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 



