VEGETATION OF SWIFTLY FLOWING WATER 43 



water. Such plants have to accommodate themselves to a rush 

 of water, and to changes in the level of the water which may 

 be considerable during flood. The organ by which the plant is 

 able to meet these conditions is the leaf. Plants floating in more 

 or less still water have, as already noticed, more or less round 

 leaves with air 

 spaces. This type of 

 leaf would be use- 

 less here ; it would 

 be swamped during 

 floods. The long, 

 narrow, upright 

 leaf is the kind 

 required. Leaves 

 of this type bend 

 with the water, 

 and, being a foot 

 or more above the 

 surface, are un- 

 affected by the 

 changes of level in 

 the water. The 

 Sweet Flag, Bur- 

 Reed, Bulrush, 

 Reeds all have 

 long, narrow, up- 

 right leaves, and 

 are all monocoty- 

 ledons with the 

 veins of the leaf 

 parallel. The Sweet 

 Flag must not be 



r i .,1 ,i FIG. 1 6. Branched Bur-Reed (Sparganium ramosum). 



confused with the 



Yellow Flag or Yellow Iris. It belongs to the same order 

 as the Common Arum, which grows under shady hedges. The 

 Sweet Flag has narrower and brighter green leaves than those 

 of the Yellow Flag, with an aromatic odour when bruised, 

 and their margin is slightly crimped. The flower-stems are 



