Glare Island Hurvc;i — Marine Algae. 15 43 



though Bporelinga and young plants of a very large number of species occur. 

 In Vancouver, Henkel ('06) has worked on the structure of rock-pools ; and 

 Skinner's paper ('03) is one of the only attempts to analyse the flora and the 

 conditions obtaining. His conclusions, equally applicable to the British coasts, 

 are as follows:— In general, the higher the elevation of the pool, and the 

 less exposed to wave-action, the fewer the species found, though the number 

 of individuals may be great. The more gradual the slope, and the rougher 

 and more irregular the sides, the more abundant the plant-life. A perpendicular 

 or receding wall is unfavourable for the location of plants. 



An attempt is made below to indicate the characteristic species of ordinary 

 rock-pools not more than 18 inches deep. A complete list is out of the 

 question. In a general way they are at their best in early spring. During 

 the dark days of winter many of the Florideae, which at other seasons thrive 

 in more shady spots, are found in shallow pools (e.g. Phyllophora rubens, 

 Ploeamiv.m eoceineum, Laurencia obtusn, Rhodomeht subfusca, Halwrua 

 equiseUfolius). Hence in winter and spring the Florideae are markedly 

 dominant. In March and April many transient species develop ; and in May 

 and June brown algae* are abundant, and also species of Cladophora. The 

 Florideae in shallow pools are in summer of poor colour; and a great decrease 

 in the number of species is noticeable. Ulva and Enteromorpha spp. 

 may be dominant in August and September. Excluding large species, such 

 as Fucus, Laminarid, &c, and small epiphytes like Myrionema, Elachista, 

 and Chantransia, the most typical species are as indicated in the list on 

 pp. 44, 45. 



Cave-Vegetation. 



As would be expected from the conditions obtaining in caves, shade-loving 

 littoral plants abound; whilst in addition species from the sub-littoral region 

 also occur. Provided wave -action is not too severe, the cave-flora is often 

 full of interest biologically, long tunnel-like caves, with pools and boulders, 

 being particularly instructive. 



Though the Clare Island caves are neither numerous nor extensive, their 

 flora is characteristic and worthy of record. They may be roughly divided 

 into two sets, namely, exposed and sheltered. The flora of caves fully 

 exposed to the sea is usually poor, incrusting species on the walls being at 

 times the only vegetation present ; but sheltered caves, especially if stocked 

 with boulders, are rich in species, and possess the shaggy growth of which 

 Plumaria is typical, as well as the mossy and incrusting coverings supplied 

 respectively by Rhodochorton Roth ii and Litliothamnium polymorpluim. 



