178 Chlorophycew 



spores are ellipsoidal, smooth or furnished with somewhat flattened 

 or conical papilla?. 



There are six British species of the genus, none of which is abundant. 

 D. Swartzii Ag. (length 14 19 p ; breadth 37 43 p ; fig. 69 A) and D. cylin- 

 dricum Grev. (fig. 69 C) are the two most general species. D. quadratum 

 Nordst. (fig. 69 B) and D. graciliceps (Nordst.) Lagerh. are two species very 

 rarely met with in the British Isles. 



Genus Gymnozyga Ehrenb., 1840. [Bambusina Kiitz., 1845.] 

 The cells are cylindrical or barrel-shaped and united by their flat 

 ends into slightly twisted filamentous colonies. There is a slight 

 median constriction and at the base of each semicell is a swelling 

 of variable size. The vertical view is circular, sometimes with 

 two opposite papillae. The cell-wall frequently possesses delicate 

 longitudinal grooves. There is one central chloroplast in each 

 semicell with six radiating longitudinal plates and one pyrenoid. 

 The zygospores are ellipsoidal and smooth. 



The only British species is G. moniliformis Ehrenb. (length 25 30 fi ; 

 breadth 17'5 22'5^; fig. 69 E), which is generally distributed in boggy 

 districts, particularly in elevated localities. 



Order IX. PBOTOCOGCQIDELffl. 



This order includes a large number of green AlgaB which are 

 mostly unicellular in character. The cells are commonly aggregated 

 to form loose irregular colonies and are often embedded in a 

 copious mucilage. In a few of the forms a small multicellular 

 expansion is developed, and in others a definite coenobium which is 

 sometimes cosnocytic in character. In some the cells are normally 

 ciliated and the plants are either motile unicells or more or less 

 complex motile colonies. 



The order includes the lowest and most primitive of the green 

 Algse, forms through which most of the other ChlorophyceaB have 

 been evolved along divergent lines. 



They are wonderfully varied in character and are found in 

 almost all possible situations. The cell-walls may be extremely 

 delicate or firm and thick, and there is often a great development 

 of the gelatinous pectose compounds. The number and disposition 

 of the . chloroplasts vary greatly in the different genera, and 

 pyrenoids may or may not be present. 



