194 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
at 53° F. it approaches the average for the higher temperatures. The 
highest temperature at which the rust will produce infection in wheat was 
found to be 8o° F. under the conditions of the experiments. The figures ob- 
tained by Johnson (1912) are cited in evidence that it is not failure of the 
spores to germinate which determines the infection limits observed in the 
experiments. Johnson (p. 48) found the optimum temperatures for the 
germination of the uredospores of the common cereal grain rusts to be low 
— 12° to 17° C. — helping to explain such observations as the difficulty of 
keeping rust in culture in the greenhouse in the summer, when the incubation 
period of the rust is shorter than at any other time of the year but it is re- 
markably difficult to obtain infection; the difficulty of finding viable uredo- 
spore material in the spring, the larger number of the spores having already 
germinated; and the favoring of rust development and epidemics by sub- 
normal temperatures at the critical infection periods in the life of the host 
plants. 
Stimulants and Depressants ; Toxic Agents 
Observations on chemical and physical agents stimulating or depressing 
the vitality of the host plant indicate that susceptibiUty to rust infection 
is affected in like manner. 
Sheldon (1903, p. 44) found that in the case of the asparagus rust and 
the carnation rust complete immunity to infection can be produced by 
lowering the vitality of the host — an end the failure to achieve which in 
the case of the brown rust of the bromes caused disappointment to Ward. 
Concerning the asparagus rust, Sheldon states (p. 44) : 
Attempts have been made repeatedly not only on asparagus but on several species of 
pinks, to inoculate them when they are not growing well. It was tried on repotted plants, 
those attacked by insects, and young seedlings. A failure was the result in nearly every 
case; while with vigorous, growing plants which had become established there were few 
failures — thirty-seven out of forty-two inoculations made at one time being successful in 
one instance — approximately 90 percent. 
In his experiments with the carnation rust Sheldon (p. 83) found attacks 
by thrips a very disturbing factor, as it was almost an impossibility to 
secure infection where the thrips had worked to any extent either before or 
after inoculation, while vigorously growing plants which were free from 
thrips were readily inoculated. 
Spinks (191 3, p. 243) and Voelcker (1912, p. 319) have made observa- 
tions on the susceptibility to rust of the wheat plants grown in the pot- 
culture experiments on the fertilizing effects of small quantities of the salts 
of the heavy metals conducted at Woburn, England. They found that 
the lithium salts of the 191 1 experiments depressed susceptibility, with the 
exception of lithium nitrate which gave increased susceptibility to rust. 
The experiments of 1912 were with zinc salts, and they were all found to 
produce increased susceptibility to rust, zinc nitrate seeming particularly 
notable in this respect. 
