474 SCIENCE PROGRESS. 



change of climate and food. This has been a custom from 

 time immemorial among the cane growers of India, and has 

 been extensively adopted in most sugar-growing countries. 



It is also a firmly established belief that the long-con- 

 tinued planting of any variety in the same locality is certain 

 to engender a diseased condition. Many interesting and 

 striking examples of this might be quoted. In 1840 the 

 Bourbon cane was introduced to Bogra, one of the chief 

 sugar-growing parts of Bengal. For eighteen years it 

 succeeded remarkably well, and practically displaced the 

 older native varieties, and those who had adopted it became 

 rapidly rich. In 1857- 1858 a disease suddenly appeared 

 which left the canes rotting on the fields, and in the suc- 

 ceeding year or two the Bourbon cane disappeared from 

 cultivation.^ 



Cases of this kind — and many others might be quoted 

 — may be explained in the following way. A variety in- 

 troduced into a new country is at once placed among a new 

 set of parasites. Sometimes these are able im.mediately 

 to assert themselves, rendering acclimatisation difficult. 

 Others require a certain number of years before they can 

 adapt themselves to the protoplasm and cell-sap of the 

 new variety, after which they rapidly destroy it, unless it 

 has a similar or greater adaptive power. This seems to 

 account for the sudden destruction of the Bourbon cane in 

 the Central Provinces of India. In the same way it was 

 several years before the Peronospora viticola, already 

 referred to as coming from America, was able to overcome 

 the natural resistance of European vines. Once able to do 

 so, it became an active and dangerous parasite. 



14. We may conclude then that continued planting in 

 the same soil need not cause a deterioration in the cane, 

 especially as there are many districts where the Bourbon 

 has been grown for years without an appreciable in- 

 crease of disease (as in Demerara). It has been urged 

 by many that the exclusive vegetative reproduction of the 

 sugar-cane, without the chance of an occasional cross, is 

 bringing about a gradual decline of vigour. This may 



1 Watt. 



