44:8 Mr Rattray on the Distribution of 



is thus at first checked, their rhizoids finally loose hold, 

 and the thallus, having become more or less pathological, 

 is carried away by the tidal waters from its position of 

 growth. After a period of sporulation young plants will 

 again appear along the new course of the fresh water 

 streamlets, but will not be found along the old course if the 

 water be continually diverted. 



The influence of pressure is evidenced by its effect on 

 the rigidity of the plants — an effect induced by the strains 

 to which they are subjected by the passage of water across 

 them during the ebb and flow of the tide. This in- 

 creased firmness, together with the longer exposure to the 

 influence of salt water in proportion as the position of fix- 

 ation is at a lower level, and consequently the less the 

 exposure to the heat rays of direct sunshine (the water 

 being colder than the air in summer and early autumn), 

 will sufficiently account for the greater lateness in the emis- 

 sion of swarmspores. 



Chaetomorjplue {C. aerea and C. melagonium) occur here 

 but rarely, but many Cladophorce of fine growth are found. 

 Among these may be noted Cladoplmra uncialis, C, arcta, 

 G. glav.cescens. This genus is exceedingly abundant at this 

 and a sliglitly higher level among the rocky buttresses to 

 the south of the May Island, and at several places they 

 present a remarkable contrast to the less conspicuous 

 species around them. 



The continuity of species between the levels of a quarter 

 and three-quarter tide over almost the entire estuary pre- 

 cludes the subdivision of this into more than a single 

 area — which from the preponderance of Fucacese may be 

 named the Fucus area. We here find the upper limits of 

 several species, e.g., Ptilota elegans, Schizymenia edulis 

 (only in pools at the lowest reaches of this area), Odon- 

 thalia dentata, Phyllophora ruhens, P. traillii (in dark and 

 shallow pools well seaward), Polysiplionia atro-ruhescens, 

 P. brodicei, P. fastigiata (common in many parts), P 

 formosa, {e.g., Fidra, in pools). P. parasitica (rare only in 

 deep pot-holes or sheltered spots). Porphyra amethystea, 

 Furcellaria fastigiata, Ahnfeldtia p)licata, Alaria esculenta 

 (at the lower reaches of the zone), Bryopsis plumosa (not 

 uncommon), Callithamnion arluscula, C. hookeri, C. ijlumula 



