THE ALGAL 2338 
some are a deep purple. Chlorophyll is present, as in all other 
alge, but is often completely obscured by the other colors. 
Sometimes all the colors are obscured by coatings of calcareous 
material. 
The red algze have basal holdfasts. The plants are exten- 
sively branched (fig. 181) and, as a rule, are smaller and 
more delicate than the brown forms.! The entire plant often 
looks like a sparsely branched stem with many finely divided 
leaves. The gelatinous material obtained from certain of the 
red algze 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 alge on the whole considered to be of 
lower organization than the green alge ? 
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 alge 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 alge (diatoms) are found in the digestive cavities of oysters ? 
5. What alge are used as human food? 
6. May polishing powders or pastes be made of fossil remains of 
alge? Use a compound microscope in examining some such powders. 
7. It is generally supposed that algae were among the first plants 
to appear on the earth in very early geological times. Does this seem 
probable ? 
8. How do the alge pass through the winter and seasons of drought? 
1 The best way for the teacher to give a general notion of brown and red 
alge 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. 
