The Animals of the Lemna Association

Abstract

This is a study of the small animals in an association of one of the smallest flowering plants, Lemna minor, in the vicinity of Ithaca, N. Y. The dominant animal forms are naturally very small, ranging in size from a fraction of a millimeter to scarcely six millimeters in length. Yet, just as with large organisms, the destruction caused by them is compensated by the prolific growth of the plant. The individual plants of Lemna mttinor are, as is well known, very small, flat, leaf-like discs, convex on the upper side and slightly concave underneath. A single water root dangles from the center of each green thallus and helps to maintain the equilibrium of the plant. Being a free-swimming hydrophyte, Lemn1a minor has many air cavities which make it buoyant. Stomata are scattered over the upper surface. From April to October may be considered the active growing period of this species in the latitude of Ithaca, New York, with the peak of rapidity in growth occurring in July and August, when an entire pond may be blanketed with these small plants. Proliferation takes place in one plane, the lateral branches emerging from two clefts or pouches on the margin of the thallus. These new thalli are connected with the parent thallus by short slender stalks and are easily detached by a slight disturbance of the water. New centers of growth are thus established in the pond by the drifting apart of the disconnected offspring. In the fall, when conditions become less favorable for growth, lateral branches develop which lack air spaces and have plenty of reserve food. These winter buds sink to the bottom of the pond where they remain until the water becomes warm again the next spring. Then with the formation of air spaces, they begin their vegetative activities for the summer and rise to the surface. Propagation takes place also by the production of seeds from diminutive flowers (fig. 1). Inasmuch as Leminia mi-nor grows in quiet waters, it is not surprising that many filamentous forms of algae, as well as colonial types, are found among its roots and attached to the under surface of the thalli. Desmids and diatoms likewise abound. The liverwort, Riccia natans, and the spermatophytes, Spir odela and Wolfia, sometimes float about with the Lem-na. Near the edges of ponds grow cat-tails (Typhaceae), bur-reeds (Sparganiaceae), and arrowheads (Alismaceae). Occasionally water-lilies (Nymphaeaceae), water cress (Cruciferae), Elodea, and CeratophyllvUn associate themselves with the duckweeds (Lemlnaceae).