra 

 and Zygnema have a habit of breaking up into short filaments, and 



