32 



PLANT STKDCTURES 



by walls, or they are coenocytic, as in the Siphon forms. 

 The characteristic asexual spores are zoospores, but these 

 may be wanting, as in the Conjugate forms. In addition 

 to asexual reproduction, both isogamy and heterogamy are 

 developed, and both zygotes and oospores are resting spores. 



Fig. 17. A group of Desmids, one-celled Conjugate forms, showing various pat- 

 terns, and the cells organized into distinct halves.— After Kebneb. 



The Green Algae are of special interest in connection 

 with the evolution of higher plants, which are supposed by 

 some to have been derived from them. 



3. PHiEOPHYCEiE (Brown Algce) 



28. General characters. — The Blue-green Algae and the 

 Green Algae are characteristic of fresh water, but the Brown 

 Algae, or "kelps," are almost all marine, being very charac- 



