184 
MILDEWS, RUSTS AND SMUTS 
takes place while the promyceliun spores are yet attached 
to the promycelium, but most frequently after they have 
become free. After conjugation a slender germ-tube is 
produced, into which the protoplasm from the two promy¬ 
celium spores passes, and this germ-tube, under favourable 
conditions, is capable of entering a host-plant and starting 
an infection. In some species the germ-tubes, produced 
by promycelium spores that have conjugated, give origin to 
a second lot of spores called secondary promycelium spores, 
these give origin to germ-tubes capable of infecting a host- 
plant. The above described formation of promycelium 
spores and germ-tubes occurs when the spores germinate 
in water, but when germination takes place in a nutrient 
solution, as for instance in a wet manure heap, the result 
is different. Here the promycelium, instead of producing 
promycelium spores, forms a dense, branched mycelium, 
which eventually produces spores, or the mycelium continues 
to grow and produces myriads of minute cells resembling 
yeast-cells in appearance. Such spores do not conjugate, 
but are capable of infecting a host-plant. 
USTILAGINACEAE 
Fungi for the most part parasitic in the above-ground 
parts of living plants ; mycelium usually wide-spread in 
the host, but soon disappearing ; teleutospores produced 
within the hyphae, which often become gelatinous and 
disappear, on germination producing a promycelium which 
bears promycelium spores at the side, or at the tip. The 
promycelium spores usually conjugate in pairs, and after¬ 
wards develop a germ-tube capable of infecting a 
host-plant. In nutritive solutions the teleutospores often 
produce yeast-like cells by continuous budding or ger¬ 
mination, in immense numbers. 
KEY TO THE GENERA 
I. Teleutospores solitary, that is, not aggregated in groitps. 
* Promycelium spores generally borne laterally on the 
promycelium, rarely at the tip. Sori powdery at 
maturity. Ustilago 
** Promycelium spores numerous, borne at the tip of the 
promycelium. 
f Sori powdery at maturity. 
Wall of teleutospores generally ornamented with a 
network . .... Tilletia. 
