20 F. E. FRITSCH. 



biseriate arrangement. The same thing may more rarely take place in the expanded 

 part of the thallus, which occasionally exhibits prominent constrictions. In spite of 

 all these irregularities there is a very sensible and gradual widening of the plant 

 after the first permanent longitudinal divisions set in. 



In many of the plants the broadening out of the thallus did not go very 

 far, so that the whole plant had a filiform appearance, with a length many times 

 exceeding its breadth. This, and the very gradual widening of the stipe into 

 the thallus, appear, according to Imhauser (" Entwicklungsgesch. u. Formenkreis v. 

 Prasiola " Flora, 1889, Heft 3, Diss. Marburg, 1889, pp. 47 and 55) to be the two most 

 prominent characteristics of the species. These two characters are also well shown 

 in exsiccafca of P. calophylla (from Tristan d'Acunha, Hb. Dickie) preserved in the 

 British Museum, although the specimens are not as elongated as those from Winter 

 Harbour. 



In the expanded thallus (text-figure J) it was often not easy to recognise the 

 longitudinal seriation of the cells, referred to in many descriptions of the species 

 (Imhauser, op. clt. ; De Toni, op. cit. ; Rabenhorst, op. cit. ; F. S. Collins, " Green Algae 

 of N. America," Tuft's College Studies, Scient. series, II., No. 3, 1909, p. 219). The 

 cells were regularly arranged in groups of four and were often practically equi- 

 distant, so that an areolation, such as that in P. crispa, was not manifest ; but 

 this grouping of the cells was frequently interrupted owing to the cells in adjacent 

 parts of the thallus showing a slightly different orientation. The cells in the 

 expanded thallus were often almost square, the cell-contents measuring 5-6 x 5-6 p. 



Some of the larger thalli attained a length of 2 cm. and a breadth of about 

 a millimetre (at their widest point). The specimens from Granite Harbour were 

 much shorter than those from the other habitat. 



No reproductive stages were observed, 'but intermingled with the other growth 

 on the stones from Winter Harbour were PraswZa-packets, which did not differ in 

 any respect from those of P. crispa. Presumably they belonged to P. calophylla. 



CONJUGATE. 



MESOT^ENIACE^E. 



Genus PENIUM Breb. 



14. PENIUM sp. 

 (PI. L, figs. 45, 46.) 



Cellulis subcylindricis vel modice ellipsoideis, latitudine circa duplo longioribus, 

 polis rotundato-deplanatis, constrictione modica in media parte cellulse munitis ; 

 membrana cellules Isevis et hyalina, modice sed sequaliter incrassata ; conteutu 



