176 



Chlorophycece 



There are seven species of the genus known to occur in the British Islands, 

 none of which is abundant. S. papillatum W. & G. S. West (length of 

 cells 8 9-5 p; breadth 9-510-5 M ; fig. 67 B) and S. pulchellum Arch, (length 

 12-5 15 n; breadth 11 12'5 /x; fig. 67 A) are the most widely distributed. 



Genus Hyalotheca Ehrenb., 1841. The cells are more or less 



cylindrical and are connected 

 by their broadly truncate 

 apices into filamentous colo- 

 nies. The median constric- 

 tion is very slight and the 

 semicells are trapezoid, sub- 

 quadrate or oblong in form, 

 with straight or convex lateral 

 margins. The filaments are 

 usually twisted and always 

 enveloped in a thick coat of 

 mucus. There are frequently 

 several slight swellings at the 

 base of each semicell near the 

 constriction, causing the cir- 

 cular vertical view to possess 

 two or three nipple-like pro- 

 jections at equal intervals 

 round the margin. There is 

 one chloroplast in each semi- 

 cell, consisting of a central 

 mass with a number of radiat- 

 ing plates, and containing one 

 pyrenoid. The zygospores are 

 globose and smooth, and in 



H. dissiliens the four empty semicells unite to form a cruciform 



structure which surrounds each spore. 



There are four British species of the genus, of which H. dissiliens (Sm.) 

 Breb. is general and often abundant; length 15 25 /x; breadth 21 33 /*; 

 fig. 68 A C. This Desmid is more frequently found with zygospores than 

 any other. H. mucosa (Dillw.) Ehrenb. is a much scarcer plant, although 

 widely distributed. H. undulata Nordst. (length 13'5 17'5/A ; breadth 7'5 

 9/n) and H. neglecta Eacib. (length 28 34'5 /*; breadth 11'5 13^; fig. 68 

 E H) are amongst the rarest of British Desmids. 



Series b. The oblique junction of the new and old halves of 

 the cell-wall (at the region of the isthmus) develops an internal 



Fig. 68. A C, Hyalotheca dissiliens 

 (Sm.) Br6b. ; A and B, from Capel Curig, 

 N. Wales (x365); C and D, zygospores 

 from Galway, Ireland ( x 365). E H, 

 H. neglecta Eacib., from the New Forest, 

 Hants. ( x 475) ; E G, showing aplano- 

 spores (a) ; H, zygospore (z). 



