adually taking up all the contents of the
cell, until a large oval spore (_sp._) is formed at the tip,
containing all the protoplasm of the cell.

Experiments have been made showing that these spores do not germinate
upon the cedar, but upon the hawthorn or crab-apple, where they
produce the cluster-cup stage often met with late in the summer. The
affected leaves show bright orange-yellow spots about a centimetre in
diameter (Fig. 47, _D_), and considerably thicker than the other parts
of the leaf. On the upper side of these spots may be seen little black
specks, which microscopic examination shows to be spermogonia,
resembling those of the lichens. Later, on the lower surface, appear
the cluster cups, whose walls are prolonged so that they form little
tubular processes of considerable length (Fig. 47, _E_).

In most rusts the teleuto spores are produced late in the summer or
autumn, and remain until the following spring before they germinate.
They are very thick-walled, the walls being dark-colored, so that in
mass they appear black, and constitute the "black-rust" stage
(Fig. 47, _J_). Associated with these, but formed earlier, and
germinating immediately, are often to be found large single-celled
spores, borne on long stalks. They are usually oval in form, rather
thin-walled, but the outer surface sometimes provided with little
points. The contents are reddish, so that in mass they appear of the
color of iron rust, and cause the "red rust" of wheat and other
plants, upon which they are growing.

The classification of the rusts is based mainly upon the size and
shape of the teleuto spores where they are known, as the cluster-cup
and red-rust stages are pretty much the same in all. Of the commoner
genera _Melampsora_, and _Uromyces_ (Fig. 47, _L_ i), have unicellular
teleuto spores; _Puccinia_ (ii) and _Gymnosporangium_, two-celled
spores; _Triphragmium_, three-celled; and _Phragmidium_ (iii), four or
more.

The rusts are so abundant that a little search can scarcely fail to
find some or all of the stages. The cluster-cup stages are best
examined fresh, or from alcoholic material; the teleuto spores may be
dried without affecting them.

Probably the best-known member of the group is the wheat rust
(_Puccinia graminis_), which causes so much damage to wheat and
sometimes to other grains. The red-rust stage may be found in early
summer; the black-rust spores in the stubble and dead leaves