406 EXPEBIMENTS WITH PLANTS 



leaves together, with the infected leaf between them, 

 in such a way that there will be a close contact all 

 along the leaf: this will ensui'e the uredo- spores reach- 

 ing the growing leaves, and will prevent them from 

 drying up. The moister the air is kept, the better; it 

 may be well, therefore, to cover the plants with a 

 paper bag (or, better still, with a bell-jar if available). 



The common Hollyhock Rust (found everywhere on 

 Hollyhocks, the Round -leaved Mallow and other Mal- 

 lows) is an example of a Rust which passes through 

 its entire life- history on one kind of plant, thus con- 

 trasting with the Rusts of Grain: it produces teleudo- 

 spores only; they germinate readily in water and may 

 be used for infection experiments (either by putting a 

 drop of water containing the spores between two leaves 

 tied together or by using the infected leaf in the 

 manner described above) . 



The yearly loss from Grain Rusts in the United 

 States is estimated at considerably more than $18,- 

 000,000. 



As an example of a Mildew we may study the com- 

 mon Mildew of the Lilac. The mycelium appears on 

 the surface of the leaf as a whitish covering (scrape 

 off a little of this, mount it in a drop of weak alcohol 

 and examine imder the microscope). After a time 

 black specks, visible to the naked eye, appear here and 

 there on the mycelium. Remove some of them and ex- 

 amine under the microscope : they have the appearance 



