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 fenestrata Kitz., which is exceedingly abundant in the 
plankton of European lakes and in our own Great Lakes, was found 
but a single time in the waters of the JIlinois. 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. 
CONJUGATA. 
This group of algz 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 alge 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 algz 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 
