15 170 Proceedings of the Royal Irish Academy. 



which the plants can endure is not reached during summer, but that in the 

 south of England this point Lb exceeded. The meteorological office reports 

 show, however, that though the atmospheric temperature in S.W. England 

 exceeds in summer that of S.W. Ireland, the sea-temperature is practically 

 the same. 1 The temperature factor then may explain the absence of the 

 Littoral C. arhueula, but hardly that of the sub-littoral Ptilota. The physical 

 nature of the coast has also to be considered. Both the algae mentioned 

 require rocky ground. Ptilota grows on the stipes of Laminaria Cloustoni (a 

 stout species demanding a very firm bottom) ; and C. arbuscula is found only 

 "ii exposed rocks in the littoral region: from shores of sand, shingle, or soft 

 rock they would therefore be absent. On the east of England, stretches of 

 hard rock in the four counties immediately south of Yorkshire are very scarce: 

 and it is most probable that both plants would extend much further south 

 were such rocks available. For Wales and S.W. England, however, this 

 difficulty does not exist. 



Another theory that presents itself is the possibility that, owing to more 

 recent geographical changes in the English coast-line, C. arbuscula and 

 P. plumosa are still advancing in the Irish and North Seas, and that they have 

 not yet reached their southern limit. The conditions (e.g., currents, nature of 

 coast, etc.) may have been less favourable to their progress than in Ireland. 

 This, however, is improbable, as the dispersal of marine algae by spores 

 appears to be an easy and comparatively rapid process. Ptilota, moreover, is 

 readily transported, being found washed ashore in abundance, and often in a 

 bleached state. Such specimens (which are possibly of distant origin) 

 frequently possess cystocarps containing presumably healthy spores. Currents 

 do not appear to offer any resistance to the southern advance of algae in the 

 Irish Channel. 



From these considerations it is probable that the geographical theory does 

 not meet the case, and that ecological factors are more likely to explain it. If 

 this be so, the scarcity of hard rocks may have limited the range of these 

 boreal species in the North Sea ; and a high summer temperature may have 

 prevented Callithamnion arbuscula from settling down on our south-west coast. 

 Ptilota remains a puzzle. Numerous points present themselves which would 

 be worthy of inquiry ; but with its precise distribution in Wales and Ireland 

 uncertain, these must be left for future consideration. 



The only geographic types recognized worthy of discussion here are the 



1 The average sea -temperatures for the past five years for Seafield (Co. Clare) and Plymouth 

 during the summer months are almost identical. But during 1911 the Plymouth average from 

 July 15 to September 9 was distinctly higher. The more eastern counties (Kent, Sussex) possess a 

 much higher sea-temperature in summer than Devon and Cornwall. 



