BY JOHN SHIRLEY, D.SC, AND C. A. LAMBERT. 41 



'Cilternates with ])eriod.s of div weather, or even of drought, 

 ill which there is little trouble from frost ; where there is a 

 depth of soil not less than ten inches in thickness, containing 

 a fair percentage of lime ; and where there is a rainfall 

 foi' the year of at least ten to twelve inches. All these 

 conditions are provided in the greater part of Queensland 

 and Northern X.S. Wales, hence the rapidity with which 

 introduced ]>lant,s of this family have sjjread in these 

 States, as well as in countries providing like conditions — 

 S. India, Ceylon, S. Africa, etc. 



An introduced plant in a new country has many 

 advantages over native species. It has left its natural 

 enemies behind, and the insects or mammals that are likelv 

 to prey upon it are some time before they discover its 

 uses. It therefore thrives and spreads at this stage more 

 readily than in its native haunts. But the Cacti are 

 specially adapted to ward off enemies. Those that are 

 proving pests in Australia have no true leaves ; the 

 stems are flattened, they retain their epidermis, and are 

 green because their cells are furnished with chlorophyll. 

 The true leaves fall off when young, or are changed into 

 spines. It is much more difficult for no.xious insects to 

 damage these cladodes, as the flattened stems are called, 

 than to damage an ordinary foliage leaf. Again, at each 

 node, the point from which leaves or flowers spring, instead 

 of leaves ov branches, there are sharp spines, in .some species 

 2-3 inches long, and a tuft of bristles or spinules, often 

 furnished with a most formidable clothing of recurved 

 barbs. These defences serve to keep awaj^ most of the 

 grazing mammals. On the grazing farms in Texas, the 

 spines are burnt off l)y means of flames from small 

 acetyleiie tanks, borne by employees, and the cattle 

 following the workmen feed upon the disarmed cactus. 



As a protection against insects, all species Net 

 e.xaniined by the authors of this paper are found to have 

 the cells of some portion of the cladode, or in Peireskia 

 of the leaf, armed with crystals of oxalate of li)ne. In 

 Opuntia inermis, D.C., the pei^t pear of Queensland and New 

 South Wales, and Opuntia avrantiaca, Ciilhes established 

 near Warwick, sphere-crystals form an almost continuous 



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