I970 



A TEXTBOOK OF THEORETICAL BOTANY 



inflorescence of Zostera consists of a linear series of alternating carpels and 

 stamens, the whole being enclosed by the sheathing margins of a leaf. 



Special organs of vegetative propagation occur in some species and 

 consist of tuber-bearing runners. Anatomically the plants have the 

 typical structure of aquatics with a well-developed aerenchyma. It has 

 been found that the quantity of sclerenchyma produced varies with the 

 habitat, for plants growing in running water have more mechanical tissue 

 than those found in stagnant water. In marine species the mechanical 

 tissue is well developed and air spaces may be absent. 



Pollination in Potamogeton (Fig. 1897) is anemophilous and the grains 



Fig. 1897. — Potamogeton crispus. Flowering shoot showing anemophilous pollination. 



{After Kernel- and Oliver.) 



are round, but in other genera, such as Zostera (Fig. 1898), the pollen is 

 filamentous and develops into alga-like threads of the same specific gravity 

 as the water. These filaments become entangled in the branching stigmas 

 (Fig. 1899) and pollination is effected. This type of hydrophilous polli- 

 nation is rare and has been only imperfectly studied. 



The family includes a number of genera of which Potamogeton is the 

 largest. The species are difficult to separate but over twenty have been 

 recognized in Britain. Among the other genera, Ruppia and ZannicheUia, 

 (Fig. 1900) which are monotypic, and two of the six species of Zostera 

 are found in British waters. Other genera occur all over temperate and 

 subtropical regions of both hemispheres. 



The Aponogetonaceae include the single genus Aponogeton with 

 twenty-five species occurring in tropical regions of the Old World and in 

 South Africa (Fig. 1901). They are water plants with sympodial rhizomes 



