AQUATIC VEGETATION 235 
The only group of flowering plants which has become 
adapted to life under the water of cascades are the 
Podostemacee 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. 
pondweed), which has floating leaves 2 to 4 inches long 
by 1 to 14 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, 
