[Chap. XLVII THE ALGAE 607 



that develop directly from vegetative cells. In one large group of algae 

 no sexual reproduction occurs. 



Algae are economically important both directly and indirectly. Com- 

 mercial transactions involving the collection, processing, and sale of 

 algae amount annually to several million dollars. The animals of the 

 oceans, lakes, and streams are for the most part dependent upon the 

 algae. 



What are algae? First of all, algae are plants. Some consist of onlv a 

 single cell, a single filament, or branched filaments. Even the more 

 complex ones have no organs exactly comparable to those of seed 

 plants. They contain chlorophyll, although other pigments may parth' 

 or entirely mask the green color. The presence of these pigments usualh 

 enables one to distinguish algae from fungi, bacteria, and small animals. 

 Many of them are easily distinguished from other green plants by the 

 number, size, and form of their chloroplasts. Many species are capable 

 of self-locomotion by twisting, bending, gliding, and swimming; and 

 others produce motile spores and gametes. 



The occurrence of motile algal plants and cells may require us to 

 change one of our notions about plants if we have been accustomed to 

 associate independent movement only with animals. It should be re- 

 membered that the names "plants" and "animals" are very old and 

 were used long before microscopes were made. It is easy to separate 

 plants from animals when we are thinking of oaks and horses or corn 

 and mice. Now that the microscope has enabled us to see minute living 

 organisms not even suspected before, it is not surprising to find that 

 the criteria we are accustomed to use in separating plants and animals 

 will not apply to many of these smaller organisms. 



Among the many organisms having flagella some are more plant-like 

 than others. When exposed to light, some of them are green and syn- 

 thesize both sugar and amino acids; others remain colorless and obtain 

 food only from external sources. Even within a single genus, such as 

 Euglena, some species are more plant-like than others. For example, 

 many species of Euglena are green and synthesize sugar when exposed 

 to light. Some of these green species can live in darkness as colorless 

 saprophytes if supplied with sugar and protein foods, but others have 

 failed to grow in continuous darkness. They differ also in their ability 

 to utilize nitrogen compounds. Some may utilize ammonia or nitrate 

 salts, which means that they can synthesize amino acids from sugar 

 and either ammonia or nitrate salts. Still others cannot utilize either of 



