Phylum Phaeophyta [ 85 



producing the haploid stage, or as gametes, initiating the diploid stage. The diploid 

 individuals produce both plurilocular and unilocular reproductive structures. The 

 swarmers from the former are spores, reproducing the diploid body. The swarmers 

 from the latter act either as spores, giving rise to haploid individuals, or as gametes, 

 reproducing the diploid body. 



It is believed that the brown algae arose by evolution from order Ochromonadalea. 

 Filamentous organisms with a facultatively complete homolgous life cycle, as just 

 described, are believed to be primitive among them : such organisms appear to be the 

 starting point of evolution in many features. The filaments have become differentiated 

 and woven into thalli, and thalli of tridimensionally placed cells have been produced. 

 The haploid and diploid stages have become differentiated. The plurilocular and 

 unilocular structures have undergone specialization. Even in the most primitive 

 brown algae, there is a physiological differentiation of gametes; this has evolved into 

 extreme morphological differentiation. Every one of these evolutionary changes ap- 

 pears to have occurred in more than one line of descent; research is constantly reveal- 

 ing intermediate examples and rather free parallel evolution. 



Conservative classification, such as that of Fritsch (1945), recognizes as orders a 

 comparatively primitive miscellany followed by a series of small derived groups 

 marked by distinctive specializations. Features of the life cycle, as applied to classi- 

 fication by Taylor (1922), Oltmanns (1922), Svedelius (1929) and Kylin (1933), 

 are not reliable as marks of natural groups. Kylin provided three classes (one of 

 them divided into two subclasses) and twelve orders. His system appears to provide 

 an excessive number of subdivisions of high category within a moderately small group 

 exhibiting no very profound evolutionary gaps. Tentatively, the seven orders dis- 

 tinguished as follows may be recognized. 



1. Producing spores, that is, cells which germi- 

 nate without syngamy. 



2. All spores bearing flagella. 



3. Having an alternation of haploid 

 and diploid stages which are alike, 

 both being filamentous; or else com- 

 pletely lacking one of these stages. 



4. The filaments uniseriate Order 1. Phaeozoosporea. 



4. The filaments becoming pluri- 



seriate Order 2. Sphacelarialea. 



3. Not as above. 



4. Haploid stage thallose, not dis- 

 tinctly less highly developed 



than the diploid stage Order 5. Cutlerialea. 



4. Haploid stage filamentous, dis- 

 tinctly less highly developed 

 than the diploid stage. 



5. Diploid stage filamentous; 

 or, if partially or com- 

 pletely thallose, the thal- 

 lose part with apical growth Order 4. SpoROCHNoroEA. 



5. Diploid stage thallose, its 



growth intercalary Order 6. Laminariea. 



2. Producing large non-motile spores Order 3. Dictyotea. 



