WATER POLLINATION 71 



spread apart, the stamens elongate until the anthers hang outside 

 of the flower, and then the pollen is shed, the whole process often 

 taking no more than fifteen or twenty minutes. As a rule each flower 

 remains open but a short time but different flowers on the same plant 

 may open progressively for several days. Sometimes unfavorable 

 weather greatly retards or prevents the opening of the flowers and in 

 that case self-pollination is likely to occur. 



45. Water Pollination.— The majority of seed plants that grow 

 in the water produce their flowers above the surface of the water 

 and are pollinated by the wind or by insects. There are some inter- 

 esting cases, how^ever, in which water is the agent of pollination. 

 In some of the pondweeds (Potamogeton) , for example, the pollen 

 grains are filamentous and without thick walls. They are just 

 heavy enough to float below the surface of the water and may come 

 in contact with the long exserted stigmas by chance. 



In the tape grass {Vallisneria spiralis) the solitary pistillate 

 flowers are borne on peduncles which elongate just enough to bring 

 the flowers to the surface of the water. Sometimes these peduncles 

 become as much as a meter in length, the growth continuing in any 

 particular case until it is checked, probably, by increased transpira- 

 tion at the surface of the water. The staminate flowers are produced 

 in clusters of several hundred. Each one is about 1 mm. in diameter 

 and consists of two stamens enclosed in a three-parted perianth. 

 These staminate flowers become detached under water and rise 

 slowly to the surface where they open and the segments of the 

 perianth curve back in such a way as to support the flower on a little 

 tripod. A pistillate flower resting on the surface of the water 

 causes a slight depression of the surface film and when staminate 

 flowers approach this depression by chance they shoot down into it 

 and the stamens are very likely to come into contact with the 

 stigmas. After pollination has taken place the peduncle coils up 

 and draws the pistillate flower below the surface again, where the 

 fruit matures. 



Another interesting case of pollination in water plants is that found 

 in some of the duckweeds. The duckweeds are among the smallest 

 and sunplest of flowering plants. One of these, Lemna minor, con- 

 sists of a little green thallus from 2 to 5 mm. in diameter which 

 floats on the surface of the water. Projecting into the water from the 

 under side of the thallus is a single little rootlet, so reduced that it 

 contains no vascular tissue. This simple little plant produces a 

 cluster of 3 flowers, 2 staminate and 1 pistillate, in a notch in one 



