488 THE SOOTY MOULD OF CITRUS TREES, 



gives an unsightly appearance to the plant. This fungus or 

 'smut' is an almost invariable indication that a plant is attacked 

 hy insects, and may, indeed, give a useful warning to tree- 

 growers." The occurrence of the fungus on the upper surface of 

 the leaf may be variously accounted for. The upper surface is 

 most readily moistened; the rain and dew are longer retained in the 

 channel over the midrib at the tip. But the main reason eA idently 

 is that the honey-dew is dropped there by the coccids generally 

 found on the under surface of the leaves. In the absence of 

 honey-dew the fungus might grow on the accumulations of the 

 excreta of insects, &c., but the general rule is that the fungus 

 follows in the wake of insects, and to get rid of the one you must 

 also get rid of the other. 



Since writing the above I have received a note from J. G. O. 

 Tepper, F.L.S., Adelaide, in which he shows how the destruction 

 of honey-eating birds may affect the prevalence of this disease. 

 He says: — "Regarding the 'Sooty Mould' and its prevalence 

 now in many localities, it may be mentioned that it appears to 

 have Ijeen practically absent, when nature was less disorganised 

 by man, and for a very simple reason. It being due to the sugary 

 exudations of scale insects, &c., coating the trees, its abundance 

 depends upon that of its producers, and this upon the reduction 

 of the sugar-loving, bi'ush-tongued parakeets and other birds 

 which formerly abounded so greatly. These I have often oljserved 

 myself busy in the earlt/ morning among the foliage of gums, kc, 

 upon which the honej'-dew appeared. Later in the day the ants 

 occupied these in overwhelming numbers, and drove the birds 

 away, protecting the insects and cleaning the foliage. 



"Now many plants have developed special oryms to attract 

 the ants as protectors against birds and animals which feed upon 

 foliage, flowers or unripe fruit, and though rendering service to the 

 jtlants by reducing superfluous quantities of either, and securing 

 thus the greatest perfection of that remaining (also controlling 

 other insect life), the lairds constantly tend to overdo the work at 

 certain critical periods. As our Eucalypts, itc , and many intro- 

 duced plants have no such organs, they make use of the scales, 



