CHAP. XXIII ARCTIC PLAXTS IX XEW ZEALAXD 513 



intervals on which they might temporarily establish them- 

 selves. The facilities afforded for the transmission of plants 

 by mountains has hardly received sufficient attention. The 

 numerous land-slips, the fresh surfaces of broken rock and 

 precij^ice, the debris of torrents, and the moraines deposited 

 by glaciers, afford numerous unoccuj^ied stations on which 

 wind -borne seeds have a good chance of germinating. It is 

 a well-known fact that fresh surfaces of soil or rock, such 

 as are presented by railway cuttings and embankments, 

 often produce plants strange to the locality, which survive 

 for a few years, and then disappear as the normal vegeta- 

 tion gains strength and permanence.^ But such a surface 



^ As this is a point of great interest in its bearing on the dispersal of 

 plants by means of mountain ranges, I have endeavoured to obtain a few 

 illustrative facts : — 



1. ]\Ir. William Mitten, of Hurstpierpoint, Sussex, informs me that when 

 the London and Brighton railway was in progress in his neighbourhood, 

 Melilotus vulgaris made its appearance on the banks, remained for several 

 years, and then altogether disappeared. Another case is that of Biplotaxis 

 muralis, which formerly occurred only near the sea-coast of Sussex, and at 

 Lewes ; but since the railway was made has spread along it, and still 

 maintains itself abundantly on the railway banks though rarely found 

 anywhere else. 



2. A correspondent in Tasmania informs me that whenever the virgin 

 forest is cleared in that island there invariably comes up a thick crop of 

 a plant locally known as fire-weed — a species of Senecio, probably S. Aus- 

 tralis. It never grows except where the fire has gone over the ground, 

 and is unknown except in such places. My correspondent adds : — "This 

 autumn I went back about thirty-five miles through a dense forest, along 

 a track marked by some prospectors the year before, and in one spot 

 where they had camped, and the fire had burnt the fallen logs, &c., there 

 was a fine crop of 'fire-weed.' All around for many miles was a forest of 

 the largest trees and dense scrub." Here we have a case in which burnt 

 soil and ashes favour the germination of a particular plant, whose seeds 

 are easily carried by the wind, and it is not difficult to see how this 

 peculiarity might favour the dispersal of the species for enormous distances, 

 by enabling it temporarily to grow and produce seeds on burnt spots. 



3. In answer to an inquiry on this subject, Mr. H. C. Watson has been 

 kind enough to send me a detailed account of the progress of vegetation 

 on the railway banks and cuttings about Thames Ditton. This account is 

 written from memory, but as Mr. Watson states that he took a great 

 interest in watching the process year by j'ear, there can be no reason to 

 doubt the accuracy of his memory. I give a few extracts which bear 

 especially on the subject we are discussing. 



" One rather remarkable biennial plant appeared early (the second year, 

 as I recollect) and renewed itself either two or three years, namely, Isatis 

 tinctoria — a species usually supposed to be one of our introduced, but 

 ]iretty well naturalised, plants. The nearest stations then or since known 

 to me for this Isatis are on chalk about Guildford, twenty miles distant, 



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