420 The Ohio Naturalist. [Vol. VI, No. 2, 



FREE-FLOATING PLANTS OF OHIO. 



Mabel Schaffxer. 



In general all hydrophytes may be classed into two groups, 

 those rooted in the soil and those which are free. The rooted 

 plants are either completely submerged or they may have part 

 of the body above and part below the surface of the water. 

 Among the latter type of plants are numerous species with only 

 the leaf blades floating on the surface, as Potamogeton natans 

 and Castalia odorata. The non-rooted vegetation consists (1) 

 of microscopic, free-floating and free-swimming Thallophytes 

 and (2) of higher plants adapted to a free-floating condition, 

 amons: which must also be included rooted forms accidentlv torn 

 from their anchorage and the specially developed propagative 

 buds known as hibernacula. The microscopic plants together 

 with the Protozoa and other low animal forms make up the 

 plankton, while the second type of societies has been called the 

 derived or secondary phyto-plankton. 



The typical members of the secondary plankton are passive, 

 free-floating plants which as appears from their general structure 

 and life cycle were evidently derived from rooted ancestors. In 

 free-floating plants like the duckweeds, which are among the most 

 highly specialized forms, the leaves are entirely absent and the 

 stem is a flattened, disc-like body, or in a few species it is nearly 

 spherical. The plants are buoyed up on the surface of the water 

 by means of air cavities developed either in the body of the stem 

 or in the leaves. The most striking of these adaptations is a 

 spongy enlargement of the petiole as in the water hyacinth. 

 The air reservoirs usually consist of spongy tissue with large 

 intercellular spaces. 



Most floating plants have a suitable counterpoise to prevent 

 the plant from being turned upside down by ripples and waves. 

 In AzoUa and most of the duckweeds the counterpoise consists 

 of one or more dangling roots. In Salvinia dissected leaves 

 looking much like hanging roots act as counterpoises. In 

 Ricciocarpus the counterpoise consists of numerous slender 

 scales. 



There are various adaptations to afford protection against 

 wetting. The larger duckweeds have a very smooth and glisten- 

 ing surface from which water rolls in the spheroidal form. Lemna 

 trisulca which is usually submerged does not have the power of 

 shedding water. In Salvinia curious, tufted hairs, the tips of 

 which spread out in three or four branches, are developed on the 

 upper surface. When the plant is overturned air is imprisoned 

 bv these tufted hairs and it is immediately turned right side up. 



Surface floating plants are exposed to intense light. Some 



