NITELLA OPACA. 101 



or scabrous with wart-like protuberances (PI. IV, f. 3). 

 Antheridium large, c. 650-775 /x. in diameter. 



Habitat. — In lakes, ponds, pools, ditches, and 

 streams ; common. 



DiSTKiBUTiON. — Throughout the British Isles from 

 the Hebrides and Shetland to Devon and Cornwall, and 

 from Bast ISTorfolk and Bast Kent to "West Cornwall ; 

 in Ireland from Donegal to Kerry and from West 

 Mayo to Down. Recorded from more than 75 per 

 cent, of the counties and vice-counties, and probably 

 occurring in almost all of the remainder. 



First record : probably Ray's 'Synopsis,' ed. 3 (1724) 

 as Ghara translucens minor flexilis. 



Outside the British Isles N. opaca is recorded from 

 Norway, Sweden, Finland, Denmark, France, Belgium, 

 Holland, Germany, Switzerland, Austria, Hungary 

 and Italy; also from N. Africa, Asia, and N. & S- 

 America. 



An extremely variable plant, generally stoutish, of medium 

 stature (about 15-25 cm.), usually of a darkish green, often 

 becoming almost black, sometimes annularly, occasionally 

 completely, incrusted. 



The branchlets on the main stems are of variable length, 

 usually J-^ the length of the internodes, but sometimes 

 abbreviated, when the general appearance of the plant is 

 altered. Extreme states from Scotland are quite moniliform. 

 The branchlets are only once-forked, the primary being 

 usually much longer than the secondary rays. In the male 

 the branchlets, and especially the secondary rays, are rather 

 stouter than in the female plant. 



Though the branchlets of the lower whorls are often also 

 fertile, the oogonia and anthei-idia are mostly produced at 

 the apex of the stem and on rather short branches, the 

 whoi'ls and internodes of the fertile whorls being abbreviated, 

 forming usually largish compact heads. 



The antheridia are larger than in N. capillaris, the ridges 

 on the oospore are less salient, and the membrane has no 

 decoration. Both antheridia and oogonia are destitute of 

 gelatinous envelopment. 



The apex of the terminal ray is very variable in shape, 

 usually bluntly mucronate, but occasionally acuminate as in 



