The Artificial Infection of Plants. 115 
they are infected. It is a good plan to establish a number 
of plants, say half a dozen, in the autumn; they will then 
be ready for use in the following spring. It is often con- 
venient to infect every alternate plant, so that the remaining 
plants may be kept as control specimens. The reason for 
using established plants is that the young foliage is so 
much more easily infected by the Uredinee than the 
older ; in fact, it is by no means uncommon for an old leaf 
to die off before the Uredine has had time to complete its 
development. 
Let us suppose we wish to perform the classical in- 
fection of the barberry with P. graminis. In the autumn, 
six young barberries, small enough to be covered with a 
bell-glass, having been planted, as soon as their leaves are 
fully developed in the spring they may be infected in the 
following manner. A quantity of P. graménds having also 
been provided in the autumn and kept during the winter 
in the mode before explained, as soon as the barberry 
foliage is ready, test the germinative power of the P. gramznis 
by placing a few fragments in water in a watch-glass. If 
it germinate freely and produce a good crop of promycelial 
spores, as proved by microscopic examination, the contents 
of the watch-glass may be at once employed. It is best to 
do your infection experiments in the evening. Water one 
of the barberries freely through the rose of a watering-can 
and then cover it with a bell-glass ; then water the outside 
of the bell-glass. By so doing the temperature of the 
enclosed air is reduced, and the inside of the bell-glass as 
well as the leaves of the barberry become bedewed with 
condensed vapour. After leaving it a few minutes, remove 
the bell-glass, and apply the germinating spores with a 
camel-hair pencil. As the promycelial spores easily become 
diffused in the water in the watch-glass, by stirring it with 
the camel-hair pencil the water becomes equally charged 
