472 THE SOOTY mould of citrus trees, 



(ill fact growers do confound it with dust), then of a dark muddy 

 grey, peeling off as a thin papery layer, and finally as a sooty 

 crust, soiling the fingers when rubbed. At times there is a 

 considerable admixture of dust with the filaments, and then it is 

 usually checked in its development. The depth of the colour is 

 evidently largely influenced by the amount of more or less 

 colourless and coloured hyphfe respectively, both of which ai'e 

 usually always present. 



Fungua described. — When examined under the microscope it is 

 seen to consist of a network of filaments and the reproductive 

 bodies which they bear. These filaments are colourless or pale 

 green, and darkly coloured, but there is a gradual transition from 

 the one to the other. The thin-walled colourless filaments 

 generally form a network in contact with the leaf, Ijut they 

 intermix with the thick-walled coloured filaments, and the more 

 or less colourless may gradually become coloured, while the 

 coloured may produce a colourless portion. When further deve- 

 loped, however, the colourless and the coloured hyph?e are distinctly 

 seen. 



.^fycelium. — At an early stage the surface of the leaf shows 

 numerous more or less colourless hyphte creeping over it, and 

 there are two kinds which may be distinguished — (a) closely 

 septate, copiously branched hyphee, in contact with each other 

 and intermixing, so that a close-set pavement of cells is formed 

 resembling a parenchymatous layer. The walls of these cells 

 may become gelatinous, and thus not only stick together, Ijut 

 attach themselves more firmly to the epidermis of the leaf; and 

 (6) at other times only creeping, colourless or pale green h3-2:)haj 

 are seen, very distantly septate and with their walls very uneven, 

 as if thereby better able to adhere to the leaf. Even at this 

 eail}' stage there are abundance of colourless or pale green 

 gonidia scattered about, which will be referred to subsequenth\ 



When further developed the dark coloured hyphse arise, and 

 now there are the two kinds plainly discernible. The more or 

 less colourless hyphre are l)ranched, septate, forming moniliform 



