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 Uredineae 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. grauiijiis. 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. graniinis 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.graminis 

 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 



