292 SVEDELIUS : LIFE-HISTORY OF 
In Elodea the pollen is nearly all discharged before the 
male flower reaches the surface, so that, according to Wylie, 
the floating male flowers themselves do not play any impor¬ 
tant. part in the pollination. But instead the pollen itself 
shows a very remarkable organization for floating. The 
tetrads remain joined together so strongly that violent 
shaking seldom breaks them apart. In organization they 
rather resemble a Pediastrum or Hydrodictyon or other such 
plankton-alga with the different cells more or less closely 
joined together with interjacent spaces between (cf. Wylie, 35, 
PL III., fig. 51, 53). The pollen grains have not only a 
well-developed intine, but also a strongly cutinized exine 
beset with spines, which retain air in their interspaces, so 
that the whole tetrad is kept floating. Thus the pollen itself 
in Elodea shows a remarkable adaptation for floating and in 
this way reaches and comes into contact with the floating 
stigmas. Consequently Elodea is strikingly epihydrogamic, 
but also represents a distinct type of hydrophilous pollina¬ 
tion owing to the pollen tetrads being especially adapted for 
floating. 
In the structure of the-pollen grains Enalus agrees much 
more closely with some of the hypohydrogamic plants. 
These might, I think, also be divided into two more or 
less different groups with respect to their pollen grains. 
One is distinguished by the pollen grains having the same 
specific gravity as the water, or at any rate not being much 
heavier. Often the pollen grains also have a remarkable 
shape, viz., vermiform. In connection with this Goebel (10, 
II., p. 367) has pointed out that the stigmas are especially 
adapted for catching such pollen, as they are more or less 
curved but never infundibuliform. This form on the 
contrary is very characteristic of hypohydrogamics -with 
pollen heavier than the water (owing to numerous starch 
grams). These hypohydrogamics are always so constructed 
that the sinking pollen is collected by a special “appareil 
collecteur,” which in different plants is of somewhat 
