6 DISEASES OF CROP-PLANTS 



Others only attack plants of one order, genus, or species, and this 

 tendency reaches its extreme in parasites which are so restricted 

 in their range that invisible characters associated with a par- 

 ticular strain of the host species may partially or completely 

 prevent their development. 



Reproduction and Infection. 



The active feeding part of a fungus, its vegetative body, is 

 the mycelium already described, typically made up of an aggre- 

 gate of hyphae. A portion of mycelium may be detached, with 

 or without fragments of the material on which it is growing, 

 and serve to reproduce the fungus in new situations, as happens 

 with the root fungi of sugar-cane transported on cane cuttings 

 or trash, and in Rosellinia root diseases when fragments of in- 

 fested wood are washed by floods to lower levels. The common 

 Sclerotium diseases of legumes, sugar-cane, etc., are caused by 

 fungi in which no method of reproduction is known save the 

 characteristic hard, shot-like grains, formed of thick-walled 

 hyphas. 



With very few exceptions, however, reproduction is further 

 provided for by the formation and release of spores — individual 

 cells or less frequently aggregates of 2, 3, or more cells — which 

 can remain, like seeds, dormant for a longer or shorter period and 

 then, under appropriate conditions, germinate and give rise to 

 a new mycelium. In a few of the more primitive fungi there are 

 sexual processes more or less preserved which resemble those of 

 algae, but in most members of the larger groups there is little 

 more than a reminiscence of sexual fusion, and in very many 

 no certain trace or no trace at all. 



Classification, as in the flowering plants, depends mainly on 

 the form of the reproductive structures, and some detail as to 

 this will be given later : only the more general aspects of repro- 

 duction are at present in question. 



The visible forms of the larger fungi, the mushroom-like, 

 bracket-like, and other shapes in multitudinous variety, are 

 structures erected by the usually obscure mycelium for the 

 production and discharge of spores. The provision of gills, 

 spines, tubes, pores, folds, convolutions, recesses, or chambers, 

 greatly increases the surface available for this purpose. A similar 

 end is served in the simpler forms by the outgrowth and branching 

 of the surface hyphge. 



Broadly speaking, spores may be divided into {a) those 

 produced within a parent cell (an ascus or a sporangium), to which 

 the term spore is sometimes technically restricted (endogenous 

 formation), and (b) those which are budded off from more or less 

 free hyphae and are known as conidia or conidiospores, or from 

 h57phal terminations of special form (basidia) and distinguished 

 as basidiospores (exogenous formation). In some very simple 

 forms the whole mycelium divides up into conidia or gemmae. 



