15 18 Proceedings of the Royal Irish Academy. 



the degree of slope influence the vegetation, as does also the nature of the 

 rock-surface ; but, as stated on p. 16, this produces merely a change in the 

 associations, rather than a fundamental difference in the plant-covering. 

 Boulders may he singled out for notice, since their vegetation is peculiar and 

 well marked. The associations on a boulder-beach are much broken up, and 

 secondary series on the vertical faces of the rock are of general occurrence 

 (see Plate II). The tops of the boulders are usually bare, owing probably 

 to immediate escape of the water, and to extreme desiccation, which 

 prohibit respectively the retention and development of spores. Points such 

 as these continually present themselves when studying the communities', 

 and several are noted in the report ; but it is too early to offer generaliza- 

 tions. 



A word may be said as to methods of attachment. In addition to the 

 stout disk typical of the formation, three are distinguishable. In the case of 

 simple filamentous species such as Bangia, Urospora, and Ulothrix, each 

 thread is fixed to the rock by the basal cell, with or without the assistance 

 of corticating filaments. Hence the association, which consists of many 

 thousands of minute plants, does not require a rough surface, but occurs also 

 on smooth rocks where other species obtain no footing. It is in reality 

 attached along its whole base. The second type is that of Ccramium 

 acanthonotum, which possesses rhizoids. Such plants cannot grow on smooth 

 rocks, but occur on mussels and barnacles, amongst which they thrust a dense 

 system of rhizoidal filaments. Where Mytilus and Balanus form extensive 

 sheets, these algae are often plentiful. The third type — a soft, spongy disk- 

 is supplied by Callitlmmnion arbuscula. This is intermediate between the 

 two former and the stout cellular disk. The spongy disk is composed of 

 filaments more or less free at the margins, but forming a dense tissue in the 

 centre. Algae possessing this type require a rough surface, and are commonly 

 found on barnacles. 



Many transitions exist between the various forms of holdfasts, and the 

 subject, which is of great importance in connexion with the rock-surface, 

 requires detailed study. Epiphytes are adfixed in a number of ways ; see 

 Tobler ('06), Menz ('10), and Delf (12). In spite of the above exceptions, the 

 majority of the algae forming associations on a rocky shore are attached by a 

 stout disk (Fucus, Porphyra, Rhodymenia). 



