THE NATURALISED PLANTS. 129 



increased, so would the best-suited species increase. All this is to 

 say that many plants are potential weeds, ready to become active 

 ones as soon as suitable conditions arise. Thus, through the long 

 centuries of cultivation in Europe, aided by the ever-expanding inter- 

 course with other lands, the great army of now almost cosmopolitan 

 weeds has been gathered together the very pick of the vegetable 

 world for thriving under the artificial conditions imposed by man 

 in temperate regions. It is the old story that when one interferes 

 with nature she exacts remorselessly her tribute. 



It is only in a virgin vegetation that we can actually witness the 

 evolution of a weed. Our own flora has furnished some rather striking 

 examples. The common bracken-fern (Pteridium csculentum) is a 

 case in point. This plant can be transported over great distances 

 by means of its tiny spores, which, light as the finest dust, are carried 

 for many miles by the wind. It has a stout far-creeping underground 

 stem full of nutritious starch. Consequently, when the farmer burns 

 its leaves and the grasses and other plants in its neighbourhood, the 

 stem remains unhurt beneath the ground, and from its store of food 

 can soon construct fresh leaves. Owing to their form and internal 

 structure, these are capable of enduring considerable drought, so 

 that the plant need not be at all concerned about its water-supply ; 

 consequently, all things considered, the bracken is a fair example of 

 a potential weed. Nor does it belie this expectation, for at the present 

 time this fern, worthless from the farmers' point of view, is far more 

 abundant than in primeval New Zealand. 



So, too, with the manuka (Leptospermum scopanum)., thanks to 

 its abundant and quickly germinating seeds, its hard seed-capsules 

 that are not easily destroyed by fire, the early blooming of the seed- 

 lings, its leaf-form and structure, its toleration of most varieties of 

 soil, and its extreme plasticity with regard to changes of environment. 

 Miles of manuka now exist where originally forest or a mixed shrubbery 

 flourished. 



Several species of piripiri (Acaena Sanguisorbae various varieties, 

 A. novae-zealandiae) have become troublesome weeds. In their case 

 the barbed 'seeds"* are easily carried by domestic animals, espe- 

 cially sheep and dogs. On bare ground they readily germinate, 



* Really a dry fruit which contains one seed, and does not split open at 

 maturity. The calyx, which is furnished with several barbed bristles, remains 

 attached to and encloses the fruit proper. 



