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INTRODUCTION
THE smut fungi, which are represented in Britain by seventy-four species, are
nearly all parasites of flowering plants. They inhabit stems, leaves, and floral
organs and are most easily recognized in the fruiting condition, when they
produce a sorus of spores, which are, in the mass, usually dark in colour and
often powdery. The sorus may be naked or covered by a membrane of fungal
origin; it may consist of spores only or be traversed by a columella of sterile
mycelium or by threads of host tissue. The spores (chlamydospores) are onecelled
and thick-walled. They are single or united in balls, which may contain
both fertile and sterile cells. The wall of the spore is smooth or ornamented in
various ways. Rarely the spores carry hyaline appendage, but they are always
without stalks and arise from a closely septate mycelium which is generally used
up completely in spore formation.
The chlamydospore, though not the only unit of dispersal, is certainly the
most important means of dissemination in many smuts. In some species germination
may occur while the spores are still embedded in the host tissue; in others,
spores, ripe for dispersal, are not necessarily ripe for germination and time must
elapse before this process begins. Mature chlamydospores in certain species of
economic importance are known to remain viable for a pericfd of several years.
Those who have studied the early development of a soruti agree in finding, two
nuclei in the very young chlamydospore. These fuse and it is generally accepted
that the mature chlamydospore contains a single diploid nucleus. Meiosis occurs
in the promycelium, the germ-tube produced when a ripe chlamydospore starts
growth. Haploid nuclei pass into the sporidia or branches arising from the
promycelium, and become paired when the appropriate gametic elements unite.
Fusion of nuclei is, however, delayed until the last stages of sporogenesis. It
seems probable, therefore, that the parasitic mycehum of many smut fungi is
dicaryophytic and that the gametophytic phase in nature is often very short,
sometimes limited to one cell. Entyloma and alUed genera are exceptional since
they produce haploid sporidia freely on the living host.
Smuts, as a rule, develop easily on culture media, and studies have been made
of their saprophj^ic growth and the segregation of gametophytic characters.
Most of the species so far investigated are heterothaUic, carrying one or more
pairs of allelomorphic genes, which govern the fusion of haplonts. While progress
has been made in genetics, our knowledge of the cytology of smuts is scant.
The Ustilaginales is a compact and clearly defined group embracing the
Ustilaginaceae and the TiLletiaceae together with a family of somewhat uncertain
afiBnities, the Graphiolaceae. The Graphiolaceae comprises three species
parasitic on the leaves of'palms. It is represented in this country by the exotic
OrapJiiola phoenicis in which the chlamydospores, united in chains within a
compact fructification, bud in situ to form sporidia which are dispersed. The
other two famiUes are differentiated by the morphology of the promyceHum
arid its branches. In the Ustilaginaceae the germ-tube soon becomes septate and
buds laterally; in the TUletiaceae it remains at least for a time non-septate and
produces a terminal whorl of branches. Whether lateral or terminal, these
branches are the gametic elements which ultimately fuse in pairs. In both