458 Mr Eattray on the Distribution of 



occurs {e.g.^ at Elie) at a somewhat higher level, and of 

 tolerable size. The influence of light as affecting plant dis- 

 tribution is thus well illustrated in the case of this some- 

 what rare species, and it also enables us to recognise the 

 changes of water pressure which so comparatively slender a 

 species can endure, provided that the other conditions of 

 environment are not of too trying a character. 



Halidrys siliquosa is especially interesting in this 

 connection. It is of very frequent occurrence in high- 

 water tidal pools, and attains here an average size of from 

 12 to 15 inches. Specimens, however, dredged in 24 

 fathoms off Anstruther, were 26 inches long. Many other 

 similar cases might be noted. 



(3) As to habit, it appears generally that all the great 

 groups are more fully formed, and present a more elegant 

 appearance as the sea is approached. This applies to the 

 strongly developed Phseophycefe no less than to the Chloro- 

 phycese and FloridesB, The detracting influence exerted in 

 this respect by influx of fresh loater is very remarkable in 

 the case of many species, as is well seen at the mouth of the 

 Almond at Cramond, of the Esk near Musselburgh, of the 

 Tyne at Tynninghame Sands, and elsewhere. Many of the 

 Chlorophycese are less affected than others, e.g., Rhizoclonia, 

 Entermori:)ha3, TJlothrices, Confervcv, &c. Ccdlithamnice and 

 other forms of the finer Floridese are more sensitive than 

 many species of Polysiphonia, while the effect of such 

 influences in producing almost varietal modifications in 

 some species {e.g., contortions in Dumontia flliformis) is re- 

 markable. It has already been stated that it is the salinity 

 of the water which chiefly determines the distribution of 

 marine plants,* and the influence which this exerts in 

 modifying the habit and in checking the downward dis- 

 tribution of Enteromorphse has already been referred to. 



II. There is a decided increase in number of species as 

 we advance seawards — the finest Florideee — Dudresnaia 

 coccinea, Nitophyllum laceratum, N. punctatum, Callith- 

 amnion gracillimurn, C. floccosum, Bonnemaisonia aspara- 

 goides, Delesseria sanguinea, &c., either being found 

 exclusively in seaward parts, or exhibiting a more graceful 

 habit and a larger number of specimens there. There are 



* r. J. r. Meyen, Grundriss dcr Pflanzengeographie. 



