12 Algce 



the cell-walls. This mucilaginous sheath is present in most of the 

 unicellular and filamentous freshwater Algae, and sometimes attains 

 a huge development. Its nature was well investigated by Haupt- 

 fleisch 1 and more recently by Schroder 2 . 



The multicellular forms consist of closely connected cells form- 

 ing a thallus, which exhibits a great variety of form. It may be 

 spherical (e.g. Ccelastrum), filamentous (e.g. Spirogyra, Ulothrix, 

 etc.), or a flattened expansion (e.g. some species of Coleochcete, 

 Protoderma). Sometimes the thallus is differentiated into a 

 " root " and a thalloid " shoot " (e.g. Botrydium, Rkizoclonium, 

 (Edogonium, Spirogyra, etc. 3 ), but the " root " is in all cases merely 

 an organ of attachment and is more correctly called a hapteron. 



The cell-wall always consists largely of cellulose, and is some- 

 times delicate, sometimes of considerable thickness and strength, 

 being cuticularized or even silicified, but it is rarely, if ever, 

 lignified. It often becomes gelatinous in its outer layers. 



The principal colouring-matter of the cell is usually arranged 

 in definite parts of the protoplasm known as chromatophores. A 

 single cell may contain one or many chromatophores. If the 

 chromatophores contain the green colouring-matter chlorophyll, 

 they are known as chloroplastids (or chloroplasts) ; if they contain 

 some other colouring-matter they are termed chromoplastids (or 

 chromoplasts). Plastids are present in all groups of Algse, but 

 those present in the Myxophyceae are of a very primitive character. 

 Chromatophores, particularly chloroplasts, often contain pyrenoids, 

 which consist of colourless masses of proteid substance. The 

 central mass of the pyrenoid is a proteid substance of crystalloidal 

 character which bears a great resemblance to an aleuron grain. 

 It is sometimes angular, sometimes rounded, or it may be quite 

 irregular in its outward form, and is often surrounded by an 

 amylaceous envelope or coat of starch. The latter sometimes 

 becomes lobed and penetrates into the chromatophore to such an 

 extent that its existence appears doubtful. In Spirogyra the 

 envelope of amylaceous material round each central mass (or 

 pyrenocrystal) is in the form of a number of grains of starch. On 



1 Hauptfleisch, ' Zellmeinbran und Hiillgallerte der Desmidiaceen,' Mitteil. aus 

 d. Naturwiss. Vereine f. Neuvorpommern und Riigen, 1888. 



2 Schroder, ' Untersuchungen iiber Gallertbildungen der Algen,' Verhand. des 

 Naturhist.-Med. Vereins zu Heidelberg, Bd vii, Heft 2, 1902. 



3 Borge, ' Ueber die Ehizoidenbildung,' Upsala nya Tidnings Akteb. Tr. 1894 ; 

 West and G. S. West in Ann. Bot. vol. xii, March 1898. 



