Cockayne. — Plants' of Chatham Island. 307 



racecourse before described, although it has become a dis- 

 tinct component of that recent plant-formation, id is no more 

 dominant than some of its indigenous competitors. On dry 

 slopes, where fires are constantly opening up room for the 

 advent of introduced xerophytes, none have yet arrived which 

 can make the slightest headway against Pteris esculenta. 



Certain other causes distinctly operate in checking the 

 spread of introduced plants, amongst which may be men- 

 tioned — the small area of cultivated land ; the absence usually 

 of roads, there being merely horse-tracks over the greater 

 part of the island ; the small amount of traffic with other 

 countx*ies ; and, finally, the large number of sheep which 

 graze on such land as introduced plants could best establish 

 themselves on. 



Effect of Fiees on the Plant-covebing. 



Of all the factors which have changed the plant physiog- 

 nomy of Chatham Island, fires have been by far the most 

 important. In order to provide young growths of grass for 

 his sheep, the farmer sets fire to the bracken fern of the heath 

 or the Dracophyllum of the bog. Such fires in dry weather 

 spread over very large areas, and the whole of the vegetation 

 is burnt right down to the ground, leaving only the blackened 

 bases of the plants. This destruction leads to the spreading 

 of certain plants which had been kept in check by others. It 

 also leads indirectly, especially when aided by the trampling 

 of stock, to the drying up of the ground in wet places. 



Of all the plants which gain an ascendency after burning 

 none can approach Pteris esculenta. As burning succeeds 

 burning so does the Pteris increase, until at the present time 

 it must occupy a very much larger area than it did originally ; 

 indeed, it seems to me hardly an exaggeration to affirm that 

 it occupies ten times its original area. In this opinion I 

 am supported by Mr. E. R. Chudleigh, who tells me that 

 " bracken has increased enormously since the advent of 

 the white man, and, owing to burning, stocking, and other 

 causes, it has replaced much of the original vegetation." 

 Even after burning the Sphagnum formation bracken takes 

 the place of Gleichenia circinata, and with repeated burnings 

 the Sphagnum altogether disappears and the ground becomes 

 dry and covered with a thick mantle of bracken. Mr. D. 

 Petrie also writes (50, p. 323), speaking of the spread of 

 certain plants in the Auckland Province : " The most aggres- 

 sive plant of all is Pteris aqivilina, which is rapidly over- 

 running much of the land that has been cleared of bush, and 

 which permanently establishes itself before roots are suffi- 

 ciently decayed to admit of ploughing." 



It is only by studying the plant-formations of the table- 



