BULLETIN OF THE UNITED STATES FISH COMMISSION. 
68 
since that time. Absence of data concerning the aquatic plants growing during those 
years makes it impossible to saj r what effect this change of level has had, but it is 
certain that data collected in any one year can not be depended upon to give the 
normal depth at which certain plants flourish. Most species of aquatics being 
perennial, they may start during periods of low water in places that will be too deep 
for them during high water, but where they can struggle along for a season or two 
before perishing. 
ECOLOGICAL GROUPS. 
Many attempts have been made in recent years to arrange all plants into groups 
according to the conditions under which they flourish and the manner in which they 
adapt themselves to their environment. 'Warming, Drude, and others have adopted 
certain groupings, but as yet, save on the main divisions, there is no agreement among 
plant geographers. All classifications, however, include the hydrophites, or water 
plants, as one of the main divisions, and split this up into a number of smaller groups, 
each group including all those plants having more or less similar life habits, although 
perhaps belonging to widely separated families. Ecological groups, indeed, are not 
in the least based on taxonomic characters. Among the free-swimming, submersed 
forms are such widely different species as Utricularia vulgaris , Lenina trimica, and 
species of Mesocarpus , Spirogyra , and Lyngbya , besides many others, both macro- 
scopic and microscopic. The attached, low-growing, submersed forms include the 
JSTaiadacece and Characeoe , while species of Potamogeton and JV-ymphcea belong to the 
group with floating leaves. It would be possible to make a large number of groups, 
attempting to express in this manner all the ways in which water plants are influenced 
by currents, depth, light, and the other factors bearing upon an aquatic habitat, and 
in so far as such groups represent biological facts they have a value. It is doubtful, 
however, whether any but the broader divisions are sufficiently stable to be of use, 
the great adaptability, within certain limits, of the aquatic organism rendering the 
boundaries of the smaller groups too indistinct to be readily recognized. This is 
illustrated in Vallisneria spiralis , the long ribbon-like leaves of which are thought 
by some to be especially well fitted for floating downstream in a rapid current, but 
it grows equally well in almost stagnant water. Potamogeton heterophylhis, when 
growing in quiet pools, produces floating leaves, but when on a surf-beaten bar it 
branches freely from the base and the floating leaves are absent. I shall therefore 
divide the water plants of our region into only five groups, as follows: 
1. Free-swimming, microscopic forms in the open lake: The Plankton. 
2. All other unattached species, macroscopic and microscopic: Utricularia vulgaris, Lcmme , Cerato- 
phyllum demersum, and many algte growing in quiet places. 
3. Attached submersed plants: Naias, Cliara, Cladophora, Vallisneria , Potamogetons. 
4. Attached plants with floating leaves: Nymphoeaceee, Potamogetons. 
5. Swamp plants. 
The Plankton includes, of course, both animal and vegetable forms, among the 
latter being species of Merisrnojjedia , Pediastrum , Sphazrozyga, Clathrocystis , and 
many diatoms. Species of Desrnidem sometimes occur, but probably by accident. 
The species of the plant Plankton being mostly without voluntary motion, are subject 
to all the currents and movements of the water. 
In the second group are many widely distant forms, all growing in quiet pools 
and sheltered places. Among the higher plants Utricularia vulgaris , Ceratophyttum 
