vi] Rusts 67 
of wheat germinates in the same way. When the germ 
tube reaches a breathing pore it enters and so infects 
the plant. The mycelium from an uredospore is said 
to produce fresh uredospores in about 7-10 days. 
These are continually produced throughout the 
summer. In late summer we shall notice that the 
appearance of the rust changes. The leaves look 
darker and on closer examination this is seen to be 
due to the production of dark brown or nearly black 
spores together with the orange coloured ones. After 
a time the latter disappear and the straw is now 
marked with black lines, due to the long patches 
which are placed one above the other in the soft tissue 
between the veins. It is this stage that gives the name 
of black rust to the fungus. 
The black spores are not of the same kind as the 
orange coloured ones and are not derived from them 
by change of colour. They are known as teleutospores 
or winter spores, and are produced from: the same 
mycelium which gave rise to the uredospores. After 
successive crops of the latter the fungus suddenly starts 
to form teleutospores, and so we get the change from the 
orange to the black colour. A teleutospore is almost 
twice as long as an uredospore and is divided into two 
almost equal cells by a thick partition across it. Its 
walls are very thick and smooth (see Fig. 24). They 
take much longer than the uredospores to germinate 
and on doing so each of the two cells sends out a tube 
from its upper part. This becomes divided into three 
or four cells by cross partitions; from each of these a 
short tube grows out and becomes swollen at the end, 
and eventually cut off as a small spherical body, 
known as a secondary spore (see Fig. 24). 
5—2 
