38 AQUATIC PLANTS. POLLEN. [chap. 



and the stigmas unfurl themselves. More or less 

 similar movements have been observed in various 

 other flowers. Thus the anthers of the Foxglove 

 {Digitalis purpurea) (Figs. 100-102), which are at 

 first transverse, become longitudinal as they ripen. 



In aquatic plants, the blossoms habitually come to 

 the surface. In Valisneria spiralis the female flower 

 has a long spiral stalk which enables it to rise to 

 the top of the water. The male flowers which are 

 small, very numerous, and attached lower down, 

 separate themselves altogether from the plant, rise 

 to the surface, and fertilise the female flowers, among 

 which they float. When this is effected, the spiral 

 stalk of the female flower again contracts, and draws 

 it down below the surface. 



While the pollen grains from each flower agree very 

 closely with one another, those of different species 

 differ greatly in form, size, character of surface, &c. 

 Doubtless there are reasons for these differences, but 

 the subject is one with reference to which we have 

 as yet very little information. 



According to Sprengel, the pollen of wind-fertilised 

 flowers is drier, and therefore more easily carried by 

 the wind, than that of most insect-fertilised flowers. 

 I say of most, because in some cases, for instance 

 in the violet, as will be shown presently, it is as 

 necessary that the pollen should separate readily 

 from the anthers, as in wind-fertilised flowers. 



Mr. Bennett states that the pollen of wind-fertilised 

 flowers is generally spherical ; while that of insect- 

 fertilised flowers is usually furrowed, the furrows 

 running along the longer axis of the grain. 



