RELATION OF SOME RUSTS TO THEIR HOSTS 
187 
have been well illustrated by Evans (1907). The vascular bundles 
are apparently never invaded by the mycelium and the parenchyma 
sheaths which surround them only occasionally have haustoria in 
their cells. 
When limited areas of the leaf are infected, an interesting phenome- 
non takes place. After the tissue has been infected for some time — 
in some cases in so short a time as nine days — the ends of the leaf 
beyond the infected areas begin to die and the regions immediately 
surrounding become yellowish, while the infected areas retain the 
green color of healthy tissue. The infected areas, of which there may 
be two or three on the same leaf, often become surrounded by dead 
tissue, except perhaps for the midrib itself. The infected areas them- 
selves still retain their original green color, and sections show that the 
cells of these regions have all the appearance of normal cells, except 
for the presence of haustoria within them. They are turgid and filled 
with green chloroplasts. The neighboring tissue, on the other hand, 
is brown and the cells are shrivelled up and dead. 
Effect of Temperature upon Development 
Puccinia coronata 
Experiments i and 2. — Two experiments were carried out to dis- 
cover the effect of temperature upon the development of Puccinia 
coronata within its host. In each, six pots of oats were used. These 
were inoculated by spraying with a spore suspension of the uredospores, 
and, after being left for 24 hours under belljars, four pots of each set 
were removed to a well-lighted room where the temperature averaged 
15° and 13° C. respectively for the two experiments. Two from each 
set were kept in a similar room where the temperature averaged 20° C. 
The incubation period of the rust at 20° C. in both experiments was 
9 days. The incubation period of the rust at 15° C. in the first ex- 
periment was 13 to 15 days, and the incubation period for the rust at 
13° C. in the second experiment was 15 days. 
The results of these two experiments thus indicate that low tem- 
peratures retard the development of the rust in its host. 
Puccinia Sorghi 
Experiment 3. — This experiment was carried out with Puccinia 
Sorghi on corn in the same manner as were the two preceding experi- 
