THE DISTRIBUTION OF PADINA PAVONICA 55 



historically distant past, then the sites of primary introduction seem likely to have been in Dorset 

 or Devon, from the favourable ambient conditions and the size and vigour of populations and 

 individuals there. Certainly the east Devon focal area of Padina possesses by far the longest 

 history of being recorded in publications, although specific populations there may have fluctuated 

 widely within particular locations. The available evidence does not permit meaningful comment on 

 the time scales involved in any changes. 



Reproductive patterns 



Demonstrably, Dorset, east Devon, and the Isle of Wight form the main foci of the present distri- 

 bution pattern, and al) these have firm, long- and well-established tetrasporangial and vegetatively 

 per ennating populations. Suitable areas peripheral to and/or within the foci carry some populations, 

 generally small, that are ephemeral and appear and disappear over variable periods, perhaps as a 

 result of the isolated or combined effects of changes in configuration or substrata on shore; of 

 temperature variations (see Ayr); of the effects of population size or population vigour; or of 

 similar phenomena. The coincidence of tolerable environmental conditions with an adequate 

 concentration of viable spores seems likely by chance to be a rare event, especially in areas outside 

 any of the well-established foci at the periphery of the general distribution range. Near that 

 periphery of its distribution, the specific habitat requirements of Padina pavonica are more rarely 

 satisfied by less abundantly available tolerable conditions present over a lesser vertical amplitude 

 than in the major centres. The alga thus shows signs of even greater dependence on local vegetative 

 perennation and spread than on spore formation. Observations by Norris (op. cit.) and in this 

 paper indicate that those individuals or lobes of largest size and/or most maturely reproductive are 

 generally found towards the centre of clumps of specimens ; perennation through adverse conditions 

 by rhizomatous portions has previously been discussed (see p. 4). Both these observations point 

 to the importance of vegetative perennation and spread in British populations. Plants in the 

 British foci of distribution are very rarely sexually (gametangial phase) reproductive and only 

 restrictedly (July-October) tetrasporangial in large numbers; this would seem to place a limit on 

 peripheral new colonisation by spore saturation except in the most favourable circumstances. 

 Recruitment of spores within the distribution foci that exist, however, may be more effective. 



At least at some times of the year (and it may be significant that it is in the months before the 

 onset of the usually least favourable time of year for the erect alga) the extent of tetraspore 

 development is locally adequate to supply whatever is required as periodic replenishment and re- 

 invigoration for the otherwise largely vegetatively maintained populations. Depletion of the main 

 vegetatively reproducing basis of the local population could possibly in this way be offset so that 

 size (? numbers or density) and vigour do not usually fall below the level required for population 

 maintenance. This postulated ideal is not always realised in practice, as population loss and gain 

 within and without the focal areas of the south clearly demonstrates. The optimal balance, what- 

 ever that may locally be, between spore supply and vegetative vigour is by no means always 

 achieved. The circumstances favourable to new colonisation do occasionally occur and can be 

 effective; this is attested by the recent ephemeral find in southern Ireland. The same conclusion 

 is suggested from what seem to be past variably ephemeral populations in Sussex, Kent and 

 Essex, and perhaps in Cornwall, Lancashire, Ayr, and north and south Wales. The reports from 

 Ayr and from comparable situations in Europe are not easy to accept as having been based on 

 attached material. 



The assessment of these populations as ephemeral is to a degree speculative. No importance has 

 usually been attached to absence of data before the first record (except as indicated within county 

 entries) nor, generally, to similar absence between records unless there has been traced comment 

 or data showing unusual significance of the absence. 'Ephemeral' therefore indicates only that 

 there are a few authenticable past records, variably spaced throughout time, from an area where 

 plants cannot now be traced. 



Validity of earlier data 



It has been rightly remarked of Padina (Harvey, 1 847) that on the whole '. . . it is difficult to imagine 

 what could have been mistaken for it, so different in appearance is it from all other Algae'; even 



