It is well known that all life depends primarily on the conversion of carbon and
nitrogen into protoplasm. This can be accomplished only by the living plants which absorb
the nutrient elements from the surroundings and with the aid of pigments in them, particularly
chlorophyll, and with the energy from sunlight build up starch, proteins, fats, etc.
In the terrestrial environment this is quite obvious-accomplished by plants such as grass,
the crops, fruit trees and so on; in the aquatic environment, small simple plants with not
much differentiation of parts, known as algae play this role
