176 



Chlorophycece 



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

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

 cells 8 9'5 p. ; breadth 9'5 1O5 p. ; fig. 67 B) and S. pidchellum Arch, (length 

 12 - 5 15 p.; breadth 11 12'5 p.; 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 1525^; 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. imdulata Nordst. (length 13'5 17'5/x ; breadth 7'5 

 9 p.) and H. neglecta Racib. (length 2834-5 p.; breadth 11 '5 13 p.; 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.) Breb. ; A and B, from Capel Curig, 

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

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

 H. neglecta Racib., from the New Forest, 

 Hants'. (x47o); E G, shosving aplano- 

 spores (); H, zygospore (z). 



