The Struggle for Life in the Swamp. 247 
shallow waters and which necessarily have a limit to their 
power of elongating, we come to another large class that 
float on the surface. These properly belong only to still 
waters, but are equally at home rooting in the mud. In 
Guiana the beautiful lilac Eichorneas, Limnobium, Pistia, 
and that unique fern Ceratopteris thalidlroides, are the 
most common and best examples. By means of hollow 
swollen leaf stalks or a spongy thickening of the under- 
side of the leaves, they are enabled to float like green 
rosettes on the surface, rising and falling, carried here 
and there, but always at home whatever may be the 
height of the flood. Except the fern they increase by 
runners, which spread in every direction and soon cover 
the whole surface if undisturbed. As long as there is 
plenty of room each rosette lies down on the water, but 
when they begin to crowd upon each other the inner 
leaves become less spongy and thinner, and rise on 
longer stalks until almost upright above the water. The 
same result follows a drought, when the plants root in 
the mud and become very luxuriant. The Ceratopteris 
differs so much from all others of its family that it is 
hardly recognizable as a fern. In its early stage it 
resembles a small lettuce with incised and almost curled 
leaves, little divided; but in a choked canal or on the 
mud the leaves grow to a length of two feet and are cut 
into narrow segments. The spores germinate on the 
leaf, and when a flood comes and breaks the plant into a 
thousand fragments almost every one of these is ready 
to reproduce others wherever the conditions are favour- 
able. 
Several small species of floating plants are much 
weaker than those just mentioned, but they manage to 
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