248 TlMEHRI. 
exist in the smallest ditches and pools where perhaps 
larger plants could not be carried. The best known are 
the duckweeds, Azolla and Salvinia. The first are not 
so common here as in England, being replaced to a 
great extent by the others, which are much larger in 
size. These are stri6lly floating plants, and are popu- 
larly looked upon as hardly anything more than scum on 
the water. They may exist for a time on very damp 
mud, but have made no arrangements like the others for 
the struggle annually carried on in the bottom of a 
canal. On examination they are found to be particu- 
larly interesting and exceedingly well-fitted for the con- 
ditions under which they live and thrive, and although 
apparently driven into the background by every flood 
appear again in their myriads when the water is still. 
Floating plants are more common in Guiana than those 
which spread under the water. This is due to the fa£t 
that the waters are not very transparent, even when 
still, and during floods decidedly opaque. In a clear 
English brook the bottom is often covered with vege- 
tation, while in some deep ponds, charas and other 
plants live and thrive under water without ever rising 
to the top. Such a thing is unknown here, as even the 
intense light of the tropics cannot penetrate far through 
the brown water. It has followed therefore that every- 
thing comes to the surface or grows on it, shutting out 
the light from the Cabombas and Utricularias whenever 
there is an opportunity. The influence of light can be 
easily understood from the entire absence of vegetation 
where a creek is shaded by a great tree, or by examining 
the water under a covering of floating plants or water- 
lilies. 
