




THE FLOWER. | 85 
those Hydrophytes, however, which are pollinated under 
water, such as the Grass-wracks (Zostera), the pollen grain has 
no outer coat. As soon as the anthers dehisce, the pollen 
grains take the form of long cylindrical tubes, and are carried 
by the flowing water to the thread-like stigmas of the flower. 
These Grass-wracks are entirely submerged water-plants, 
abundant in the lagoons around Venice. Many Hydrophytes 
—in fact, the majority—discharge their pollen above the 
surface of the water. In this case the pollen grains, as in 
Mesophytes, have the usual external coat, and are round or 
elliptical, not cylindrical or tube-like. They are conveyed, 

Fras, 54-56.—PoLLEN GRAINS OF ANGIOSPERMS. 
not by flowing water, but by wind or insects. Under these 
circumstances the pollen is dust-like, the grains being separate 
from each other. In a few cases pollen grains occur in sticky 
masses in both Hydrophytes and Mesophytes, as in Vallisneria 
and the Orchidacee (Fig. 133). The pollen grains of water- 
plants must be of the same specific gravity as the water, 
otherwise they would float or sink. , 
In Mesophytes the pollen grains vary very much in size, in 
shape, and in the structure of the outer coat; thus, in the 
Boraginaceze the pollen grains are small, in Malvacee and in 
Geraniacez large, and in those flowers which remain open but 
