IOWA ACADEMY OF SCIENCES. 169 



to some varieties of carnations, while several other varieties are 

 nearly exempt from its attacks. One variety (Wm. 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 

 Roestelia. In the spring of 1894 G. macropus was fairly 

 abundant at Ames, but the spricg and summer were very dry, 

 and, as a consequence of the drouth, Roestelia lyirata on Plrus 

 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 tivo 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. 



