34 DISEASES OF CROP-PLANTS 



closely adjacent to an infested tree often receive a share of the 

 secretions which give rise to it. Apprehensions of the general 

 spread of the affection from such a source — of the infestation of 

 cacao from a blackened mango tree, for instance — are baseless 

 as regards the fungus, and, where the plants are of unrelated 

 species, are usually so in regard to the insect as well. 



Spraying with fungicides for the removal of black blight, 

 where, as is most commonly the case, this is due to scale insect 

 infestations, makes matters worse in the end by checking the 

 beneficial fungi which in favourable weather reduce the prevalence 

 of the insect. The remedy for black blight must be sought 

 from the entomologist. 



The Classification of Fungi. 



The current system of classification of fungi is in a very 

 imperfect condition in many of its sections. In its general lines 

 it is as nearly natural as the state of knowledge permits, but 

 in many of its subdivisions becomes frankly artificial. Judged 

 as a natural system it not infrequently brings together species 

 that are obviously not nearly related, and separates many that 

 are obviously akin. As an artificial system its weakness is 

 demonstrated by the facility with which half a dozen different 

 authorities can place the same fungus in as many or even a 

 larger number of genera. 



The confusion which is so frequently apparent arises from the 

 lack of stable morphological features on which distinctions can 

 be based, on the difficulties of preservation in recognisable 

 condition, and not least on the irresponsible way in which new 

 species have been so often founded with a minimum of enquiry 

 on imperfect material imperfectly described. 



Among the fiowerless plants three groups occur of which 

 the members are wholly destitute of chlorophyll and depend 

 (with a few exceptions among the bacteria) on parasitism or 

 saprophytism for a living. Each group contains parasites 

 which cause plant diseases. 



Myxomycetes, Slime-fungi. 



In the vegetative condition, during which they grow and 

 feed, these consist of naked protoplasm, either in small units or 

 in masses (plasmodia). In their final stage they come to rest 

 and produce spores, often in elaborate sporangia, from which 

 the vegetative form is again derived. The order Plasmodio- 

 phorales contains the only parasites of plants, the best known 

 being of the genus Plasmodiophora. The amoeba-like plasmodia 

 occupy soft tissues in the roots of certain plants, and give rise 

 to galls or distortions, as in the club-root disease of turnip, 

 cabbage, and other Cruciferae in temperate countries. Very 

 many saprophytic myxomycetes occur in the West Indies on 

 rotting wood and decaying vegetable matter generally. 



