336 



Myxophycece 



in all kinds of wet situations, and sometimes on damp ground, or 



in hot springs. Flocculent masses 

 of Oscillatoria are often found float- 

 ing on the surfaces of ponds and 

 ditches after sunshine, and they 

 commonly give off a more or less 

 distinctive odour. The movements 

 exhibited by the trichomes of this 

 genus are discussed on page 315. 



There are about 20 British species of 

 the genus, one of which (0. decolorata 

 G. S. West) is destitute of pigment and 

 lives a saprophytic existence in stagnant 

 ditches. 0. princeps Vauch. is the largest 

 species (thickness of trichomes 25 50 ^i) 

 and 0. angustissima W. & G. S. West is 

 the narrowest (thickness of trichomes 

 0'6 p). 0. limosa Vauch. (thickness of 

 trichomes 12 17 n ; fig. 154 A) and 0. 

 tennis Ag. (thickness of trichomes 4 

 10 n; fig. 154 C) are the most abundant 

 species. 0. irrigua Kiitz. (fig. 154 B) is 

 common in quickly running water. 



A B C D 



Fig. 154. A, Oscillatoria limosa 

 Ag., from Wimbledon Common, 

 Surrey. B, 0. irrigua Kiitz., from 

 Shipley Glen, W. Yorks. C, 0. 

 tenuis Ag., from Sheep's Green, 

 Cambridge. D, 0. splendidaGrev. 

 var. attenuata W. & G. S. West, 

 from Baildon Moor, W. Yorks. 

 E, 0. acuminata Gom., from 



Sheep's Green, Cambridge, 

 x 460.) 



(All 



Genus Arthrospira Stizenberger, 

 1852. The trichomes are cylindrical, 

 commonly devoid of a sheath, and twisted into a regular spiral. 

 The latter character is the only distinction from Oscillatoria. 

 The cells are generally shorter than their diameter. 



A. Jenneri (Hass.) Stizenb. is a rare Alga, which occurs in stagnant 

 water, or forms a dark-green mucous stratum in trickling water ; diam. of 

 trichomes 5 8 /i ; diam. of spiral whorls 9 15 p. 



Genus Spirulina Turpin, 1827. The trichomes are very narrow 

 and are twisted into a regular spiral. 

 They consist of a single elongated cell, 

 sometimes of great length, and without 

 transverse septa. This feature at once 

 distinguishes the genus from Arthrospira. 



S. major Kiitz. is frequent in stagnant water 

 (diam. trich. 1-5 2'5/i ; fig. 155 B), and S. tenu- 

 issima Kiitz. ( = S. subsalsa CErsted) is often 



abundant in salt and brackish water, rarely occurring in fresh water (diam. 



trich. 12 *. 



B 



Fig. 155. A, Spirulina 

 turf osa Cram., from Lough 

 Neagh, Ireland. B, Sp. 

 major Kiitz., from Wicken 

 Fen, Cambridge ( x 480). 



