322 Transactions. — Botany. 



usually so powerful in taking possession of the soil." This is 

 a rather overstated case. The Hypochceris, or cat's-ear, 

 usually misnamed Cape-weed with us, is only troublesome 

 where sheep cannot get at it. But on sheep-runs and farms 

 on which sheep are fed the plant disappears, as these animals 

 eat it down to the ground, and so completely eradicate it. 



The following notes by Mr. D. Petrie are supplementary 

 to the above paper : — 



(1.) A leading fact which might have been emphasized 

 more is that the spread of weeds is mainly due to useful 

 plants — their competitors — being regularly checked and eaten 

 down, while the weeds are mostly allowed to grow without 

 check of any important nature. Almost all weeds found in 

 •our northern pastures owe their spread to this — e.g., several 

 buttercups, numerous docks, pennyroyal, Holcjis mollis and 

 H. lanatus, and many weedy grasses, various spurges, mallows, 

 mulleins, and so forth. In many cases their spread is facili- 

 tated by the ready germination of their seeds, by the long 

 time that the seeds retain their vitality in the soil, and by the 

 readiness with which their earliest roots strike deep down into 

 the soil, which allows the plants to establish themselves in 

 hot, dry weather. Black medick (Medicago lupulina), meadow 

 plantain (Plantago lanceolata), the docks and spurges, all 

 start and thrive in hot, dry weather, when more superficial- 

 rooting seedlings die off. The introduced speedwells and poor 

 man's weather-glass (Anagallis arvensis) are much in the 

 same case. 



(2.) The decline of plants that have taken possession of a 

 district for some years is no doubt due to temporary exhaus- 

 tion of some element of plant-food needful for their vigorous 

 growth. This principle lies at the base of the theory of the 

 rotation of crops. In Central Otago. when I first knew it, 

 Carduus lanceolatus was the prevailing weed on open downs 

 and dry hill-slopes. Some years after C. paiccifloras com- 

 pletely replaced it, and this will, no doubt, be now giving way 

 to something else. The doctrine that the Scandinavian plants 

 possess extraordinary vigour, which is the cause of their 

 aggressive character, seems to me very doubtful. In each 

 single species particular advantages can generally be assigned 

 that will readily explain their rapid spread. In the peninsula 

 north of Auckland there are very large areas of land on which 

 European weeds have but slightly established themselves, 

 though the ground is frequently cleared of all native vegeta- 

 tion by fires. In these areas native plants mostly grow up 

 with great readiness, especially species of Leptospermum and 

 Pomaderris, Haloragis tetragyna and H. minuta, besides 

 various cyperaceous plants. The pre-eminence in aggressive 



