nmm sP

BIOLOGY 21
genus, and the manner of growth can sometimes be altered by Regulating the
environment.
It is now generally accepted that meiosis normally occurs in both families at
the onset of germination, and that segments of the promycelium and the firstformed
sporidia are haploid. The dicaryophytic condition arises by the fusion of
either promycelial cells, sporidia, or hyphae derived from them. Cultural conditions
affect conjugation, and the absence of fusions in any one species may only
indicate that the right conditions have not been found. It is clear, however,
that fusions occur more readily in some species than in others, and that even
physiologic races differ in this res'pect.
DEVELOPMENT OF SPOBIDIA ON THE HOST
In many smut diseases the parasite disappears from view after the initial
infection, and only becomes visible to the eye when sori have developed and
chlamydospores are exposed. In a few species, all members of the Tilletiaceae,
the parasitic mycehum emerges through the stomata or between the epidermal
cells and develops sporidia, sometimes in such profusion that infected organs
are powdery with spores.
The first clear account of this so-called 'conidial stage' was given by Woronin
(1882). Plants of Trientalis europaea infected by Tuburcinia trientalis produce,
after the winter rest, shoots which are white on the lower surface. Woronin
described the sporidiophores, which grow in tufts through the stomata ^nd
between the cells, as non-septate, thin, and bent in such a way that the terminal
sporidia lie horizontally. The sporidia are pyriform, 11-15 (j,, hyahne, with
finely granular protoplasm or a small vacuole. They fall easily and a second
sporidium is produced, but the method of discharge is unknown. If sown on the
surface of a leaf, the germ-tube enters and in 12 to 20 days black flecks, the
young sori, appear. Fusions between sporidia were not observed and the number
of their nuclei is unknown.
Kiihn (1883) germinated the spores of a parasite oi Primula, which he named
Paipalopsis irmischiae, but gave no details as to size or mode of origin of the
spores. Schroeter (1887), listing this as a doubtful member of the Ustilaginales,
stated that the parasite passes through the flowering stem into the floral organs,
forming white powdery spore masses which often fill the whole corolla tube.
The spore, which ha^ a smooth, COIOUEIQSS epispore, is spherical (3-6 /a), and
germinates to form a thin germ-tuKe, the tip of which again forms sporidia. It
is not clear if these Sporidia are like the spores from the corolla tube. Wilson
(1915), recording the fungus from Kent, referred to large numbers of small
unicellular spores present as meal-like masses in the open flower, glueing the
stamens together and partially filling the base of the corolla tube. Viable pollen
was, also present, and Wilson suggested that insects carry spores with poUen to
healthy flowers, but inoculation experiments were unsuccessful. Fusions between
sporidia were observed and, after the passage of one nucleus through the
connecting bridge, the binucleate sporidium developed one or more germ-tubes.
It is thought that members of the genus Thecaphora also form sporidia on stamens
of the host, but no good account of this behaviour has been published (see p. 81,
and Brett, 1940).
Sporidia develop freely on the foliage of plants attacked by some species of