46 THE BOOK OF NATURE STUDY 



with them. Under these circumstances the name of the Plant 

 association is based on the type of leaf, and it is found best to 

 speak of the Upright-leaf, the Floating-leaf, and the Submerged- 

 leaf associations, and under these headings the different plants are 

 arranged as dominant and subdominant species. 



The marshy ground within the banks of the river has also its 

 characteristic vegetation. The plants in this belt, though hardly 

 aquatic, are distinctly hydrophytic ; that is to say, they live in 

 soil which has a very high percentage of water, without being 

 constantly bathed by water as the banks of a river are. The 

 Butter-Bur, the Wild Angelica, the Yellow Meadow Rue, Water- 

 cresses, the Water Starwort, certain Willow herbs and certain 

 species of Orchis are among the most common in this situation. 



An account of the vegetation by river sides is hardly complete 

 without some mention, however slight, of the trees that most 

 frequently border their banks. Pollard Willows (Salix alba and 

 5. fragilis generally) are very common ; the trees are kept cut 

 down to a particular height to thicken the growth at the top, 

 from the stool-shoots thus produced osier-rods are obtained, 

 although in some parts of the country another species of Willow, 

 Salix viminalis, is specially cultivated for this purpose. The 

 ground vegetation of an osier plantation is a combination of 

 marsh and meadow plants. 



Another tree, belonging to the same order as the Willow, and 

 commonly planted near streams, is the White Poplar, which 

 reaches a height of sixty or even a hundred feet. Its branches 

 spread horizontally, and the leaves are on long, slender stalks. 

 The catkins appear in March and April ; the staminate ones are 

 about four inches in length, each flower consisting of six stamens 

 with purple anthers. The pistillate catkins are much shorter, and 

 the bracts are less hairy than in the staminate flowers. 



The Alder is even more generally associated in one's mind 

 with streams than the Poplar. It likes not only the moist loam 

 which is usually to be had near a river, but it enjoys the damp 

 mist which rises from it. The flowers of the Alder and Willow 

 have already been described (Vol. IV.) 



Rivers, it must be remembered, affect vegetation not only 

 through the influence they exert on the degree of moisture supplied 



