CHAPTER EIGHTEEN 



THE ALG^ 



The plants that we have discussed in the preceding chapters 

 are all seed plants, — plants with well-developed roots, stems, 

 leaves, flowers, and seeds. In these plants the physiological 

 processes — photosynthesis, digestion, absorption, conduction, 

 accumulation, and reproduction — are carried on in tissues 

 and organs that are specialized to varying degrees, and the 

 plant body is a complex structure in which all the organs are 

 mutually dependent. The seed plants make up by far the 

 largest and most conspicuous part of the earth's vegetation, and 

 it is with them that the word " plant " is ordinarily associated. 



There are thousands of other plants, however, that are 

 far simpler in structure. They usually pass unnoticed among 

 the larger plants that crowd the landscape ; but this does, 

 not imply that they are unimportant in the world of plants 

 and animals. Indeed, quite the reverse is true. They plaiy 

 a very definite role in nature : they modify the earth's surface, 

 supply food, and produce other effects which are of great con- 

 sequence to the seed plants, to animals, and to man. In this 

 and the ^succeeding x;hapters we shall make a brief study of 

 these simple plants in order that our conception of the plant 

 kingdom as a whole may be more complete, and also that we 

 may gain some appreciation of the relations of man, of the 

 animals, and of the complex plants to the lower forms of 

 plant life. The first group that we shall study are the algcB, 

 small green plants that are very common in ponds, brooks, 

 and pools, but which are found in other habitats also. The 

 algae occur in all parts of the earth. 



The simple structure of the algae. The algse are simple 

 plants that consist of single cells, groups of cells, or masses of 



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