212 Mr John Rattray on some 



organisms than by these organisms considered separately. 

 This factor becomes an exceedingly significant one when 

 the epiphytes, as not unfrequently happens, are relatively 

 of large size, e.g., Fuci, Ascophylla, Laminarke, or the 

 like. 



Although it is not possible iu most cases to determine 

 any law underlying the association of seaweeds with respect 

 to their relative sizes, it is worthy of remark that the great 

 elasticity of some species is of importance in preventing 

 such forms, when acting the part of hosts, from being 

 detached from their places, when more rigid forms of 

 corresponding size would be torn away. This is especially 

 the case in the Poiyhyras., where the toughness of the fronds 

 constitutes one of the most noteworthy characters. 



Perhaps one of the most intimate cases of association 

 between different weeds is to be found between the forms 

 Poly^iphonia faatigioia'^ (Roth.), Grev., and Ascophyllum 

 nodosum (L.) Le Jol. That we have here to deal with a 

 case of partial parasitism can hardly be questioned. The 

 thallus of the host (AscopJojUitm) is modified in its external 

 appearance, where the Polysiphonia is attached, and is 

 distinctly penetrated by the rhizoids of the latter, the 

 cells of which thus come into close contact with the 

 comparatively thin-walled subepidermal cells of the former. 

 It is true, indeed, that it is not absolutely necessary for 

 the life of this epiphyte that it should thus become fixed 

 to any host, as it occurs, though comparatively rarely, 

 growing on a rocky substratum, but, in such cases, its 

 vegetative growtli is less perfect — tliough it be otherwise 

 exposed to similar external conditions, with respect to 

 depth, and general environment — thereby showing that a 

 direct advantage is gained by an epiphytic habit. 



In addition to the extensive lists of epiphytes or 

 parasites that have already been recorded by Mr George 

 W. Traill f among the algse of the basin of the Firth of 

 Forth, the following cases have been observed by me during 

 the past season in the same area: — 



* According to Professors Magnus and Areschoug, this plant is absent from 

 the Scandinavian coast of the Skager Rack, though it is abundant in corre- 

 sponding latitudes on the coast of Scotland. 



t Proc. Roy. Diohlin Hoc, April 17, 1882 ; Monograph of the Algas of the 

 Firth of Forth, Edinburgh, 1885. 



