376 THE STORY OF PLANT LIFE 



testa. No endosperm is formed. The embryo may 

 be straight or curved. 



Some flowers, as in Triglochin, are wind-polHnated. 

 Zostera and Naias are unique in their methods, 

 pollination being effected under water. The pollen- 

 grains are long threads of the same specific gravity 

 as water. In Ruppia pollination is largely as in 

 Vallisneria. The flowers in Potamogetonaceae are 

 wind-pollinated. In Zannichellia pollination is under 

 water as in Zostera. The pollen is, however, not 

 confervoid, but spherical. 



The fruit is dispersed by the agency of water. 



The leaves of Zostera are used commercially, other- 

 wise the group is unimportant from the economic 

 standpoint. 



Arrow Grass (Triglochin palustre). 



In the English name and first scientific name, 

 from the Greek, reference is made to the three-pointed 

 carpels, in the fresh-water marsh type. The second 

 Latin name denotes the habitat. 



This plant is of universal occurrence in the British 

 Isles in suitable habitats up to the Shetlands. It is 

 found also at an altitude of 2000 ft. in the Highlands. 

 It is natifie in Ireland and the Channel Islands. 



Marshes, wet meadows, or the shallow margins of 

 streams, are the habitat of the Arrow Grass. It is 

 found on clay and loam in neutral grass-land, in sub- 



