190 ALG& 



&c. The whole of the marine vegetation of the globe, with the excep- 

 tion of a very small number of species of Flowering Plants, belongs 

 either to the Algae or to the chlorophyllous Protophytes. They vary in 

 size from the microscopic Desmidieae and Pediastreae to that of shrubs 

 or trees in the case especially of some genera of Laminariaceae and 

 Fucaceae ; and in these classes, as well as in the Florideae, we find a 

 rudimentary differentiation, not only of tissues, but of organs, which 

 leads the way to the much more complete development in the higher 

 classes of the vegetable kingdom. Fresh- water Algae are, with very few 

 exceptions (species of Bangia, Hildenbrandtia, Lithoderma, Hydrurus, 

 &c.), green. Among marine Algae there are many genera of green sea- 

 weeds, belonging chiefly to the families Confervaceae, Siphonocladaceae, 

 and Ulvaceae ; but these mostly grow in shallow water. As regards all 

 organisms growing in deep seas, it appears to be essential to them that 

 the green colour of the chorophyll should be masked by a coloured 

 pigment, red in the case of the Florideae, brown in those of the 

 Phaeosporeae and Fucaceae ; the nature of these pigments will be dis- 

 cussed under the separate families. A few of the smaller species, 

 belonging to the Coleochaetaceae, Chordariaceae, and Squamariaceae, 

 grow attached to stones, larger Algae, or other marine objects, as flat 

 discs, gelatinous cushions, or calcareous incrustations, and the deposition 

 of lime takes place to a much larger extent in the corallines. The propor- 

 tion of ash to the organic constituents is much larger in marine than in 

 land or fresh-water plants, seaweeds having the power of extracting from 

 the salt water large quantities of the soluble salts contained in it. The 

 larger species of Fucaceae and Laminariaceae are largely used in the 

 north of Europe for manuring the land and for foddering cattle ; and in 

 former times the manufacture from their ashes of kelp and barilla was 

 an important industry. They are also an important commercial source 

 of iodine. From the quantity of gelatine contained in their thallus, 

 some species of Ulvaceae, Porphyraceae, Fucaceae, and Laminariaceae are 

 also occasionally used as articles of food or for medicinal purposes. 



LITERATURE. 



Greville Algae, in Scottish Cryptogamic Flora, 1823-28; Algae Britannicae, 1830. 

 Kiitzing Phycologia generalis, 1843 ; Tabulae Phycologicae, 1845-69 ; and Species 



Algarum, 1849. 

 Harvey Phycologia Britannica, 1846-51 ; Nereis Australia, 1847-49 ; British Marine 



Algae, 1 849; Nereis Boreali- Americana, 1851-58; Phycologia Australica, 1858-63. 

 Hassall British Fresh-water Algae, 1845. 

 Nageli Die neuern Algensysteme, 1847. 

 Agardh Species, Genera, et Ordines Algarum, 1848-80 ; Till Algernes Systematik, 



1872-87. 

 Thuret Antheridies cles Cryptogames, 1851.; Zoospores des Algues, 1851 ; and 



Etudes Phycologiques, 1878. 



