THE MARINE ALGAE OF THE L.M.B.C. DISTRICT 129 



Isthmoplea sphaerophora, first observed by 

 Carmichael about 1824, is described by Harvey (Phyc. 

 Brit. pi. 'CXXVI) under the name of Ectocarpus 

 sphaerophorus , but was placed in the genus Isthmoplea 

 by Kjellman (Algenv. Murm. Meer) more especially on 

 account of the mode of origin of the asexual reproductive 

 organs. 



The gonidangia arise singly or in pairs as lateral 

 segments, formed by longitudinal division of an ordinary 

 vegetative cell. Gametangia have not, so far as we are 

 aware, been found. 



At Port Erin, as elsewhere, I. sphaerophora grows 

 as an epiphyte on Plumaria elegans. Amongst material 

 collected in April, 1911, we found, on subsequent micro- 

 scopic examination, many plants some of which bore 

 gonidangia and others gametangia. These latter were 

 growing side by side on the same host, the sexual plants 

 being inextricably intertwined with the asexual forms. 

 Both forms corresponded in every detail, as may be seen 

 by comparing the vegetative cells figured on Plate, 

 figs. 4, 5. The gametangia are of the normal Ectocarpus 

 type, and are developed at the ends of short lateral 

 branches composed of two or three cells. Each game- 

 tangium is on an average 0'07 mm. long, and tapers from 

 base to apex. It would appear, therefore, that the only 

 distinctive feature of Isthmoplea, as a genus, is the 

 somewhat peculiar mode of origin of the gonidangia and 

 their semi-imbedded nature when mature. 



Hauck (Meeresalgen, p. 331) records his having 

 occasionally found gonidangia and gametangia on the 

 same plant of Ectocarpus confervoides. On an undeter- 

 mined species of Ectocarpus we also found gametangia 

 and gonidangia on the same branch not infrequently 

 formed from contiguous cells (Plate, fig. 7). 

 i 



