262 FOUNDATIONS OF BOTANY 
prepared cross-sections through the sorus are available, especially if 
the material is fresh. Examine the spores and note the shape, color, 
and surface. If the spores are shrunken, a drop of potash solution 
will restore the natural plumpness. Draw. Spore-measurements are 
important in determining species. The wredospores of Puccinia 
graminis may be distinguished from those of other species common 
t on grasses by the greater proportionate 
length. 
The structure of the black spores 
(teleutospores) can be made out with- 
out difficulty. Some should be found 
attached at the base. Note the parts 
and the differences in color in different 
portions. Make careful drawings to 
show shape and structure of both kinds 
of spores. 
Boil a portion of a rust-injured plant 
in potash solution, pick it to pieces on 
a slide under the magnifier or dissect- 
ing microscope, use a cover-glass and . 
examine the preparation for mycelium, 
using a high power. 
313. Cultivation on a Host-Plant. — 
If practicable, find some wheat or grass 
which has remained over winter with 
Fic. 193.—A Group of Spores : - 
of Wheat Rust (Puccinia the black rust upon it. Tie a bunch 
graminis). (x about 440.) of this to a barberry bush while the 
u, u, uredospores ; t,ateleu- = leaves are young or unexpanded. When 
tospore. 
the time arrives for the appearance of 
the cluster-cups, note whether they are any more abundant on this 
bush than on others. Are you sure that the rust you have is the 
one to which the barberry cluster-cups belong? 
