The Algae 245 



rapidly by means of little hairlike appendages, seeming more 

 hke animals than plants. 



The pond scums include many blue-green algai, which are 

 much simpler in structure than the green plants that we have 

 studied. These contain, in addition to chlorophyll, a blue- 

 green pigment which gives them the color from which the 

 group takes its name. The 'cells are generally smaller than 

 those in the green algai, and distinct nucleii and chloroplasts 

 are absent. These small blue-green forms are common in all 

 kinds of water habitats and also in the upper layers of the 

 soil. Sometimes during wet weather they become sufficiently 

 abundant in fields to give a blue-green color to the soil. 



The importance of the pond scums. Both green and blue- 

 green algae are generally considered a nuisance in ponds and 

 streams, and they are commonly thought to have no economic 

 importance ; but the fact is that these despised pond scums 

 are the primary food supply of all the water animals. They 

 bear the same relation to aquatic animal life that the her- 

 baceous plants bear to animal life on the land. Nearly all the 

 water animals, from minute insects and crustaceans to the 

 largest fishes, ultimately depend upon them for their supply 

 of food. For, like the land plants, these small water plants 

 manufacture food, and the animals that live in the water 

 must feed either on them or on other animals that get their 

 living from the plants. Without the pond scums the lish 

 would soon disappear from our waters, because their food 

 supply would be cut off. A decrease in the number of fish in 

 a lake frequently follows the drainmg of its swampy margins, 

 for the alga3 thrive best in shallow water, and it is from the 

 alga^ that the small animals on which the fish feed secure 

 their food. The time is not far distant when lish will be 



