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TEXTBOOK OF BOTANY 



red and browii algae are used directly as food by human beings in many 

 parts of the world, particularly in the Orient. Among algae, as among 

 other plants, some species are much more important than others as 

 sources of food, whether the consumer be man, fish, insect, or whale. 



Fig. 275. Diagram of representative food chains from algae to man. 



The giant kelps and other seaweeds are sources of iodine and potash. 

 They have been used for many years by farmers along the coast as 

 fertilizers because of their potassium content. Dried and treated in 

 various ways, they are used as stabilizers in making ice cream, candy, 

 shaving cream, various other creams, jellies, salads, and emulsions; and 

 also as the source of agar-agar, so important in the culture of bacteria 

 and fungi. 



The algae of the soil are very numerous, but the extent to which they 

 contribute to soil fertility and aeration is only partly known. Soils have 

 definite algal floras. Those living on the surface grow and multiply 

 rapidly whenever the soil is moist. The algae below the surface grow 

 very slowly; and since they live in darkness tliey are obviously living as 

 saprophytes, using carbohydrates from the soil solution. Algae cer- 

 tainly contribute organic matter to soils, and it has been proved that cer- 

 tain nitrogen-fixing bacteria are much more eftective when they grow 

 in association with soil algae. The fungi and algae of lichens are the 



