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HOW PLANTS FEED
Duckweed is a simple plant which grows in ponds. Its leaves float on the surface of the water and its roots hang down in the water. The minerals that the duckweed needs for growth are absorbed from the water that surrounds its roots.
The following experiment was carried out on duckweed growing in two different mineral waters (Contrix and Hevion), distilled water and Knop's solution. (Knop's solution is a solution of minerals especially prepared for growing plants. It has been used since 1865.)
The numbers of leaves which the duckweed plants grew were counted regularly over four weeks. The results are shown as a graph:
On each bottle of mineral water is a label which gives the concentrations of the different minerals in cubic decimetres of water. The table below shows these concentrations and also the minerals found in Knop's solution and distilled water.
Note: Nitrates are mineral salts which contain nitrogen, sulphates contain sulphur and phosphates contain phosphorus.
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