136 Chlorophycece 



dylosium the cells are united into filaments by mucous threads 

 passing between their apposed ends, and the filaments break much 

 more readily across the isthmus of a cell than at the points of 

 apical attachment. 



The minute structure of the cell-wall was first studied by 

 Klebs 1 who examined the nature of the gelatinous outer coat and 

 also demonstrated the presence of pores in the cell-wall. Shortly 

 afterwards Liitkemiiller 2 described the pores in the cell-wall of 

 Closterittm and quite recently he has published a very important 

 account of the structure of the cell-wall of Desmids 3 . 



In a few Desmids those belonging to the Spirotseniese the 

 cell-wall consists of a single layer of cellulose of homogeneous 

 structure, but in the majority of Desmids it is composed of two 

 well differentiated layers. The inner layer is structureless and 

 consists of cellulose ; the outer layer is stronger and thicker, con- 

 sisting of a ground substance of cellulose through which pass 

 numerous tube-like structures or 'pore-organs.' The latter are 

 not composed of cellulose, and a small pore or canal runs through 

 the middle of each one. The contents of these canals also traverse 

 the inner layer of the cell-wall and often terminate on its inner 

 surface in lens-shaped or bulbous swellings. From the outer end 

 of the pore-organs there often arises a delicate flower-like or club- 

 shaped structure through which the canal passes. More often this 

 structure is entirely wanting or is replaced by a small perforated 

 button or rod. In many of the larger Desmids there are numerous, 

 smaller, thread-like pores between the larger canals and only 

 traversing the outer layer of the cell-wall. Outside the differen- 

 tiated parts of the cell-wall is the mass of gelatinous pectose 

 compounds present in so many species, and which is secreted by 

 the protoplasm and passed outwards through the pores. This 

 outer gelatinous coat often exhibits a prismatic or fibrillar struc- 

 ture and is frequently the home of epiphytes or of numerous rod- 

 shaped bacteria. The cell-wall sometimes contains a trace of 

 silica. 



With the exception of the lining primordial utricle the 



1 Klebs, 'Ueber Bewegung und Schleimbildung der Desmidiaceen,' Biol. Centralbl. 

 Bd v, 1885. 



2 Liitkemiiller, 'Die Poren der Desmidiaceen Gatt. Closterium Nitzsch,' Oesteir. 

 botan. Zeitschr. Bd 44, 1894. 



3 Liitkemiiller, 'Die Zellmembran der Desmidiaceen,' Beitrage zur Biol. der 

 Pflanzen, herausgegeberi von F. Cohn, Bd viii, Breslau, 1902. 



