CROP-PLANTS 337 



more diversified agriculture has been adopted both among Euro- 

 pean settlers and natives in every territory. 



The influence of organizations in England, such as Kew Gar- 

 dens and the Imperial Institute, has been active in stimulating 

 interest in subsidiary crops. Through their assistance the cultiva- 

 tion of tung oil, quinine, essential oils, and insecticides has been 

 developed in different parts of Africa. 



In breeding and selection, three objects have always to be con- 

 sidered — higher yield, better quality and resistance to disease. 

 There are many examples from Africa where one or two of these 

 aims have been achieved with success, but the combination of all 

 three has yet to be realized, except perhaps in the case of certain 

 strains of cotton. As mentioned in Chapter VI, diseases can be 

 divided into those caused by insects, fungi or other plant organisms, 

 virus diseases and eel- worm diseases. The role of insects not only 

 in directly bringing about disease, but in transferring the causative 

 organisms of other troubles, has been discussed in Chapter X. 

 Another aspect of disease concerns the relationships of crops 

 to wild plants which may serve as alternative hosts for pests, 

 etc. The methods of cultivation offer numerous problems for 

 each crop individually, some of which are mentioned in the 

 following pages. In this connection it is important to recognize 

 the change which has taken place in crop research during recent 

 years. As pointed out by the recent Secretary of State for the 

 Colonies (Ormsby-Gore 1934), the old idea of protection from 

 pests by spraying and similar methods for the destruction of the 

 organism involved, is giving place to the view that the best defence 

 is through suitable nutrition resulting from improved agricultural 

 practice. In this there are three lines of approach: those of the 

 geneticist or plant breeder, the physiologist, and the nutritionist, 

 to which may perhaps be added that of the ecologist who studies 

 the environment and the optimum conditions of climate, soil, etc. 

 for each crop. He points out that team work is essential for pro- 

 gress in these subjects and that probably most has been achieved 

 in the past by research stations concentrating on one crop. Quick 

 results are impossible even with a large team of workers, and hence 

 there is need for continuity in research. 



There seems to be a paucity of reference works on African crops, 



M 



