CHAPTER XXV. 

 ALGJE. 



360. The algae are plants of a low grade of organization. 



They live in the water, or a few of them in moist situations. 

 Those growing in the sea are popularly called " sea weeds,"* while 

 those growing in fresh water are called "pond scums," " water 

 nets," etc. The simplest algae are single-celled plants. From 

 this simple condition single individuals or cells are easily asso- 

 ciated into colonies, or firmly united into filaments, or cell plates, 

 which reach massive size in the rockweeds and kelps. 



361. The plant body of the algae thus varies greatly in 

 size as it does in form. The plant body of the algae is not divided 

 into true stems, roots, and leaves. There are, it is true, algae 

 which possess root-like, stem-like, and leaf -like organs, but they 

 do not belong to the same part in the plant's life cycle that the 

 true roots, stems, and leaves of the ferns and seed plants do. For 

 this reason they are not regarded as true roots, stems, or leaves 

 from the standpoint of comparative morphology, although from 

 the standpoint of physiology or function such algae possess stems 

 and leaves. Such a plant body, which is not differentiated into 

 true roots, shoots, and leaves, is called a thallus. The plant body 

 of the algae as well as of the fungi is a thallus. They are charac- 

 teristically the thallus plants or Thallophytes. 



The algae possess chlorophyll^ and are thus able to live inde- 

 pendently of other plants. They obtain their mineral and nitrog- 



* Also called "sea mosses." It should be understood that the algae are not 

 true mosses. They are all simpler in structure and lower in the scale of 

 classification than the mosses. 



j" There are a few parasitic algre which lack chlorophyll, and their method 

 of nutrition is similar to that of the fungi, although some of them store starch 

 which they obtain from their green host. Example, Rhodochytrium, parasitic 

 on the ragweed in North Carolina, and probably other Southern and Atlantic 

 States, and on one of the milkweeds in Kansas. 



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