THE DISTRIBUTION OF PADINA PAVONICA 5 



implies a potentially normal life-history. By contrast, Umezaki & Yoneda (1962) and Allender 

 (19770, b) indicated the presence of occasional non-meiotic spores in Padina spp. This apparent 

 incompleteness in British P. pavonica could be a phenomenon of superficial and insufficiently 

 persistent observation at appropriate (possibly very short) periods and in appropriate places 

 (? perhaps the infralittoral) rather than the reflection of complete absence of the gametangial phase 

 from British shores. Indeed, there is some rather dated evidence of the existence of antheridial 

 material, collected in September 1892 (Weymouth, BM slide 9535) and in September 1894 

 (Chapman's Pool, 9538; Sidmouth, 9539). These are the only traced authentic examples of 

 gametangial material from British shores. Since culture methods have not been employed to 

 complete the life-history, we have been unable to add more recent British records of antheridial 

 or oogonial plants. We are certainly able to confirm field rarity in all commonly exploited eco- 

 logical niches and seasons. 



This situation provides an interesting parallel, in view of the confirmed gametangial rarity, with 

 that detailed by Edwards (1973) for certain species of Ceramium in the northern parts of their 

 distribution ranges, and mentioned in general terms for Padina by Thivy (1959), Umezaki & 

 Yoneda (1962), Liddle (1971, 1972, 1975), Gaillard (1972, 1973), Fagerberg & Dawes (1973), and 

 Allender (19770, b}. Edwards was commenting on Dixon's (1965) hypothesis of '. . . physiological 

 expression of reproductive capacity . . .'; this suggests that sampling successively further from a 

 centre of distribution of a species would initially detect general production of gametangial and 

 sporangial material, grading through loss of gametangial and then sporangial production, to a 

 terminal peripheral zone of solely vegetative material, all reproductive potential being inhibited 

 and maintenance of populations depending on vegetative fragments or spores carried from areas 

 nearer to the distribution centres. Edwards found that (a) he could produce in culture both male 

 and female gametophytes from tetrasporophytes, using plants from the northern part of the 

 range of C.flabelligerum J. Ag. ; and (b) there appeared (rare) carposporophytic specimens in field 

 material of C. shuttleworthianum (Kiitz.) Silva, from two localities in the northern periphery of its 

 range. The latter establishes the field occurrence, however spasmodic, of fertile gametophytes, 

 which Edwards was also able to demonstrate in cultures isolated from one of the two localities (the 

 other could not be tested further due to culture contamination). Edwards therefore expressed the 

 view that, at least in the species tested and possibly in general terms, the constraints on realisation 

 of reproductive (gametangial) potential are not entirely inbuilt and physiological but are imposed 

 from without by the ambient environment. This constraint is not always complete as occasional 

 carposporophytic specimens can be detected by detailed study. Fagerberg & Dawes (1973) 

 examined down to organelle levels the morphology of gametophytes and sporophytes, from 

 Florida, of P. vickersiae; there the gametophytes habitually develop during the period November 

 to March, alongside sporophytes which are present throughout the year. Gametophytes and 

 sporophytes were isomorphic to a very high degree, the only differences (the gametophyte possess- 

 ing nucleoli and lacking osmiophilic globules) being interpreted by Fagerberg & Dawes as indi- 

 cating more active growth in the gametophytes. Allender (1977a) noted considerable behavioural 

 differences between sporophyte and gametophyte of Padina japonica Yamada, as well as pro- 

 nounced differences in cell dimensions. Sporophyte cells were significantly larger and thicker- 

 walled than those of gametophytes from the same locations on Oahu (Hawaii). Gametophytes 

 never formed more than 7 % of the populations sampled and were usually fewer than 4 %, depend- 

 ing on time of year. Nevertheless, gametophytes grew faster than sporophytes in a temperature 

 (20 C) lower than area ambient ; in extremely high light conditions ; and in all levels of water move- 

 ment, although they were more susceptible to being damaged or destroyed where the latter was 

 appreciable. Possibly plants in more rapid active growth are more demanding as to environmental 

 characteristics, leading to sterility in or eradication of the haploid state unless the environmental 

 balance is entirely favourable. Quite what this means in terms of life-history is not clear and, as 

 indicated by the authors, a great deal more ecological, physiological, and biochemical work is 

 required. In general terms, data from Edwards, Fagerberg & Dawes, Allender, and others support 

 the Dixon hypothesis, as do all traced data for Padina reproductive pattern in the present area. 

 Although a zone of solely vegetative P. pavonica appears to be lacking at the range periphery here, 

 the proportion of even tetrasporangial plants detectable is relatively small by comparison with 



