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which, if sufficiently numerous, appear as a fine white powder 

 through the lens. The conidia usually fall off or are blown off by 

 the wind almost as soon as they are formed, small swellings in 

 the hyphse indicating the places where they originated. See 

 Plate XVI (2). The mycelium or main body of the fungus 

 consists of a branched net- work of fine tubular hyphse 

 which spread and branch in all directions in the intercellular 

 spaces of the leaf-tissue. If a single hypha is examined with 

 the microscope it will be seen to consist of a long tube, with a 

 transparent thin wall of cellulose, containing water, protoplasm 

 and other substances. The hyphse are, as a rule, not septate, 

 but transverse partition walls are occasionally developed at 

 irregular intervals to separate the parts of the hyphse which 

 are still growing and full of protoplasm from those which have 

 completed their growth and are empty. These hyphae push 

 their way between the cells of the leaf, and as a rule do not 

 directly enter the cells. Occasionally, however, they develop 

 small branches which dissolve their way through the neighbouring 

 cell-walls by means of an enzyme which they secrete, and which, 

 acting as absorbing organs, are called haustoria, or suckers. 



Whether the hyphse enter the cells or not, however, the 

 result is the same ; they consume a quantity of the oxygen and 

 water which is required by the living leaf -cells, they directly ab- 

 sorb the products of assimilation which would ordinarily have 

 been devoted to the development of the tubers, and finally they 

 destroy the living protoplasm and kill all the living cells with 

 which they come in contact. The cell-walls turn brown wherever 

 the hyphae touch them, the colour extending for some distance 

 along the walls beyond the actual point of contact. Here and 

 there the ends of the hyphse emerge from the leaf tissues and 

 develop the branched conidiophores as above described. The 

 conidia being very minute and falling rapidly are blown about 

 and distributed by the wind like fine powder, thus enabling the 

 fungus to spread rapidly and infect fresh leaves and plants. 



The conidia may germinate in one of the three following 

 ways : 



(1) If the conidium falls into a drop of rain or dew the proto- 

 plasm divides into from 6-16 parts, the tip of the 

 conidium dissolves, and these little masses of proto- 

 plasm escape. These are called zoospores, and each 

 is provided with two very fine hair-like appendages 

 called cilia which lash the water and enable the zoos- 

 pore to swim and move about in the water. In a 

 quarter of an hour or so these zoospores lose their 



