AQUATIC VEGETATION 235 



The only group of flowering plants which has become 

 adapted to life under the water of cascades are the 

 Podostemaceae of Ceylon and Central Africa. In most 

 of these the stems and leaves are absent, and the plant 

 consists of a small, flat, green root which is fixed to the 

 rock by special holdfasts called haptera. The minute 

 flowers spring from the root, and in this stage the plant 

 appears more like a fruiting moss than a flowering plant. 

 This is a remarkable adaptation to environment. 



2. Slowly-Flowing Water. 



On the bed of rivers and streams there is usually an 

 abundance of fine material in which plants can root, and 

 the vegetation consequently reaches a much greater 

 degree of development than in swiftly-flowing water. Free- 

 floating plants naturally cannot exist, except under the 

 shelter of reeds near the banks, owing to the current. 

 In the centre of the stream, where the current is strongest, 

 if the water is not more than 3 or 4 feet deep, plants 

 with long ribbon-like leaves are found (pp. 50, 51). 

 These leaves offer but a slight resistance to the current, 

 and the plants are in little danger of being uprooted. 

 Most of the plants in this situation grow usually in 

 standing water, but, owing to the different conditions, 

 the form of the plant in stagnant water is not the same 

 as in flowing water. Sagittaria sagittifolia (arrow-head), 

 for example, grows commonly in ponds, and possesses 

 large arrow-shaped leaves which stand up erect in the 

 air, but in flowing water all the leaves are long and 

 narrow, sometimes reaching a length of over 2 feet, yet 

 only | inch wide. Similarly, Potamogeton natans (broad 

 pond weed), which has floating leaves 2 to 4 inches long 

 by 1 to 1^ inches broad when growing in standing 

 water, produces narrow current-leaves over 18 inches 

 long. Other plants associated with the foregoing are : 

 Myriophyllum spicatum (water-milfoil), forms of the 

 water-crowfoot e.g., Ranunculus fluitans, which has 

 submerged leaves only, the segments of which are long, 

 narrow, and parallel and Potamogeton pectinatus (fennel- 

 leaved pondweed), a submerged plant with very thin 

 stems and narrow grass-like leaves. Nearer the river- 

 margin, in shallower and more slowly-moving water, 



