CHAPTER XLVII 

 THE ALGAE 



Of all the plants in the biological communities of both fresh and salt 

 water the algae (sing, alga) are the most important synthesizers of 

 food. They are commonly called pond scums and seaweeds. Algae, how- 

 ever, are not confined to water habitats. In the rainy tropics and sub- 

 tropics they grow on moist surfaces everywhere, and even within the 

 leaves of many plants. Some of them may live in snow as shown by 

 the pink color of polar and high alpine snowfields ( Plate I ) . The trunks 

 of many woody plants of the moist temperate zone have green patches 

 of algae, usually located on the shaded side. The algae of the soil are 

 so numerous that one gram of earth from a heavily fertilized field may 

 contain as many as a million individuals. A few algae grow in intimate 

 association with certain fungi, forming compound plants known as 

 lichens. Algae grow within the aerial roots of cycads and in the hair 

 follicles of the three-toed sloth. They are attached to the appendages 

 of crustaceans and to the backs of turtles. They live within the bodies 

 of many minute animals, and even in the intestines of some mammals, 

 including man. 



The algae, moreover, exhibit as much variation in color, in size, in 

 form, and in method of reproduction as any of the other groups of 

 plants. Thev may be red, orange, brown, yellow, yellow-green, green, 

 blue-green, purple, and violet. Rarely are they colorless. Some algae are 

 so minute that their fomi is just discernible with a microscope. The 

 giant kelps of the Pacific, on the other hand, attain lengths of more 

 than 200 feet. In form algae mav be globular, disk-shaped, thread-like, 

 sheet-like, leaf-like, or large and paddle-shaped; branched or un- 

 branched; attached or free-floating. 



The structures associated with sexual reproduction in some species 

 are comparatively simple; in others, they are remarkably complex. 

 Among the various species of algae, vegetative propagation may result 

 from the division and separation of cells, from the breaking of fila- 

 ments, from specialized thick-walled donnant cells, or from motile spores 



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