In developing, the mold forms a white felt-like mass, covering the
medium on which it is growing; then as development proceeds, it changes
to bluish-green, and finally to a darker, duller color. The change in
color is accompanied by a change in structure, the surface becoming
powdery in appearance, a slight current of air being sufficient to
dislodge a cloud of fine dust. This fine dust is formed of small,
spherical bodies, the spores or conidia (from the Greek meaning
_dust_). These need no resting period, but are able to develop at
once. When the conidia lodge on a moist substance they swell to a much
greater size and then send out a tube from some part of their surface.
The tube lengthens and septa form, dividing the tube into sections, or
cells. At the same time branches are sent out, which again form other
branches. The original conidium sends out a second branch shortly after
the first one, and usually from the opposite side, and may even send
out a third one. The formation of the septa and the subbranching goes
on in all, so that in a short time the branches mat together and form a
felt-like cover.