PH HOPHYCEA 47 
dise, more rarely (asin Turbinaria, Phyllospora, &c.) 
by root-like fibres (haptera), and in the case of 
Notheia, by a haustorium, which it drives into the 
thallus of its host (Hormosira, and rarely Fucodium), 
much in the fashion of parasitic flowering plants. 
The apical cell, by which growth in length is 
effected, is generally situated at the bottom of a 
Fic. 5.—a, Notheia anomala growing on Hormosira, half natural size ; b, sec- 
tion of Notheia, showing origin of lateral branch from the base of a conceptacle ; 
¢, oogonium ; d, section of junction of Notheia with Hormosira. b,c, and d 
highly magnified. (After M. O. Mitchell, in Phyc. Mem.) 
dimple in the surrounding tissue, and both three- 
and four-sided cells occur in different genera. 
Oltmanns, in his classification of the genera of this 
order, attaches an importance to this three- or four- 
sided character which is hardly justified by its proved 
inconstancy in less plastic forms than those of Algze. 
The occurrence of air-vesicles is not characteristic 
of particular genera, and hardly even of species, 
