1920] St. John,— Elodea in New England 23 
It is clear from the context that his observations were made on the 
species here treated as occidentalis. “The staminate flowers are 
borne entirely beneath the surface of the water, and these, as is well 
known, break off and rise to the surface, there shedding the pollen. 
It is probable that with the ripening of the sporangia, in the still 
submerged flower, gases given off by the plant fill the spaces about 
the spores as well as any other cavities developed in the flower. At 
maturity a bubble of oxygen forms at the tip of the flower, and with 
its enlargement the sepals open slightly. At this time, looking down 
into the flower one can see that the sporangia have opened, and that 
many of the spores have been shed into the central space. "The 
oxygen bubble may finally become nearly as large as the flower, 
and, when conditions are proper, the buoyancy of the enclosed gas, 
aided by the low specific gravity of the flower itself, overcomes the 
weakened attachment, and the flower darts to the surface. Upon 
reaching the surface the bubble disappears, the sepals snap back 
quickly, and in their recurved position form three boat-like floats 
which support the sporangia above the water; these catch the breeze 
and the flower sails away. While such float devices for the staminate 
flower are thought to be of great importance in the pollination of 
Vallisneria, it is doubtful if any significance can be attached to them 
in Elodea. The pollen was nearly all discharged at the moment the 
flower came to the surface, and any remaining portion would have 
no better opportunity for reaching the stigma of the pistillate flower. 
The snow-white tetrads are quite conspicuous floating on the water, 
or scudding along the surface with the wind. 
The floating of the pollen grains is due to the nature of the outer 
spore coat. In a previous paragraph it was mentioned that the 
exine was covered with spines, each bearing at its tip a slight enlarge- 
ment; these spines tend to hold back the surface film from contact 
with the body of the spore, and thus imprison enough air to keep it 
afloat. The microspore has a greater specific gravity than water, 
and will sink at once if wetted. * * * 
While the gas bubbles may not be necessary for pollination, they 
are certainly very helpful. Their buoyancy aids in detaching the 
flowers, raises them quickly to the surface, and the sudden recurving 
of the sepals may be related in some way to the escape of the bubbles 
on reaching the air. The accumulation of gas about the spores of 
the submerged flower is also of significance in that it prevents the 
