424 Mr Eattray on the Distrihdion of 



so many different means for bringing about the dissemi- 

 nation of the species, as they facilitate the removal of any 

 spore from the place occupied by its parent ; while simi- 

 larly the great degree of elasticity possessed by many 

 species, combined with the tenacity of the rhizoids for their 

 substrata of all forms, are sometimes the means of lifting 

 from its bed the boulder upon which the species may be 

 fixed, and so, by presenting a larger surface to the current, 

 often bring about a transference of position, which is but 

 another means of extending, it may be, the bathy- 

 metrical range of the species, which is the usual case, or 

 its horizontal extension. Adult specimens,* in preference 

 to younger forms, where buoyant power is less, in this 

 way often spend a part of their existence at one given 

 level, and other parts of it either, as is usual, at different — 

 higher or lower — levels, or more rarely in other places at 

 corresponding levels, and so a disturbing factor must be 

 taken into account in dealing with the algoid zones or 

 regions, to which further reference will be made below. 

 The eddy gyrations of the water, moreover, which result 

 from the passage of a current among the fronds of larger 

 Algse, as well as of those of medium size, have to a certain 

 extent a well-marked tendency to cause tetraspores, and 

 to a more limited degree zoospores, even though actively 

 motile, to be retained either among or in the immediate 

 vicinity of the parent weeds. They thus facilitate the 

 formation of patches of one given species in a given 

 locality, to the exclusion of other forms, where apparently 

 other conditions of environment would not prevent the 

 growth of the latter, and they thereby favour what is 

 at first sight a distinctly selective or exclusive property 

 possessed by many species. Among the smaller and more 

 delicate species this tendency is well seen in the growth of 

 patches of the filamentous forms of Gallitliamnion (e.g., 

 C. floriduluni, C. rothii, C. graciUimum. C. corymhosum, C. 

 arbuscula, &c.) ; whilst the larger Gladophorce {G. arcta, C. 

 uncialis), and Enteromorijhce (e.g., E. compressa, E. intes- 

 tinalis, E. erecta, LaurencicB, Gigartince, PhyllophorcB^ as 



In this WE}' specimens of Cystoclonium purpurascens, Ddesseria alata, D. 

 sinuosa,, Plocamium coccineum, &c., are sometimes foiind of unusual size on 

 stones that have been cast up by the waves during strong gales. 



