Seaweeds and Leaf-green 
The great Sargasso Sea is supposed by some authors 
to occupy an area as large as Europe, and to be formed 
entirely of algal fragments torn by storms from the 
coast-line. But the common Sargasso weed has not 
yet been discovered anywhere on the shore-line of 
America, It is well able to float, for it has hundreds 
of little buoys about the size of a pea along its fronds, 
Such seaweeds are undoubtedly dangerous to navigation, 
for if twined round the screws or paddle-wheels they 
might stop the engines suddenly and cause an explosion ; 
but the lurid tales about its awful inhabitants and ancient 
derelicts have not been scientifically confirmed. 
Great as must be the amount of shore seaweeds, they 
are by no means the most important of the order. 
Algze occur also in fresh-water lakes and in all kinds of 
pools, rivers, streams, and rivulets. The bottom, if not 
more than 30 feet deep, and the sides of all such fresh- 
water surfaces, are usually densely covered with them. 
But the most important in the world of all alge are 
those which are not attached to the shore at all, but 
which float free in the water and are carried about by 
the currents and tides. They occur in almost all fresh- 
water lakes, and especially in the open sea. They are 
perhaps less numerous in the “deep blue sea” far 
from the coast-line, though even there they are by 
no means entirely absent. 
This free-floating life of the ocean is usually called 
plankton (the name includes the marine animalculze 
which occur along with the seaweeds). They are 
commonest at a short depth, though they have been 
found 1000 feet below the surface. In the Arctic and 
Antarctic Ocean depths of 120 to 250 feet seem to 
yield the largest amount.® 
One can obtain samples of this plankton by attaching 
a small bag of miller’s gauze to the taps of a sea-water 
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