1 905-1906.] Formation and Flora of a Shingle Island. 307 



these differences of flora, it will serve our purpose best to 

 omit strays and restrict ourselves to those plants that are 

 growing really well. First, then, why are the following 20 

 plants flourishing in the fields or the riparian strip, and 

 missing or very poor on our island ? — Ranunculus Flammula, 

 Viola tricolor. Lychnis Flos - cuculi, Spergula arvensis, 

 Trifolium dubium, Chrysanthemum segetum, Crepis virens, 

 Calluna Erica, Solanum tuberosum, Euphrasia officinalis, 

 Galeopsis versicolor, Teucrium Scorodonia, Polygonum Per- 

 sicaria, Mercurialis perennis. Orchis mascula, Habenaria 

 conopsea, H. bifolia, Avena sativa, Athyrium Filix-foemina, 

 and Lastriea Filix-mas. 



Of these 20 species, Solanum tuberosum and Avena 

 sativa are cultivated plants ; and Viola tricolor, Spergula 

 arvensis, Crepis virens, Chrysanthemu.m segetum, and Galeopsis 

 versicolor may fairly be considered weeds of cultivation ; 

 Lychnis Flos-cuculi, Trifolium dubium, Calluna Erica, 

 Euphrasia officinalis, Mercurialis perennis. Orchis mascula, 

 Habenaria conopsea, H. bifolia, Athyrium Filix-foemina, and 

 Lastrcea Filix-mas grow either in pasture, meadow, heath, 

 or wood. All these 17 species, then, are accustomed to very 

 different soil from that provided by the coarse shingle or 

 pure sand of our island, which would clearly be an unsuitable 

 habitat for them at present. 



There is probably too little moisture for Pianunculus 

 Flammula, Lychnis Flos-cuculi, Orchis mascula, Habenaria 

 conopsea, H. bifolia, Athyrium Filix-foemina, and Lastraea 

 Filix-mas ; and too little shade for the two last named, and 

 for Mercurialis perennis. 



Six of our 20 species — Spergula arvensis. Chrysan- 

 themum segetum, Solanum tuberosum, Galeopsis versicolor. 

 Polygonum Persicaria, and Avena sativa — were rather far re- 

 moved from the island ; and it may be that, in the absence 

 of special facilities for dispersal, they were really too far 

 off to reach it.^ In their case, then, difficulty in reaching 

 the island may be the true explanation of their absence from 



^ We are told, however, that "corn " was grown close to the island three years 

 ago ; and that fact would probably have enabled Spergula arvensis. Chrysan- 

 themum segetum, Galeopsis versicolor, and Avena sativa to establish themselves 

 on the island had it been a suitable habitat. 



