250 ESSENTIALS OF BOTANY 
MICROSPH.ERA, A SAC FUNGUS 
322. Occurrence. — Species of the lilac mildew, Miero- 
sphera 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 Microsphera 
there are about five other genera. All these fungi, from 
the appearance which they present in their earlier stages, 
are called powdery mildews. 
323. Gross Structure.1— 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. , 
(0) 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 fruits. 
$24. Minute Structure.? — 
I. Tue Mycerium. 
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 Microsphera should be 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 fall of the leaves. 
They should be pressed between sheets of porous paper. 
2Tfthe 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 be cut 
perpendicular to the surface of one of the hollows of the morel. 
