The most important vegetable planktonic organisms are the Diatoms. Their accumulated shells form important deep-sea deposits. The numerous varieties of planktonic life can thus be divided into two groups: those minute animal and vegetable organisms that pass the whole of their existence at the surface of the sea—the true constituents of plankton all the year round—and the eggs and larvae of many species of fish that are found among the plankton only at certain times of the year—notably in spring and summer. The quantity of organic food substances such as albumen, fat and carbohydrate, that is contained in the plankton produced annually by a given area of the sea, has been compared with the quantity of such substances produced by a similar area of land in crops such as pasture, hay, lupine and peas. In this way, it has been estimated that the productivity of the sea is about 20 per cent less than that of average land. [Illustration: PLANKTON: LARVAE 1. Crab zoea; 2. Fish egg; 3. Sea Urchin pluteus; 4. Barnacle nauplius; 5. Fish larva; 6. Mussel larva; 7. Copepod nauplius; 8. Worm larva.] [Illustration: PLANKTON: UNICELLULAR ORGANISMS 1, 2, 3, 7, 10, 11, 12, 13, 16, 17, 18. Diatoms; 4, 5, 7, 9, Peridinians 8. An Algal spore; 14. Noctiluca; 15. A Radiolarian. FIG. 6] Unlike that of the land, the productivity of the sea is greater in colder latitudes than in the tropics. This somewhat unexpected fact is attributable to the action of denitrifying bacteria which, flourishing more readily in warm, tropical waters, effectively reduce the amount of available nitrogen compounds in the water. In colder waters, denitrifying bacteria are less active, and nitrates and nitrites are available in larger quantities for the nourishment of the plankton. All the great fisheries of the world are prosecuted in cold or temperate seas; as examples of this we have the Banks of Newfoundland, the cod fisheries of Norway, and the great trawling grounds of the North Sea and the North Atlantic.