THE ALG^E 233 



some are a deep purple. Chlorophyll is present, as in all other 

 algae, but is often completely obscured by the other colors. 

 Sometimes all the colors are obscured by coatings of calcareous 

 material. 



The red algee have basal holdfasts. The plants are exten- 

 sively branched (fig. 181) and, as a rule, are smaller and 

 more delicate than the brown forms. 1 The entire plant often 

 looks like a sparsely branched stem with many finely divided 

 leaves. The gelatinous material obtained from certain of the 

 red algfe is by some regarded as a delicacy. In the North Sea 

 and elsewhere in the Atlantic Ocean occurs a red alga known 

 as " Irish moss," which is collected in large quantities and 

 employed in the preparation of jelly, to be used both directly 

 as food and as a basis for the preparation of other foods. 



PROBLEMS 



1. Why are the blue-green algae on the whole considered to be of 

 lower organization than the green algae ? 



2. Which of these groups is more injurious in its effect on reservoirs 

 of drinking water? Why? 



3. Why is it often found desirable to build roofs over such reservoirs? 



4. In what ways do algae help or hinder the life of aquatic animals? 

 How does an aquarium aid in answering this question? What bearing 

 on it has the fact that the flinty cell coverings of some of the micro- 

 scopic algae (diatoms) are found in the digestive cavities of oysters ? 



5. What algae are used as human food? 



6. May polishing powders or pastes be made of fossil remains of 

 algae ? Use a compound microscope in examining some such powders. 



7. It is generally supposed that algae were among the first plants 

 to apjiear on the earth in very early geological times. Does this seem 

 probable ? 



8. How do the algae pass through the winter and seasons of drought ? 



1 The best way for the teacher to give a general notion of brown and red 

 algse is to secure card mounts or bottled material for class demonstrations 

 of a few of the leading types in each group. These may be obtained from 

 the Woods Hole Biological Laboratory, Woods Hole, Massachusetts, and 

 from other reliable supply houses. Well-prepared card mounts preserve 

 the natural colors, and may be kept indefinitely for laboratory use. 



