Parsons : Alpine Plants on Lowland Heaths. 



115 



menon, is nevertheless not an unprecedented one, although I do not 

 remember any instance of plants so characteristically alpine as Lyco- 

 podium alpinum and Getraria islandica having been so found. For 

 example, two allies of the latter plant, Cetraria aculeata and Platysma 

 glaucum^ whose usual habitat is mountain rocks, grow on Riccall 

 Common — a low-lying sandy heath between Selby and York ; Baco- 

 mitrium lanuginosum, an abundant and characteristic moss of mountain 

 districts, grows sometimes on heaths in the plains, as in Norfolk 

 (Hooker, Engl., Fl., pt. 5, vol. 1), and, as I am informed, at Strensall 

 Common, near York. 



The occurrence of plants in outlying stations at a distance from the 

 territory usually inhabited by them, may be explained in two ways — 

 either by supposing them to be intruders, introduced by some means 

 into the situation which they now occupy, or by looking upon them 

 as stragglers left behind in the general retreat, and able to hold their 

 ground under favourable circumstances against the invading host of 

 new species brought in by a change in physical conditions. Which 

 explanation are we to accept in the present case ? I think, with Dr. 

 Lees, the second. It is one of the advantages which the Cryptogamia 

 afford to the student of geographical botany, that with them he may 

 usually dismiss from his mind the supposition of their being artificially 

 introduced by man's agency into the stations where he finds them. 

 Of natural agencies by which they might have been introduced, there 

 is one which at first sight looks not improbable : could they have been 

 brought on boulders transported by ice from the mountain districts of 

 the north of England and Scotland ? Alpine species of mosses are 

 known to occur on the boulders derived from the Scandinavian 

 mountains, which lie scattered about over the level plains of North 

 Germany. I do not think, however, that this is the explanation. In 

 the vale of York at any rate there is this difficulty — that the boulder 

 clay is separated from the surface of the soil on which the mountain 

 plants grow by a considerable thickness of newer strata, gravel, 

 laminated clay (in some places nearly 60ft. thick), and sand ; it is 

 upon the sand, or a bed of peat above it, that the plants of which I 

 have spoken grow. The boulder clay, however, comes to the surface 

 within a few miles, but it is not very likely that seeds or spores would 

 retain their vitality after so long a submersion and lapse of time as 

 the intermediate strata indicate to have taken place. 



The explanation which I should offer is, that these plants formerly 

 extended over an area wider than that which they now occupy, but 

 that, under alter.ed physical aad climatic conditions, they have been 



