Glare Island Survey — Marine Algae. 15 35 



higher than on bare rocks. If the shade be not too dense, its growth is quite 

 as luxuriant as when fully exposed to light and air. 



Another species, L. hybrida, is a characteristic element in the upper part 

 of the association. This plant is more restricted in its range, and appears to 

 be more sensitive to desiccation. On a flat shore it reaches its maximum 

 development somewhere about the middle of the L. pinnatifida band, where 

 it is at times eo-dominant. In the lowest parts it is practically absent; and 

 in the upper it is found only in the damper positions. On very exposed shores 

 L. hybrida is scarce (see p. 136). 



L. pinnatifida reaches its maximum development about April or May ; 

 and at this season sloping rocks are carpeted with a growth 4 to 6 inches high. 

 After May many of the larger fronds disappear, and the association as a 

 whole becomes inconspicuous. There remain, however, numerous small 

 shoots, which are continually augmented during the summer, the plant being 

 apparently perennial. L. hybrida, on the other hand, is an annual. Young 

 sporelings 1 to 2 mm. long are discernible in September ; they develop during 

 winter, fruit in spring, and disappear in May. 



Very marked colour-changes due to illumination are shown by L. pinna- 

 tifida, and to a less extent by G-igartina. The normal colour of the former 

 is a deep purplish red; but during summer the growth in sunny situations 

 is pale yellow green. This gradually gives place to purplish green in 

 autumn, which becomes redder as the days shorten. The production of 

 antheridia by Laurencia in March and April gives the plants a golden hue, 

 owing mainly to the yellow substance found in the antheridial cavities. As 

 a large number of male plants are often found together, sheets of bright yellow 

 fronds are frequent. 



Callithamnion arbuscula Association. 



The plant-community formed either by Callithamnion arbuscula, or 

 C'eramium acanthonotum, or a mixture of both, has been described by several 

 writers under various names. Callithamnion is the more conspicuous plant, 

 and most writers have dealt largely with this species ; Ceramium acanthonotum 

 is, however, an equally important element. Clare Island is peculiar in 

 possessing in addition a small quantity of the southern Callithamnion 

 gro/iiulatum. 



The association is well developed on many parts of the island and exposed 

 mainland. The relative amount of the two species varied during the 

 different seasons — a fact which probably explains discrepancies in accounts of 

 the same area by different writers. In July, 1910, C'eramium acanthonotum 

 was the dominant species on Clare Island, whereas in August, 1911, 



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