CROP-PLANTS 365 



plant virus, but when no disease organism could be discovered, 

 experiments were made with fertilizers. This resulted in the dis- 

 covery that the disease is due solely to a deficiency of sulphur in 

 the soil and can be completely cured by the application of very 

 small quantities of pure sulphur or sulphur-containing fertilizers. 

 Sulphur is an element of which only very small quantities are neces- 

 sary for plant life, and the solution of the tea-yellows problem sug- 

 gests that deficiency in sulphur or other elements of which small 

 amounts are required by plants, may be a cause of disease in 

 other parts of tropical Africa. It is worth noting in this connection 

 that Miss Sherbatofif, at the Imperial Bureau of Soil Science, is 

 preparing a special report on all plant diseases which are known 

 to be caused by specific soil deficiencies, a work which should be 

 of much value to African scientists. 



As a general matter concerning the cultivation of tea in Africa, 

 it is important that the old-established tea areas of India, Ceylon, 

 and Java have had to limit their output severely in recent years, 

 and Russia is apparently increasing her output rapidly. Hence 

 the chances for Africa in the world market may be relatively 

 small. 



Cacao. The development of cacao ( Theobroma cacao) cultivation 

 is a remarkable example of native enterprise and initiative. The 

 methods by which this crop has been established and maintained, 

 have been evolved by the native farmer practically unaided. The 

 principal centre of cultivation is the Gold Coast, which derives a 

 large part of its revenue from cacao exports; developments have 

 recently taken place in Nigeria also, where cacao grows well in 

 the high rainfall of the south-western areas. In the east, in spite 

 of adequate rainfall, the poor soil renders cacao cultivation impos- 

 sible. 



The subject of cacao cultivation cannot be separated from that 

 of forestry {see Chapter VII). Nearly all the cacao is grown on 

 small farms, an area of forest being felled and burnt before plant- 

 ing. Sometimes a few large trees are left to provide shade, but the 

 general effect in replacing a mixed evergreen forest with a single 

 stand of cacao trees is to remove the two upper layers of forest 

 cover. The single stand which remains is generally recognized 

 to be far less efficient in retaining soil and atmospheric moisture 



