236 Algce and their Study, [Sess. 



I must state what kind of plants — and even animals, as some 

 would call these — are usually included among the Algae. 

 When these lowly organisms first attracted the attention of 

 naturalists, which occurred after the time of Linnaeus, they 

 were divided by their outward appearance and colour only, as 

 their method of reproduction was then unknown. Thus we 

 find the Eed Algae or Khodophyceae, the Brown Algae or 

 Phaeophyceae, — these two classes being, with only few excep- 

 tions, inhabitants of the sea, and mostly of large size. Other 

 marine types, such, for instance, as the Sea Lettuce or Ulva 

 and the genus Enteromorpha, both well known, were grouped 

 under the Green Algae or Chlorophyceae, and this class com- 

 prises also the greater part of the fresh-water types. The 

 Diatoms were for a long time looked upon as animals ; later, 

 on account of their brown cell contents, they were ranged 

 under the Brown Algae, but nowadays it is usual to place 

 them under a separate class and to regard them as true plants. 

 Finally, we have the Blue-green Algae or Cyanophyceae, illus- 

 trating the lowest type, and found in great abundance in the 

 sea as well as in fresh water. Owing to the primitive struc- 

 ture of their cells, they have frequently been placed together 

 with the Bacteria, and separated entirely from the Algae. 



Nature has now assisted us to a great extent in the difficult 

 work of classifying the Algae, in so far as a difference in colour 

 indicates also a different method of reproduction, and there- 

 fore the old divisions based on coloration only have held good 

 even after modern research had made out their life-histories. 

 Thus we find the highest type of reproduction in the Bed 

 Algae, another mode in the Brown Algae, and so forth. All 

 true Algae, leaving aside the doubtful members, such as most 

 of the riagellatae, contain chlorophyll. However, in the Eed 

 Algae this chlorophyll is masked by a red pigment called 

 Phycoerythrin, in the Brown Algae by a brown pigment with 

 another high-sounding name, and the same pigment we find 

 also in the Diatom cell. The true Green Algae contain chloro- 

 phyll only, and show, therefore, the well-known chlorophyll- 

 green. This fact is easily demonstrated in the larger marine 

 Eed and Brown seaweeds. As all the pigments with the 

 exception of chlorophyll are soluble in fresh water, the dead 

 cells of these seaweeds lose their brown or red colour and 



