THE ALGAE 



87 



reproduction is unknown. The vegetative cells are generally large, and 

 possess large, elaborately shaped chloroplasts. The plants may be unicellular 

 or the cells may be united together into filaments, which may fragment into 

 separate unicellular parts. 



Included in this group are the Desmids, which are among the most 

 beautiful of the Green Algae. Most of them are unicellular, but the wall of 

 the cell is divided into two halves, or semi-cells, each of which is often 

 extremely complex in outline and is connected to the other by a narrow 

 region called the isthmus. 



We shall consider only one type of the Conjugales, Spirogyra. 



Spirogyra 



Spirogyra is a very common bright-green Alga which may be found in 

 large masses growing in ponds and slow-flowing rivers. The filaments are 



Chloroplast 



^ Nucleus 



A B 



Fig. 68. — Spirogyra sp. Vegetative structure of fila- 

 ment. A, Surface view. B, In optical section. 



usually free, but are sometimes attached at one end to a substratum bv a 

 branched and colourless rhizoidal cell or holdfast. I'hese masses are very 

 slimy to the touch, owing to a mucous sheath surrounding the cells, and 

 consist of enormous numbers of long, unbranched, green filaments. 



There is no morphological difl^erentiation between the cells of the 

 filament (Fig. 68). A delicate sheath of cuticle surrounds the entire filament 

 inside the mucous sheath, while each cell has its own polysaccharide wall, 



