IOWA ACADEMY OP SCIENCES. 
77 
probable that the glaciers advanced and receded with the 
changes of seasons, and with each recession of the ice the 
lichens were given a new lease of life. Thus while the higher 
plants from the east and the west met on the barren prairies of 
Iowa, those from the west at first predominating, and while 
their advance was probably respectively from the southeast 
and southwest, the lichens of the rocky ledges and boulders 
came to us from the north and represent the oldest flora in the 
state. 
NOTES ON AQUATIC PLANTS PROM NORTHERN IOWA. 
BY B. SHIMEK. 
The aquatic flora of northern and northwestern Iowa is of 
great interest, and it deserves especial attention because the 
occupancy of that part of the state by agricultural man is 
rapidly transforming the ‘‘ Thousand Lake Region of Iowa,” as 
the early settlers called it, into thousands of pastures, flax flelds 
and wheat fields. 
The lakes and ponds are being drained either artificially or 
by the changes in surface conditions, and while it is probable 
that the aquatic flora will persist in the larger lakes for a long 
time, it will certainly be restricted; it is, in fact, already 
restricted, and if these large lakes change as rapidly as Clear 
lake. Spirit lake and Lake Okoboji (to say nothing of smaller 
ones) have in the past few years, Iowa will soon know no lakes. 
It is important, therefore, that the history of the aquatic plants 
of the northern part of the state be as complete as possible, and 
that specimens of these plants be preserved for future 
reference. 
Various scattered notes on this flora have been published, 
but thus far only one paper specially devoted to it has appeared. 
Early in the year Mr. R. I. Cratty published’^ an admirable 
paper on the aquatic flowering plants of Iowa, and these notes 
are practically merely supplementary to that paper. A part of 
the field work at Spirit lake and Lake Okoboji, the results of 
which are here given, was made in company with Mr. Cratty, 
and his experience and enthusiasm added much to the interest 
and the value of the work. 
* Bulletin Lab. of Nat. His., State Univ. of Iowa, vol. HI, No. 4. 
