462 Mr Piattray on the Distribution of 



Among the causes that are at work in aflfecting the dis- 

 tribution of species in such estuaries, the following general 

 summary may be given : — 



(1) Heliotropic tendencies of the swarmspores (see p. 

 437). 



(2) The i^hysical character of the substratum as stony, 

 rocky, sandy, muddy, &c. In the words of Meneghini,* 

 " The ground on which algae grow certainly exercises some 

 influence on their nutrition, for although the greater 

 number of the species grow indifferently on any submerged 

 body, some occur exclusively on calcareous rock, others 

 on granitic or basaltic. The Lamiuarise and Chordae have 

 a preference for sandy bottoms .... Some species of 

 Callithamnion grow only on rock, others are exclusively 

 parasites of Furcellaria, just as Polysiphonia fastigiata is 

 only met with on the fronds of Fucus nodosus .... But 

 when the same species are met with in different condi- 

 tions, they always exhibit in their appearance a perfectly 

 distinctive character." 



(3) The effect of storms in shifting boulders to which 

 sea-weeds are attached. Species growing in considerable 

 depths thus become raised or lowered in position, and in 

 the case of a shingle-covered beach, which has not been 

 disturbed by storms for a considerable period, the effect 

 may be very marked. 



(4) The purity of the water is also on shore localities 

 an important factor, the influx of chemical refuse of oily 

 or, other constituents in great quantities acting as deterrants 

 to the growth of the finer Floridecc. The oily scum found 

 in some cases prevents the carrying out of the normal 

 heliotropism of the swarmspores by impeding the freedom 

 of their movements, besides, at the same time, producing 

 an indifferent medium for the performance of their other 

 vital processes. 



(5) Shifting sands or deposition of mud banks are also 

 capable of modifying the distribution of species by tritu- 

 rating or otherwise covering the plants, and especially 

 in the latter case by cutting off adequate light, and by 

 pressure. 



* Meuegliini, Cenni sulla OrganograpTiie e Fisiologia delle alghe. Padxjva, 

 1838, p. 50. 



