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and is called an '^ oospore/^
WHITE RUSTS. 131
The membrane wliicK
at first invests tMs organ is very thin, but by
deposits from the surrounding fluid it attains to
a greater thickness, and is at length of a yellowishbrown
colour, having
its surface studded with large
obtuse warts (plate
X. fig. 206). One of these
warts, larger than the rest, forms a kind of thick
sheath around the fecundating tube.
The oospores do not give evidence of any appreciable
change for some months. For instance,
those collected by De Bary
in June did not attain
their ulterior development until the commencement
of December. The method adopted was as follows
:— Parts of the plants containing ripe oospores
were preserved in the dried state. When examination
was considered desirable, the portion to be
employed was immersed in water for a day or two ;
it was then placed on a humid soil, or mould
covered with blotting-paper. The tissues enclosing
the oospores were decomposed, and at the end of
from four to eight days their germination might
be observed when placed in a drop of water. This
method again corresponds with the ordinary processes
by which the plant naturally decays on
exposure to the influences of the atmosphere, and
the oospores germinate under the favour of a
shower of rain.
If the oospore, after the decay of the tissues,
is isolated and placed in a drop of water, the
brown investing membrane will be seen to rupture
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