IOWA ACADEMY OF SCIENCES. 
169 
to some varieties of carnations, while several other varieties are 
nearly exempt from its attacks. One variety (W m. Scott) is nota- 
bly immune. We know of no well authenticated case in which 
the true rust (Uromyces) has been found upon this variety, 
although we have repeatedly observed it growing in green- 
houses where other varieties were badly rusted. 
In the present state of knowledge concerning the conditions 
of parasitism, it is impossible to completely explain the 
immunity of varieties. The structure and chemical composi- 
tion of a variety are intimately associated with its suscepti- 
bility or non- susceptibility to the attacks of a particular 
fungus; but what is the relative importance of these, or what 
part is played by the mysterious factor called “inherent 
vigor ” we do not know. 
In conclusion we will record our observations on the effect 
of moisture on the prevalence of Gymnosporangium and 
Koestelia. In the spring of 1894 G. macroious was fairly 
abundant at Ames, but the spring and summer were very dry, 
and, as a consequence of the drouth, Roestelia pirata on Pirus 
coronaria was rare. As previously stated, even attempts at 
inoculation of P. coronaria failed that season. In the spring of 
1895 showers were frequent during the month of May. This 
season Roestelia was so abundant on P. coronaria that it was 
difficult to find leaves which were not affected. Everywhere 
the wild crab trees were conspicuous because of the Roestelia 
on their leaves. 
On Long Island the summer of 1894 was very dry. The Red 
cedar grows spontaneously here and is very common. May 15, 
1895, we searched very carefully through a large grove of Red 
cedars standing near an orchard and found only three cedar 
apples. At Westbury, N. Y., a Red cedar standing in the 
midst of a nursery bore only tioo cedar apples. At Queens, 
N. Y. , three Red cedar trees grew on one side of a road, on the 
other side of which was an orchard; not a single cedar apple 
^could be found on the cedars. 
