156 DISEASES OF CROP-PLANTS 



fibrous roots, known in Grenada as bichet, is formed close to the 

 surface. Tlie use of vegetable mulches is objected to by some 

 planters on the ground that it encourages this formation of 

 feeding roots where they are liable to be killed in dry weather. 

 The effect of such a loss is probably not so serious as it is thought 

 to be : some plants, notably the vine, habitually produce feeding 

 roots of short duration. 



These effects of rainfall, of depth of soil, and of dry seasons 

 all depend on the conservation of moisture, and to some extent 

 this is capable of being artificially regulated by the traditional 

 method of growing trees for shade. The question of shade or 

 no shade is often debated as though it were capable of a single 

 and universal answer, whereas it depends entirely on local, and 

 sometimes very narrowly local, conditions. 



The control of the various pathological conditions arising 

 from lack of vegetative vigour on the one hand and from infec- 

 tions favoured by too great humidity on the other, is largely 

 dependent upon the adjustment of shade conditions, so that it is 

 relevant to consider the subject somewhat closely. 



A correct estimation of the factors involved and their effects 

 requires systematic physiological studies which so far as the 

 writer knows have never been made. In default of such studies 

 the indications obtained from empirical observation and from 

 analogy must be accepted. 



That shade in some circumstances exercises a most beneficial 

 and even essential influence is certain. The most essential fact, 

 which is quite clear, is that the value of shade depends on its 

 effect in remedying otherwise defective conditions and thus 

 extending the range of situations within which cacao can be 

 successfully grown. Where the natural conditions are in all 

 respects good the introduction of shade is a definite disadvantage ; 

 it is generally admitted that where shade can be dispensed with 

 the trees bear more heavily. With soils of equal quality the 

 estates which have least need of shade are those situated in 

 sheltered humid situations, with no strongly marked dry season 

 — conditions found in the neighbourhood of hills and forest — 

 while the need becomes more and more imperative with the 

 increase of exposure and of inequality in the distribution of 

 rainfall. Contributory differences in the same direction are seen 

 in the transition from deep to shallow soils and from strong 

 retentive soils to those which are light and very permeable. 



The most convincing case of dependence on shade known to the 

 writer is that of plantations on soils of the last-named character 

 in St. Vincent. In spite of deep roots and of a high rainfall, the 

 trees are poorly nourished, and the response they make to pen 

 manure suggests that lack of organic matter, due to the non- 

 retentive nature of the soil, is largely the cause of this. The 

 provision of effective shade very distinctly increases the vigour 

 of the trees. 



