, one beginning to germinate, x 300. _C_, a spore
that has germinated, each cell producing a short, divided filament
(basidium), which in turn gives rise to secondary spores (_sp._),
x 300. _D_, part of the leaf of a hawthorn attacked by the cluster cup
stage of the same fungus, upper side showing spermogonia, natural
size. _E_, cluster cups (_Roestelia_) of the same fungus, natural
size. _F_, tip of a leaf of the Indian turnip (_Arisaema_), bearing the
cluster cup (_AEcidium_) stage of a rust, x 2. _G_, vertical section
through a young cluster cup. _H_, similar section through a mature
one, x 50. _I_, germinating spores of _H_, x 300. _J_, part of a corn
leaf, with black rust, natural size. _K_, red rust spore of the wheat
rust (_Puccinia graminis_), x 300. _L_, forms of black-rust spores: i,
_Uromyces_; ii, _Puccinia_; iii, _Phragmidium_.]

The first form met with in most rusts is sometimes called the
"cluster-cup" stage, and in many species is the only stage known. In
Figure 47, _F_, is shown a bit of the leaf of the Indian turnip
(_Arisaema_) affected by one of these "cluster-cup" forms. To the naked
eye, or when slightly magnified, the masses of spores appear as bright
orange spots, mostly upon the lower surface. The affected leaves are
more or less checked in their growth, and the upper surface shows
lighter blotches, corresponding to the areas below that bear the
cluster cups. These at first appear as little elevations of a
yellowish color, and covered with the epidermis; but as the spores
ripen they break through the epidermis, which is turned back around
the opening, the whole forming a little cup filled with a bright
orange red powder, composed of the loose masses of spores.

Putting a piece of the affected leaf between two pieces of pith so
as to hold it firmly, with a little care thin vertical sections of
the leaf, including one of the cups, may be made, and mounted,
either in water or glycerine, removing the air with alcohol. We find
that the leaf is thickened at this point owing to a diseased growth
of the cells of the leaf, induced by the action of the fungus. The
mass of spores (Fig. 47, _G_) is surrounded by a closely woven mass
of filaments, forming a nearly globular cavity. Occupying the bottom
of the cup are closely set, upright filaments, each bearing a row of
spores, arranged like those of the white rusts, but so closely
crowded as to be flattened at the sides. The outer rows