15 '- 1 Proceedings of the Royal Irish Academy. 



we find that in fully exposed localities it is composed of three species only 



namely — Pelvetia, F. spiralis, and F. vesiculosus, var. cvcsiculosus, a precise 

 combination which has not previously been described, unless it be that referred 

 to by Boye ('94-5). Pelvetia forms a narrow zone about li-2 feet deep. 

 /-'. spiralis follows with a band only slightly deeper, and composed of fronds 

 seldom more than 6 or 8 inches in length ; whilst the lowermost zone of 

 l\ vesiculosus, var. cvcsiculosus, has a greater bathymetric range, and therefore 

 covers a wider horizontal surface. The fronds are very narrow and the 

 growth short, though, except in the most boisterous spots, it is longer than 

 F. spiralis. This type of vegetation is found more or less along the whole 

 south shore of Clare Island, where there is full exposure ; and it forms a band 

 covering the entire upper part of the littoral region. The shore is for the 

 most part steep ; hence the bands are narrow. With a natter shore and less 

 exposure, the association spreads out horizontally, and other differences are 

 noticeable as mentioned later. 



Of the three species, Pelvetia is perhaps the most susceptible to severe 

 exposure, and F. spiralis the least so. In a general way Pelvetia forms a band 

 just above the ordinary high- water level, but it is splashed daily, except in spells 

 of exceptionally calm weather. In very open spots, or where waves break 

 and there is much spray, it ascends much higher. Everywhere it forms a band 

 that is very conspicuous in summer and autumn by its orange-yellow 

 receptacles. Owing to their bathymetric position, the two species of Fuci, 

 unlike Pelvetia, feel the full force of the waves, but it is only in places with 

 the severest exposure that the association is wanting. The attachment to the 

 rock is exceedingly secure, and, though often torn by the waves, none but old 

 and worn-out specimens become detached. Several spots were noted where 

 F. spiralis only occurred, and it appeared that the exposure was too great 

 for F. vesiculosus. When fruit is absent it is difficult to distinguish the two 

 species ; but in fertile specimens the short, rounded, hermaphrodite fruits of 

 F. spiralis, and the narrow, pointed, unisexual receptacles of F. vesiculosus 

 separate them at once. On exposed coasts the former plant fruits from spring 

 to late autumn, the latter in winter, spring, and early summer. 



Moderately exposed Coasts. — On passing to less exposed rocks or to bays 

 where there is a measure of shelter, a difference in form and in composition is 

 at once apparent. Pelvetia remains the same, but takes a somewhat lower 

 level. I. spiralis becomes longer and the fronds wider, but still retains its 

 typical spiralis form. F. vesiculosus is also larger, and often possesses a few 

 bladders, the size of the frond and the number of bladders increasing with 

 the amount of shelter. A fourth species, F. serratus, now enters the association. 

 This begins in the lower part of the F. vesiculosus zone (slightly above 



