23 i 



not be confused with the black discoloration of the blossom buds passing 

 under the same name. These buds finally fall off unopened. Different kinds 

 of root moulds have been described and considered as the cause of root-rot^. 

 I think it will be necessary to study here the question whether parasitic 

 fungous forms can attack the plant injuriously only when the roots have 

 already been weakened by unfavorable nutritive conditions. 



Cocoa and Tea. 



Fesca says in regard to the cocoa tree "extremes of soil structure, poor 

 sand, as well as tough clay, are not favorable to the cocoa tree. Rather it 

 demands greater soil depth and freshness, without the necessity of enduring 

 standing water, as well as greater humus and nutrition content, than does 

 coffee." The same author, who himself has analyzed good tea soils in Japan, 

 say of tea, that he found in a more compact soil, 30 to 40 per cent, of water 

 as capillary water. Tea demands a sufficiently deep soil which is free from 

 standing water, to which it is very sensitive. Here too a still little understood 

 fungus is described as the cause of a root disease. It is said to result in the 

 early death of the bushes, especially when growing on damp soil. Neverthe- 

 less Fesca- assures us that he has never yet seen the disease on well aerated 

 soils. We might also trace the diseases of young tea plants described by 

 Zimmermann^ to an unfavorable place of growth, although a fungus bearing 

 iobed haustoria has been observed at the disease centres. The leaves become 

 flabby and discolored ; the stems turn brown at the base or higher up where 

 the root seems healthy. Often only the leaves show brown spots, especiahy 

 on the midrib. The fungi developed from the diseased parts of the stem 

 (Nectrieae) could not produce the disease even in infection experiments. 

 In dry weather the disease decreases considerably. A-lso transplanting the 

 seedlings from the closely planted seed bed arrested the disease. If we have 

 considered here with the greatest brevity the soil demands of our most impor- 

 tant cultivated tropical plants, it must still be added that naturally the climate 

 remains the decisive factor. Among these climatic factors especial attention 

 must be given to humidity since the quality of the harvest often depends 

 considerably upon this. In cocoa plantations in Kamerun, for instance, it 

 may be observed that the quantitative production of the trees is unusually 

 abundant, but the quality of the fruit is only mediocre as the result of great 

 dampness. The trees also are short-lived here. 



Other Tropical Plants. 



Of grains. Maize requires, first of all, a deep, mellow soil free from 

 standing water and cannot thrive on tough clay. Sorghum behaves similarly, 

 but is still more sensitive to cold and dampness and, because of its deep root 



1 BoUetim del Institto Fisico-Geographico de Costa Rica, 1901. 



2 Loc. cit., p. 273. 



3 Zimmermann, Untersuchungen iiber tropische Pflanzenkrankheiten. Sonder- 

 berichte iiber Lan- und Forstwirtschaft in Deutsch-Ostafrika, VoL II, Part 1, 1904. 



