50 



ELEMENTARY STUDIES IN BOTANY 



Powdery mildews. There is another group of mildews 

 (sometimes called " powdery mildews " to distinguish them 

 from the " downy mildews ") that will illustrate 

 the third relation of the mycelium to the food- 

 supply. One of the most commonly observed 

 among them is the lilac mildew. It is seen 

 on every lilac bush as whitish, dusty-looking 

 patches on the leaves (Fig. 32) ; in fact, whole 

 bushes sometimes appear as if completely 

 covered by street dust. Under the microscope 

 it is seen that this whitish material on the 

 leaves is the mycelium of a fungus, which in 

 this case is an external 

 parasite. The haustoria 

 penetrate the walls of the 

 epidermal cells of the host, 

 which are really not vital 

 cells, so that such mildews 

 may be very abundant 

 upon a plant without destroying it or seriously interfering 

 with its vigor ; in fact, almost all plants have mildews. 



The mycelium produces sporo- 

 phores abundantly, and it is really 

 the numerous spores that give the 

 dusty appearance to the leaves. 

 These spores are formed as are 

 those of the downy mildews de- 

 scribed above, except that they 

 are cut off in chains by an un- 

 branched sporophore (Fig. 33). 



Later the sex-organs appear, 

 very minute and not often seen, 

 but the result of their work is 

 always seen. This result is a 

 heavy- walled case (Fig. 34), which 



FIG. 33. A sporophore of a mildew with 

 its row of spores. After TULASNE. 



FIG. 34. A spore-case of a mil- 

 dew, showing its heavy wall, its 

 conspicuous appendages, and 

 the sacs (containing spores) 

 squeezed out through a break 

 in the wall. 



