270 



SEEDLESS PLANTS 



metry; that is, both sides of a cell are just alike. They 

 are found only in fresh water ; diatoms inhabit either salt 

 water or fresh. 



^ , ." - / 



514 S15 



514-517. — Desmids (highly magnified) : ^x/^, Micrasterlas papilUfera ; ^\^,Micra- 

 sterias Ttiorsa ; 516, Cosmarium polygonum ; ^vj , Xanthidium aculeatum, 



383. Place in Nature. — Algae exist in vast multitudes 

 both as to the number of species and of individuals. They 

 all contain chlorophyll, but in a few fresh-water forms 

 and in most seaweeds it is obscured by pigments of brown 

 or red to which the brilliant coloration of these plants is 

 due. The presence of these pigments probably has some 

 relation to their peculiar environment, especially in the 

 case of those growing in deep water, where the action of 

 light upon the chlorophyll is greatly diminished and altered 

 by refraction. Their variations in color form a convenient 

 basis of classification, and botanists divide algae into six 

 great orders, according to their color. The spirogyra and 

 most fresh-water species belong to the order of Chloro- 

 phycece, or Green Algae. This class is of special interest 

 because from it all the higher forms of vegetable life are 

 beheved to have been derived. 



PRACTICAL QUESTIONS 



1. Are any of the green algae parasitic ? How do you know ? (25.) 



2. Wliat is their effect upon the atmosphere ; that is, do they tend 

 to purify it by giving off oxygen, or the reverse ? (24, 25.) 



3. Why is their presence in water regarded as denoting unhygienic 

 conditions ? 



4. Refer to the experiment in Section 22, and account for tlie bub- 

 bles and froth that usually accompany these plants in the water. 



5. Can you suggest any other causes than tlie elimination of oxygen 

 that might produce the same effect ? 



6. Is the presence of these gas bubbles of any use to the plants? 



