220 GENERAL BOTANY 



The marine algae inhabit the salt water of the ocean and the 

 brackish Avater of salt marshes and ponds. The lowest and sim- 

 plest forms resemble closely the green fresh- water algae, to which 

 they are closely related. Among the higher forms, however, the 

 plant body attains to much greater complexity than any known 

 fresh- water species, and the colors are often very striking, ranging 

 from bright reds to dull browns and greens. Some larger brown 

 algae may attain to a length of a hundred feet or more, and 

 are attached at the base by large and strong holdfasts resem- 

 bling roots. These large forms are usually long, flattened, strap- 

 shaped bodies which float out upon the surface of the water by 

 means of special air cavities, or spaces within their tissues, called 

 floats. The form thus assumed and the floating habit are both 

 adapted to the needs of these organisms in their manufacture 

 of foods by photosynthesis. The red and brown colors are due 

 to pigments secreted in the chloroplasts, which partially or 

 wholly mask the green chlorophyll pigment ; but the significance 

 of these additional pigments is not yet fully understood. 



PROTOCOCCUS 



Protococcus occurs in the form of single cells or of loose 

 colonies growing on bark and moist surfaces of all kinds, such 

 as boards, stones, brick walls, etc. Its genetic relationship to 

 the other algae is not certainly known, since it is a xerophytic 

 type whose form and life history has become greatly modified 

 during its gradual adaptation to its present habitat. It is used 

 here for our initial study of the simplest fresh-water algae on 

 account of its wide distribution and availability for class use. 



Structure and mode of life. Each cell of Protococcus (Fig. 115) 

 has a cellulose cell wall which contains a many-lobed chloroplast 

 and a nucleus embedded in the cell cytoplasm. The plants grow 

 in masses, forming green incrustations on the surfaces which 

 they inhabit. They are essentially air plants, absorbing carbon 

 dioxide and oxygen from the air and taking up soil salts from 

 the dust particles dissolved in the rain or dew which moistens 

 their surface. In this way they are able to subsist without the 



