MILDEWS, RUSTS AND SMUTS 
56 
a brownish wall generally ornamented with warts or minute 
spines. Two to several pale round spots on the wall 
correspond to the points through which the germ-tubes 
spring when the spores germinate. These pale spots or 
germ-pores are sometimes of importance as specihc charac¬ 
ters, hence it is of importance to determine with accuracy 
the number present. They can be rendered more distinct 
by placing the spores on a glass slip in a small drop of equal 
quantities of lactic acid and water, which should be brought 
just to boiling-point for a moment over a spirit-lamp. 
When the wall of the spore is pale a dilute stain sometimes 
clearly dehnes the germ-pores. Bismarck brown answers 
this purpose well. Uredospores on germination produce 
germ-tubes which grow directly into mycelium. Uredo¬ 
spores, like aecidiospores are capable of germinating the 
moment they are mature, and as a rule do not retain their 
power of germination for a long period. 
When the first crop of uredospores is mature, they are 
dispersed by wind and infect neighbouring host-plants, 
and this rapid production and dispersion of uredospores 
continues for a considerable period of time, usually as long 
as the host-plant continues to grow vigorously. For this 
reason, uredospores are often called summer-spores. The 
principal function of uredospores is to enable the fungus 
to extend its area of distribution, but, as the power of 
germination is usually limited in its duration, that is, 
uredospores will not germinate the season after their 
production, they are not capable of enabling the fungus 
to extend its period in point of time, or to survive from one 
year to another, or during that period of the year when the 
host-plant is absent. This function is reserved for the spore 
form to be considered next. Nowhere else in the plant 
world do we find such sharply-defined division of labour 
amongst reproductive bodies, as in certain fungi included 
in this group. The uredospores, as explained, enable the 
fungus to extend its area of distribution by being dispersed 
broadcast by wind and other agents, and being capable of 
infecting suitable host-plants at once. Such infections 
form new centres of disease, as within a week or ten days 
mature spores are ready for dispersion, which in turn act 
as centres from which spores are scattered. It will thus 
be realised that in those cases where a suitable host-plant 
is present in abundance, such as a field of wheat, if a single 
plant becomes infected, its spread, under favourable con¬ 
ditions may be very rapid, resulting in an epidemic, which 
