420 
The Ohio Naturalist. 
[Vol. VI, No. 2, 
FREE-FLOATING PLANTS OF OHIO. 
Mabel Schaffxer. 
In general all h^’drophytes 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 onlv 
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, 
among which must also be included rooted forms accidently 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 Azolla 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 hav’e the power of 
shedding water. In Salvinia curious, tufted hairs, the tips of 
which s])read 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 
