By L. R. WILSON
COE COLLEGE, CEDAR RAPIDS, IOWA
If we examine the rooted hydrophytes as a biological group, we are impressed by the fact that they come from practically all parts of the vascular plant kingdom. The Pteridophytes contribute members from the genera Eqitisetum, Isoetes, and the water ferns. The monocotyledons which contribute great numbers of species are from the genera Typha, Sparganium, Potamogeton, Zostera, Najas, Juncus, Sagittaria, Alisma, Elodea, Anacharis, and Vallisnera, as well as some grasses and sedges. The dicotvledons are represented by such families as the Buckwheat, Hornwort, Water Lily, Crowfoot, Waterwort, Evening Primrose, Water Milfoil, Parsley, Primrose, Convolvulus, Figwort, Bladderwort, Lobelia, and Composite.
Such an assemblage of families and genera from all parts of the vascular plant kingdom, except from the Gymnosperms, raises the question of origin for these hydrophytes. It is sufficient to state here that there is reason to believe that the Angiosperms, at least, were originally terrestial and that some members have become semiaquatic or entirely aquatic.
The close relationship between the anatomy of the hydrophytes and the waters in which they live is not only of prime importance to the plants in question, but is also important to those smaller plants and animals which depend upon them for support, protection, or food. Probably the most important modification in their anatomy is the development of spongy tissues called aërenchyma. These tissues provide air spaces in the various parts of the plants, and serve in various capacities in their physiology, and in the life histories of the invertebrates which live upon them. Most of these plants are perennial, and coupled with their more or less uniform environment, tend to produce somewhat permanent communities of plants and associated animals. Therefore, a more static ecology is likely to exist in water than on land.
It is common knowledge that the geology of any particular region determines the soils and the characteristics of lakes which are formed in it. The soils which have evolved from the bedrock, or from the materials that have been variously transported to a particular region and modified by climate, determine to a large extent the vegetation of the region. This statement applies to both the vegetation of the land and of the water.
The relationship of bedrock to lake chemistry appears to be well illustrated in Wisconsin. An examination of the geological map of that state shows a large area in the southern part to be covered by limestones, while in the northern part there is a smaller area where granites are exposed. It may be generally stated that the lake waters upon the limestones are harder than those upon the granites. This difference of lake waters may also be a direct reflection of the glacial drifts in regions where the bed rocks are deeply buried. In regions, as in north central Wisconsin, where the glacial drift is derived largely from acid rocks, the lake waters are softer than in regions where the drift is derived from limestone. Lakes that were formed by some geologic agent which left the bed or basin developed on solid rock, or with little or no fine elastic material, show different physical and chemical characteristics than those that