58 L. COCKAYNE 



subalpine scrub association. But cultivate 0. cymbifolia in moist 

 air and dim light or let it grow in the open in excessive shade, 

 and the boat-shaped leaf is replaced by one that is flat. Here 

 certainly there is greater stability of form than in the species of 

 Gleichenia dealt with above, and the question arises as to what 

 degree of constancy may be required in plants generally as a 

 specific mark. 



Up to the present we have dealt only with examples of natural 

 distribution where the species have worked out their destinies 

 without the disturbing influence of man, but in the case of the 

 next fern to be spoken of, Pteridium aquilinum var. esculentum, 

 its present distribution, even in recently colonized New Zealand, 

 is certainly no measure of the part it played in the primeval vege- 

 tation. 



The plant in question is confined to the Southern Hemisphere, 

 and, though it may be considered a geographical species, the dis- 

 tinctions between it and the cosmopolitan P. aquilina are but 

 slight. As to the abundance of the plant in virgin New Zealand, 

 I am of opinion that, though it might be more or less common, it 

 did not form that aggressive, almost pure, association which is the 

 dominant feature of many parts of the region at the present time. 

 No sooner is the forest destroyed in some localities, the heath 

 burned or the moor drained ever so little than, as if by magic, 

 the bracken fern gains possession of the ground. "Fern" to the 

 settler is a specific term and in a land of ferns refers only to the 

 bracken, which he knows to his cost is so difficult to eradicate. 

 Where a bracken heath is well developed, it is a trial both to one's 

 patience and muscles to force a path through the entangled mass 

 of fronds, at times more than man-high. 



The great spread of the plant is due to its rapid vegetative 

 increase, its easy dissemination by spores, its capacity for reproduc- 

 tion after fire, and, not least, its rapid response to change in outer 

 stimuli. Thus, according to reaction to wind, moisture, and light, 

 it speedily varies from a strong xerophyte to almost a hygrophyte, 

 so far as form and structure of leaf go. Less well known is the 

 fact that it can become a scrambing and almost a twining liane. 

 Mr. H. Carse and myself recently found a number of climbing 



