Trfbottemacece 



255 



compounds. Vegetative division does not occur. Asexual repro- 

 duction takes place by the division of the cell-contents to form 

 ellipsoidal aplanospores, or to form eight ovoidal zoogonidia with 

 two cilia. In those 

 species which are at- 

 tached the zoogonidia 

 generally come to rest 

 on the rim of the empty 

 tube-like cell and there 

 grow into adult cells. 

 A repetition of this pro- 

 cess produces a curious 

 branch-system. Some- 

 times the zoogonidia 

 develop on the apices 

 of other cells which 

 contain the cell-con- 

 tents. No gametes have 

 been observed. The 



genus was monographed 

 by Lemmermann 1 , but 

 the validity of the thir- 

 teen species he puts 

 forward is questionable. 



Fig. 120. A, Ophiocytium majus Nag., from 

 Bowness, Westmoreland. B G, O. cochleare 

 (Eichw.) A. Br. , from same locality. H and I, 

 0. bicuspidatum (Borge) Lemm. forma longispina 

 Lemm. , from Pilmoor, N. Yorks. J, O. Arbus- 

 cula (A. Br.) Eabenh., from Mitcham Common, 

 Surrey. (All x450.) K, 0. graciliceps (A. Br.) 

 Babenh., after treatment with potassium hydrate 

 (after Bohlin, x 570). 



Several species are widely distributed in the British Islands. 0. Arbuscula 

 (A. Br.) Rabenh. is an attached species, often with very pretty branch- 

 systems ; diam. of cells 3 8 p. ; fig. 120 J. 0. majus Nag. is the largest 

 species, the cells reaching a diameter of 17 /* ; fig. 120 A. 0. cochleare (Eichw.) 

 A. Br., 0. capitatum Wolle, 0. bicuspidatum (Borge) Lemm. and 0. parvulum 

 (Perty) A. Br. are also frequent species. 



Genus Tribonema Derbes & Solier, 1856. {Conferva in the 

 sense used by Lagerheim, 1888.] The plants of this genus are 

 simple filaments of cylindrical or slightly barrel-shaped cells with 

 strong cell-walls. The latter are often of considerable thickness 

 and the filaments frequently break up into H -pieces. Each 

 H -piece consists of a transverse cell- wall with a cylindrical piece 

 on either side, and the whole is composed of a number of layers of 

 pectose compounds. Each cell is thus bounded by the halves of 

 two H -pieces. The cells contain one (or sometimes two) nuclei 

 and a variable number of parietal chromatophores. In some 

 1 Lemmermann in Hedwigia, Bd xxxviii, 1899, pp. 20 38, t. iii & iv. 



