T2 BOTANICAL GAZETTE [JANUARY 
if any significance can be attached to them in Elodea. The pol- 
len 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 enlargement; 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. 
This can be demonstrated easily by placing the spores in dilute 
alcohol, then transferring quickly to water. A simpler and 
more striking method is to float a quantity of these spores on 
the surface of water half filling a stender dish; then cover and 
shake vigorously for a moment. The violent agitation of the 
water breaks the surface film, permitting the liquid to come into 
more intimate contact with the body of the spores, which sink at 
once to the bottom of the vessel. 
While the gas bubbles may not be necessary for pollination, 
they are certainly very helpful. Their buoyancy aids in detach- 
ing 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 accumu- 
lation of gas about the spores in the submerged flower is also of 
significance in that it prevents the moistening of the ripe spores 
while yet submerged; for this, as we have seen, would lead to 
their sinking upon release. The general relation of these accumu- 
lations of oxygen is shown by the fact that in quiet waters of 
aquaria the rapid appearance of the staminate flowers at the sur- 
face of the water is simultaneous with the first marked photo- 
synthesis of the day. During the early part of the forenoon 
the flowers which have ripened during the hours of darkness are 
rapidly brought up as the liberation of gas by the plant is 
increased, and very few appear in afternoon or evening. 
