DISEASES OF CROP-PLANTS 



The Mycelium. 



The way of life characteristic of the great majority of fungi 

 is for the invaded substance to be covered or interpenetrated 

 with an abundant development of slender filaments, which 

 absorb from it the food materials required. This structure is 

 the vegetative part of the fungus ; it is known collectively as 

 the mycelium, and its individual filaments as hyphae. In mass 

 the mycelium is frequently visible to the naked eye as a cobweb- 

 like, threadlike, fluffy or papery investment. Sometimes the 

 hyphae are combined into threads, as in the black and shining 

 strands of "horse-hair blight" and the whitish threads or strings 

 of thread blight. In certain species cord-like strands of mycelium 

 ramify through soU or vegetable mould, and such structures 

 in general have been named rhizomorphs, from their resemblance 

 in these cases to roots. In certain fungi 

 the mycelium collects into hard masses 

 of various shape and size, sclerotia, which 

 by their resistant nature serve to carry 

 the fungus through adverse conditions 

 or aid in its distribution. 



If the abstraction of food materials 

 by the mycelium takes place from living 

 tissues or non-living but functioning 

 parts of a plant or animal the mode of 

 life constitutes parasitism ; if from ma- 

 terial no longer forming part of a living 

 structure, it is known as saprophytism. 



The Place of Fungi in Nature. 

 P^S- 1 Fungi have a place in the economy 



Fungus Hyphae in Cells of Nature far wider than that arising 



from their part in the causation of plant 

 disease. Chiefly by their operation the great mass of vegetable 

 material formed each year in the shape of leaf and stem, flower 

 and fruit, is broken down and returned to the soil. Their 

 activity in this direction has a harmful side, from the point of 

 view of man, when it concerns such commodities as timber or 

 stored foods. 



The rapid destruction of humus-forming materials in the 

 tropics as compared with temperate countries — resulting in the 

 absence of notable deposits of leaf-mould even from the forests — 

 appears to be connected with the suitability of tropical conditions 

 for the continuous activity of soil fungi. The only lasting 

 natural accumulations of vegetable material in the West Indies 

 known to the writer are in swampy situations, where the water, 

 by excluding air, prevents their operation. In the biological 

 processes of soils under forest growth or in orchard cultivation 

 it is believed that fungi replace to a considerable extent the 

 bacteria of arable land. 



