THE DISEASES OF THE SUGAR-CANE. 471 



ance. A careful analysis of diseased canes has been made 

 in various countries, one frequent result being- a deficiency 

 of silica in the ash, Stutzer found that the plants suffering 

 from sereh had deficient silica and excess of potash, and 

 suggested that, among other remedies, a rational system of 

 manuring might be applied to sereh. ^ Francis, in canes 

 attacked by rust in Trinidad, found 36*01 per cent, of silica 

 in the ash instead of 45 "84 per cent. ; and Liversedge, 

 examining Queensland canes also suffering from rust, found 

 the deficiency in silica much greater, the diseased canes 

 containing only 25 per cent, or 30 per cent, of the normal 

 quantity." It does not appear how this state of things is 

 to be altered. 



The idea has been advanced in certain directions of 

 altering the cell-sap by the addition of special substances, 

 such as copper sulphate to the soil,"^ and thus rendering a 

 plant more capable of resisting the attacks of a parasitic 

 fungus. But sufficient stress does not seem to have been 

 laid on the selective power of the plant itself. Changes are 

 probably constantly taking place in Nature by which a plant 

 renders its juices less palatable or nutritious to its parasite, 

 but it is doubtful whether any system of manuring will ever 

 succeed in thus directly altering the cell-sap. 



II. It is quite certain that the cells of plants are more 

 or less resistant to disease according to their age. Young 

 cells by their greater vitality are able to resist the action of 

 the ferment emitted by an attacking fungus. The older 

 cells, with their stores of reserve materials and less active 

 protoplasm, on the other hand, are frequently guarded by 

 the presence of special substances, e.o-., tannin in many 

 cells, ethereal oils, resins, etc. The tendency of cultivation 

 is, however, to eliminate these latter substances ; and this 

 will possibly account for the time of appearance of certain 

 diseases. 



The rind-fungus, TrichospJiaeria Sacckari, may be pre- 

 sent in the young tissues, but it is one of the characters of 

 the disease that it becomes first evident by the discolora- 

 tion of the mature joints in the middle of the cane, where 



^ Stulzer. - D'Albuquerque, see also Eoname. ^ Berlese. 



