128 DISEASES OF FIELD & GARDEN CROPS. [OH. 



ment. If, however, infected grass is kept in damp air, a 

 fragment of a leaf may generally be successfully cut, 

 examined without water with a low power of the micro- 

 scope, and the Oidium seen in a growing condition. When 

 thus examined the profuse chainlike growth may be easily 

 observed. If placed under a cover glass for examination 

 under the higher powers of the microscope, the beadlike 

 spores or conidia instantly break away from each other, 

 so inconceivably slight is their attachment. The monili- 

 form habit can, therefore, only be seen with a low power 

 applied to the dry living fungus whilst in situ. In water 

 or damp air the conidia, and especially the topmost coni- 

 dium, quickly germinate and produce thin threads, as 

 illustrated at Fig. 56, A, enlarged 1000 diameters. This 

 constant spore production and germination is incessantly 

 continued through the summer months, till at last a thick 

 grayish-white coat of mildew more or less covers all in- 

 fected plants. The conidia are so small that it would 

 take about a million to cover a square inch. The Oidium 

 state of grass blight may be compared with the Oidium 

 of the turnip, illustrated to the same scale at Figs. 27 and 

 28. The grass Oidium is somewhat taller, but the spores of 

 the turnip Oidium are more than twice the length of those 

 of our present plant. Oidium monilioides, Lk., is a typical 

 plant. The Oidium growth of the fungus, however, and 

 the production of the profuse mycelium is only a prepara- 

 tory stage of growth for the perfect Erysiphe which gene- 

 rally follows ; it has been observed that when the Oidium 

 does not appear till late in the summer, the Erysiphe or 

 perfect condition is never produced, and the whole growth 

 of the fungus is confined to the Oidium stage. Under 

 favourable conditions of growth, the Oidium threads of 

 the summer produce in the autumn little brown globose 

 bodies termed conceptacles. This condition of the mildew 

 is shown on the wheat stem in Fig. 55. When examined 

 even with a powerful lens, these little blackish dots, termed 

 conceptacles, appear less in size than fine grains of dust ; 



