FROMME: THE CULTURE OF CEREAL RUSTS 505 
sorium and germ tube are poured. One or more infecting hyphae 
are now sent off from the substomatal vesicle and these imme- 
diately establish connections with the surrounding host cells by 
means of haustoria. The early stages of development and the 
position and shape of the substomatal vesicle, and the number of 
infecting hyphae arising from it, are morphological distinctions 
between the different species. 
In the rusts the period between inoculation and sporulation is 
known as the incubation period. This apparently varies some- 
what with different species but the normal range for the uredo as 
reported by a number of authors lies between eight and twelve 
days. Marshall Ward has noted that the normal incubation 
period is shortened during clear sunny weather and Iwanoff (16) 
found that shading delayed aecidium-formation in P. graminis. 
What stimulus, or stimuli, determine the entrance of the germ 
tube into the stomata of the leaf has not been established. It is 
perhaps most generally held that the host exerts a chemical influ- 
ence on the germ tube. If this is true it is apparently not a specific 
influence, since germ tubes have been shown to enter the stomata 
of quite the wrong host. Massee (22) endeavored to demonstrate 
‘a positive chemotropism but was unable to eliminate the effects 
of hydrotropism from the experiment. Marshall Ward (37) has 
noted an apparent heliotropic curvature in the germ tubes of P. 
dispersa. Balls (1) placed uredospores on a rubber film provided 
with small holes. Laboratory air was on one side of the film while 
the air on the other side was saturated with moisture at 23°. The 
germ tubes entered the holes and grew through into the region of 
higher pressure of water vapor. He believes that growth towards 
greater moisture will explain the entrange of the germ tube into 
the host. 
A recent article by Melhus (24) on the culture of parasitic 
. fungi deals with the culture of P. Helianthi, P. coronata, P. graminis 
and P. Sorght. The-methods employed by him consist in spraying 
the plants to be inoculated with a spore suspension, covering 
with a hood and placing in a refrigerator or humidity-box for 
twenty-four hours, at 14° for P. Helianthi, 16° for P. coronata, and 
18° for P. Sorghi. He finds that P. coronata will not maintain itself 
- even though supplied with plenty of host material. It is more 
