90 SECRETS OF ANIMAL LIFE 
distinctive population of their own. From the 
shore to a depth of two or three fathoms are the 
meadows of the sea-grass, with true roots and 
flowers and very long ribbon-like leaves, familiar as 
a packing and stuffing material and as a covering for 
Italian flasks of wine or oil. Mixed with this Zostera 
are the seaweeds proper, attached but without 
true roots, enlivening the grass-green with beautiful 
reds, browns, and olives. Farther out the sea- 
weed vegetation thins, until it disappears at a 
depth of about twenty-five fathoms. It is a crowded 
vegetative area, able to support a crowded animal 
life; the waving sea-grass is often as thick as the 
stems in a cornfield, and Professor Petersen notes 
that the total annual yield in Danish waters is over 
8000 million kilograms of dry matter, about four 
times the quantity of hay produced in a year in 
Denmark. This suggests further utilization—the 
Zostera is already used for fodder, for paper-mak- 
ing, for making explosives, and in other ways—but 
sea-grass is a difficult harvest to reap, and perhaps 
its greatest value is the indirect one, that it forms a 
basal food supply for animals on which many food 
fishes mostly depend. For what Professor Petersen 
and his colleagues have discovered is that the surface 
of the mud (or clay farther from shore) is covered 
by a thin layer of detritus of very nutritive quality, 
and that this is mainly produced by fragments of 
sea-grass and littoral seaweeds, the downward sink- 
ing Plankton counting for little. Examination of 
the stomach contents of common non-predatory 
