THE DISPERSION OF SEEDS AND SPORES. 65 



itself possesses the power of movement, marine 

 plants must, as a rule, depend for the distribution 

 of their reproductive cells upon water-currents, as 

 we see in the tetraspores of Florideae, and in the 

 fertilised oospheres of Fucus and other olive sea- 

 weeds. 



Water-fertilised flowers, like Zoatera, Ceratojihyllum, 

 Ruppia, etc., in their arrangements for crossing, very 

 much resemble the wind-fertilised class. The same 

 unspecialised character belongs to the flowers of 

 Callitriche, Hippuris, and Myriophyllum, which may 

 be either anemophilous or hydrophilous. Vallisneria 

 is especially interesting, for although for the most 

 part immersed, its fertilisation takes place in the 

 air. The plant is submerged all but the female 

 flowers, "which are attached to long spiral stalks. 

 The small male flowers grow near the bottom, but 

 become detached, rise, and swim on the surface 

 of the water. They may now be blown in the 

 direction of the pistillate flowers ; but whether the 

 pollen is floated to the stigmas, carried by 

 ■wind, or transferred by insects, is not very clearly 

 made out. Herman Miiller considers this a transi- 

 tion from the water-fertilised to the insect-fertil- 

 ised class of flowers. 



Fruits and seeds specialised for this mode of 

 distribution show adaptation — flrst, in having their 

 specific gravity reduced by the presence of air 

 cavities in their tissues ; and secondly, in their 

 being impermeable to water to prevent injury in 

 course of transit. The fruit of the water lily 

 {Nymphoza alba) is matured under water ; when 

 ripe it breaks away from the peduncle, and, 

 according to Hildebrand, its walls separate, setting 

 free a globular mucilaginous mass in which the 

 seeds are imbedded. This spongy, gelatinous mass 

 encloses a number of air-bells, and floats for a 

 time. Ultimately, however, it disintegrates, the air 

 bubbles escape, and the seeds separate. Each seed 

 now continues its voyage alone, kept afloat by" the 



