44 PLANT ASSOCIATIONS 
increase in depth, we reach an aquatic desert region, 
where the reduction of light renders plant growth 
difficult, and eventually inhibits it. Let us consider 
the conditions prevailing in the reed zone. Here the 
plants are essentially aerial, and though they have 
their feet in water, the stems and leaves rise far above 
it. Water-level is variable in lakes and rivers; the 
plants are usually tall, so that even in case of flood 
the leaves and flowers will not be drowned. Wave 
action on lake-shores is somewhat violent, and in 
flooded rivers a strong current may sweep through 
the vegetation; we see the advantage of the slender 
elastic stems and narrow leaves that characterize 
the plants: compare Reed (Phragmites), Reed-mace 
(Typha), Flag (ris), Bur-reed (Spargarium), Bulrush 
(Scirpus); and these characters also fit them for the 
windy nature of their habitat. The denuding effect of 
wave or current action is countered by the network of 
creeping stems and abundant roots which the plants 
possess, forming a tough felt which floats, and by its 
growth and decay helps materially to form fresh land. 
Another effect of the creeping and branching stem- 
systems is the production of extensive and dense 
groves of many of the species. 
When we pass beyond the reed zone, a completely 
different type of vegetation prevails. Here the plants 
are essentially aquatic. They make no effort to raise 
their stems and leaves above the water surface; but 
almost all of them raise their flowers into the air, 
though the seed is often ripened below the surface by 
a downward curving of the stem. These plants, sur- 
rounded by water, use their roots chiefly as anchors, 
and absorb through their stems and leaves the water 
