16 THE WEEDS OF NEW SOUTH WALES. 



(n) Some weeds disappear naturally. 



This may be attributed to two separate causes, or to a combination of 

 them. 



1. In course of time they exhaust the soil of the substances necessary for 

 their healthy existence, and so perish. 



2. Absence of rain at a critical period of the weed's history. It is not the 

 total rainfall of a district that counts, but whether it falls at periods critical 

 for the development of the plant. For example, at the period of germina- 

 tion, a little later to make leaf, a little later to form the flower and plump 

 the seed. 



Ten times as much raiu may fall as is necessary for the absolute require- 

 ments of the plant, yet, if there are lengthy periods between the falls, 

 development may be arrested, and either the weed may die in that particular 

 season or it may not form seed for the next. In any case the weed dis- 

 appears, and, I think, this lack of fortuitous rainfall is the key to the so- 

 called mysterious disappearance of weeds in certain areas. 



In the Agricultural Gazette for January, 1896, p. 39, Mr. John F. Tabrett 

 gives an example of the migration of the common Black Thistle (Carduus 

 lanceolate) from a locality in the Hartley district where it was formerly 

 abundant. 



In Wallace's " Island Life," 3rd Edition, p. 513, is an instructive account 

 of the temporary establishment of weeds which are (usually) later on over- 

 come by the surrounding indigenous vegetation. 



It is a matter of common knowledge that weed-infested areas become 

 clean again or replaced by other weeds, and my readers are invited to record 

 cases which have come under their notice. If land once infested by weeds 

 were always infested, large areas of ground would be permanently in this 

 condition except through the restless intervention of man. 



Weeds will destroy each other if kept on the same block of land without 

 outside interference. For example, it is sometimes the practice to fence a 

 piece of land infested with Bathurst Burr (Xanthium spinosum), so as to 

 prevent any interference with the plants. Left alone they grow luxuriantly 

 for a time and then, struggling for existence on the same plot of soil, the 

 plants become debilitated, and the weeds die out in patches and sometimes 

 altogether. 



SECTION 5. MISCELLANEOUS. 



(a) Annual and perennial weeds, 

 (fo) Native plants as weeds. 



(c) Places that harbour weeds. 



(d) Weed legislation. 



There are three useful Australian weed manuals, as follows: 



1. " The Naturalised Flora of South Australia," by J. M. Black, Ade- 



laide, published by the author. 



2. " The Weeds, Poison-plants, and Naturalised Aliens of Victoria," 



by Alfred J. Ewart, assisted by J. E. Tovey, published by the 

 Government Printer, Melbourne. 



3. " The Weeds and Suspected Poisonous Plants of Queensland," by 



F. M. Bailey, published by H. Pole & Co., Brisbane. 



