314 Marine Alga: of Berwick-on-Tweed. 



Petrocelis Hennedyi (Harv.) Batters, in Holmes's Alg. Brit. 

 Exsicc. — Actinococcus Hennedyi, Harv., in Nat. Hist. Rev., 

 vol. iv. (18-57), p. 202, pi. 13a, fig. 1. 

 Descr. Actinococcus Hennedyi, Harv., I.e. 



Fig. „ „ Harv., I.e.; Tab. Nost. XL, fig. 3, 4. 



Exsicc. Petrocelis Hennedyi, Holmes, Alg. Brit. Rar., Exsicc., no. 89. 

 Syn. Cruoria pellita, (Lyngbyei) Bupr., Tang. d. Ochotskischen Meeres., 

 p. 138, t. 18c-e. 

 Petrocelis Ruprechtii, Haucl-, Meeresalg., p. 30 (1883) (ex ipso 

 Hanck.) 

 Hab. On rocks near low-water mark or more freqnently on the stems 

 of Laminarin. hyperborea. Fruit Jan. and Feb. Not uncommon. 

 Berwick Bay, Sharper Head. 



To the naked eye this plant is indistinguishable from 

 Petrocelis cruenta, but whereas in the last-named species only 

 one tetraspore is formed in a single filament of the thallus, in 

 Petrocelis Hennedyi, from six to eight or even twelve tetraspores 

 are formed from adjacent cells of the same filament. 



The plant forms glossy, dark purple, almost black, fleshy 

 patches, roundish or irregular in outline, on the stems of Lam- 

 inar ia hyperborea, or more rarely on rocks near low- water mark. 

 The frond is composed of simple, vertical, parallel filaments 

 firmly united below, but above rather loosely held together by 

 a gelatinous substance. The tetraspores are large and cruciate, 

 subquadrate rather than oval. The cystocarps, which I believe 

 have never been described, are composed of numerous rather 

 small spores placed one above the other in one, two, or three 

 rows (vide Plate xi., fig. 4.) Most of the specimens of this 

 species which. I gathered in January 1887 were fruited, tetra- 

 sporic plants being much more plentiful than those bearing 

 cystocarps. 



I am indebted to Prof. E. Perceval Wright, of Dublin, for 

 comparing a specimen of the Berwick plant with Harvey's 

 original specimens of Actinococcus Hennedyi in the Herbarium of 

 Trinity College, and to Mr. Gr. W. Traill for comparing one with 

 Hennedy's specimens in Edinburgh. 



The fronds of the present species and of Cruoria pellita are, with 

 us, frequently infested by a green unicellular parasite which grows 

 amongst the vertical filaments of the thallus. It forms ovoid or 

 pyriform green sacs surrounded by a gelatinous limbus, drawn 

 out below into a slender stem. It appears to be the plant 

 discovered by Kjellman in the fronds of Sarcophyllis, to which 



