MILDEWS, RUSTS AND SMUTS 
183 
branchlets, or intercalary, that is at intervals in the length 
of the mycelium. From the protoplasm contained in these 
swellings the spores are formed, and develop their own cell- 
wall while yet enclosed in the wall of the swelling. In 
the genus Entyloma spores are produced in all portions of 
the vegetative mycelium, whereas in Ustilago the spores 
are produced in special branches only. These branches, 
called sporogenous hyphae, are very much branched, and 
produced in great numbers at definite points of the host- 
plant. When about to produce spores, the sporogenous 
hyphae become broken up into a row of short cells by the 
formation of transverse cell-walls or septa, then the cell- 
walls of the hypha become much swollen and form a 
gelatinous envelope enclosing the protoplasm, which forms 
a single spore in each component cell of the hypha. The 
spores secrete their own cell-wall while yet enclosed in the 
gelatinised wall of the hypha, which eventually disappears, 
leaving the fully formed spores as a dry, powdery mass. 
In the genera Urocystis, Tuhercinia, and Sorosporium, the 
spores form compact groups or clusters, known as spore- 
halls, surrounded by a special envelope formed of hyphae, 
which either persists or soon disappears. In Urocystis the 
envelope consists of empty pale coloured cells, which sur¬ 
round the cluster of dark coloured cells capable of germina¬ 
tion. In Doassanaia the groups of cells are enclosed in 
a covering composed of closely compacted sterile cells, 
arranged in a parallel manner resembling the palisade 
tissue of a leaf. The mature teleutospores are usually 
brown, often with a tinge of violet or olive, and the epispore 
is frequently ornamented with warts or a raised network. 
Although uredospores are absent, delicate mildew-like 
tufts of conidiophores bearing minute, colourless conidia 
are present in the life-cycle of some species. The generic 
characters depend mainly on the mode of germination 
of the teleutospores, in fact it may be truly said that no 
member of the Ustilaginaceae can, with certainty, be re¬ 
ferred to its genus until such measures have been taken. 
Details of the various modes of germination are given 
under the generic characters of the genera. A point of 
importance respecting the promycelium spores is, that they 
usually conjugate in pairs ; that is, a pair of promycelium 
spores lying near each other, become organically joined by a 
short tube originating from one and becoming joined to the 
other, thus bringing the protoplasm of the two promy¬ 
celium spores into contact. Such conjugation sometimes 
