CHAPTER XIII 
THALLOPHYTES 
Algae (Thallophytes with a Food-making Pigment) 
General Characteristics. — The Algae are a familiar group of 
Thallophytes, for in nearly every lake, pond, and stream, and 
along the sea coast some forms of them can be found. They 
commonly appear in fresh water as a green scum or as floating 
mats of green threads on or near the surface of the water. They 
often occur in abundance in watering troughs, and sometimes 
become troublesome by clogging sewers and water mains. Along 
the sea coast occur the large brown and red forms known as 
Seaweeds. 
Algae are of some economic importance. The Seaweeds are 
much used as food in some countries, especially in Japan, and 
from some Seaweeds iodine and potassium are extracted. Along 
the Pacific Coast of the United States, Seaweeds are an impor- 
tant source of potassium for fertilizers. However, the interest 
in the study of Algae is not due so much to their economic 
importance as it is to the fact that a knowledge of them is 
essential to an understanding of the evolution of the higher 
plant forms. 
Although Algae are water plants, not all Algae live in the water, 
for there are some forms which live on moist soil or rocks where 
water is easily obtained, and a few exceptional forms, such as 
those that live on the bark of trees, have very dry surroundings 
much of the time. Algae differ from other groups of Thallophytes 
in having food-making pigments by which they make their car- 
bohydrates. Consequently, they are not saprophytes or para- 
sites, that is, plants which have to depend directly upon other 
plants for food, but are equipped to live independently. Among 
them there is a wide range of variation in plant body and 
methods of reproduction, and four groups of Algae are commonly 
recognized — Blue-green, Green, Brown, and Red Algae. 
296 
