vi] Rusts 65 
fungus is black or orange rust of wheat, and it appears 
commonly in wheat fields in late spring or early summer. 
The leaves of the wheat first of all lose their character- 
istic green colour and become slightly yellowish. 
Dark orange patches are then formed on them, and are 
found between the veins of the leaf or on the stem. 
This is the first spore stage that the fungus produces 
on wheat and it is this which gives the popular name 
of rust. 
Examination with a lens shows that these patches 
are composed of a number of minute bodies or spores 
clustered together, and that they are partly covered at 
the sides by the colourless skin or epidermis of the 
wheat leaf. They are known as uredospores or summer 
spores (see Fig. 24). On shaking the leaf some of them 
fall out as a powder. They are ovoid in shape, the 
coat or wall being of moderate thickness and covered 
with a number of very short blunt spikes. Midway 
between the apex and the point of attachment are 
some thin places in the wall. A section through one 
of the orange patches shows that each spore is borne 
on a short stalk (see Fig. 24). The mycelium which 
produces them does not extend throughout the whole 
plant, but is confined to small areas near the spores. 
It consists of a dense web of very fine hyphae growing 
in the spaces between the cells. Small tubes or 
haustoria grow into these cells for the purpose of 
obtaining food. The hyphae have occasional transverse 
walls. If the uredospores are placed in a hanging drop 
under suitable conditions for germination they send out 
tubes through the thin places in their walls (see Fig. 24). 
One of these usually grows much longer than the 
others and branches. An uredospore falling on a blade 
P.F. 5 
