SEAWEEDS AND THBIE ALLIES. 107 



who are likely to use this book their study has not been 

 introduced. The various text-books and manuals give the 

 necessary help for beginning their study. 



REVIEW AND SUMMARY, 



The algEe include seaweeds and numerous fresh-water 

 species, together with many other flowerless plants of sim- 

 ple structure that grow on trees, rocks, and Theeronp 

 other exposed places. They contain chlorophyll, defined, 

 by means of which they elaborate their own food, thus 

 standing in sharp contrast with the fungi, which are 

 dependent on other organisms for their sustenance. 



Many algse, like Spirogyra and Vaucheria, are of a sim- 

 ple green color, while in others the chlorophyll which they 

 contain is disguised by the presence of differ- 



. mi • IT., ■ Classification 



ent coloring matters. iheir classiiication is and distrihu- 

 based partly on this fact and partly on peculiari- *'™' 

 ties of reproduction. The green algas include the least 

 complex forms structurally, and are characterized by such 

 simple processes of reproduction as we have observed in 

 the species selected for laboratory study. They belong to 

 fresh water and also occur to a limited extent, but only to 

 a slight depth, in marine waters. The brown algse belong 

 to the sea, occurring chiefly at lower depths than the 

 green ones, are structurally more highly differentiated, and 

 exhibit a variety in their modes of reproduction that on the 

 whole indicates a more advanced stage of development. 

 The red algse are still more advanced, as is plainly shown 

 by their complicated and peculiar reproductive processes. 

 They belong almost exclusively to salt water, and occur as 

 a rule at lower depths than the brown algse. From their 

 wide distribution, variety, and beauty they are universally 



