308 Formation and Flora of a Shingle Island. [Sess. 



it; but in the case of the other 14 species there is apparently 

 no such difficulty. 



We have not accounted for the absence of Teucrium 

 Scorodonia. 



Summing up, then, the various circumstances responsible for 

 the absence of these 20 species from the island, we find 

 that unsuitability of soil seems to be a factor in 17 cases; 

 lack of moisture or shade in 8 ; and perhaps difficulty of 

 access in 6 cases. 



Why cei'tain plants thriving on the island are absent or 

 poor in the fields and the riparian strip. — We come now to the 

 11 species more or less peculiar to the island — namely, Silene 

 acaulis, Alchemilla vulgaris, Saxifraga aizoides, Aster hybrid, 

 Tanacetum vulgare, Cnicus heterophyllus, C. arvensis, Mentha 

 sylvestris. Polygonum cuspidatum, Oxyria digyna, and Tritonia 

 crocosmiflora. Why are these flourishing on the island and 

 absent or poor in the fields and riparian strip ? 



Five of them — the Aster hybrid, Tanacetum vulgare, Mentha 

 sylvestris, Polygonum cuspidatum, and Tritonia crocosmiflora 

 — are escapes from gardens bordering on the river ; ^ and 

 four — Silene acaulis, Saxifraga aizoides, Cnicus heterophyllus, 

 and Oxyria digyna — are mountain plants, the last three of 

 which habitually grow by streams. It is pretty clear that 

 all these 9 species owe their presence on the island to 

 the fact of having been brought down by the river and its 

 tributaries. The island, owing to its position, would have a 

 much better chance than the river-bank of intercepting water- 

 borne plants. 



Three of the 1 1 species — Silene acaulis, Saxifraga 

 aizoides, and Oxyria digyna — would not find the fields or 

 the riparian strip a suitable habitat; but there seems no 

 reason why the remaining 8 species should not grow there 

 if they got the chance. Cnicus arvensis would certainly do 

 so, had it not been carefully eradicated. We are told that 

 Tritonia crocosmiflora has in fact established itself on the 

 river - bank some distance lower down. Why Alchemilla 

 vulgaris is not growing better in the fields and the riparian 

 strip, it is not easy to say ; but this species habitually grows 



^ Four from gardens ou the bank of the Orchy itself ; and one, Tritonia 

 crocosmiflora, from a garden on the bank of the Donachain burn. 



