250 ESSENTIALS OF BOTANY 



MICROSPHaERA, A SAC EUNGUS 



322. Occurrence. — Species of the lilac mildew, Micro- 

 sphcera Alni and allied forms, occur during the summer 

 and autumn on leaves of various herbaceous and woody 

 plants. The growth is confined to the surface of the leaf. 

 Among the commonest species are those which grow upon 

 lilac, oak, grape, cherry, willow, and wild plants of the 

 Composite family. Some species are known to occur on 

 only one host^plant, others on several, and the hosts may 

 belong to more than one family. Besides Microsphcera 

 there are about five other genera. All these fungi, from 

 the appearance which they present in their earlier stages, 

 are called powdery mildews. 



333. Gross Structure.^ — Examine with the magnifying glass the 

 upper surface of a lilac leaf infected ■with M. Alni. Note : 



(a) The color and distribution of the fungus on the upper surface 

 of the leaf. 



(6) The powdery or moldy appearance of the leaves first gathered, 

 due to the abundant conidia (or conidiospores). 



(c) The yellow or black dots on late-gathered leaves, sac friii/s. 



324. Minute Structure. ^ — 



I. The Mycelium. 



Examine with h.p. some scrapings or thin sections parallel to the 

 leaf surface, or fungus spots from a leaf gathered early in the sum- 

 mer. Dried leaves may be moistened with five-per-cent solution of 



1 Material for all the studies of this Microsphxra should he gathered at 

 three periods, — when the lilac leaves first begin to appear powdery (in June 

 or July), then in early September, and finally just before the tall of the leaves. 

 They should be pressed between sheets of porous paper. 



2 If the teacher prefers to use material which will show larger sac fruits he 

 may use fresh or dried fructifications of the morel (Fig. 176). Dried morels 

 must be soaked in warm water before sectioning. The sections should he cut 

 perpendicular to the surface of one of the hollows of the morel. 



