386 iMINNESOTA BOTANICAL STUDIES 



The variation in weight of the enlarging paper was found to be 

 not greater than 5 per cent. This error, however, automatically re- 

 duced itself to a negligible quantity when the pustular area based 

 on the entire transpiring surface was determined. 



While the whitish areas which develop about the pustules may 

 increase transpiration, Stakman^" has shown that at least for a long 

 time in non-rust-resistant varieties (such as those used in these ex- 

 periments) the cells remain intact, and although the normal stomatal 

 activity may be afifected, it is certain that the epidermis is not rup- 

 tured except at the pustules. When physiologists have devised a 

 method of measuring the real internal transpiring svirface of a leaf, 

 we may be able more nearly to calculate the affected transpiring area 

 of rusted plants. 



EXPERIMEXTS WITH WhEAT 



The wheat used in these experiments was a bluestem, Minnesota 

 No. 169, which was obtained from Mr. John Parker of the Bureau 

 of Plant Industry, who was then stationed at the ^Minnesota Agri- 

 cultural Experiment Station. He also furnished the seed of the 

 other cereals used. This wheat was inoculated with uredospores of 

 Puccinia graminis tritici. This rust, as well as the others used on 

 the small grains, was obtained from the stock cultures of Dr. E. C. 

 Stakman at the University Agricultural Experiment Station, and 

 afterwards kept growing on stock materials in the greenhouse physi- 

 ological laboratory of the University of Minnesota, where the ex- 

 periments w^ere performed. 



The first weighings were made at 3 p.m., January 29, and follow- 

 !ng weighings on alternate days imtil 10 a.m.. Eebruary 12, when 

 the experiment was concluded. The results have been ])lotted in 

 the form of graphs in figure 1. The light solid line gives the trans- 

 piration in grams per sq. dm., as indicated on the y axis. It rep- 

 resents the water loss of six healthy plants in two containers. The 

 abscissae represent the intervals of time between weighings. Simi- 

 larly, the heavy solid line gives the same data for seven rusted plants 

 in three containers. The broken line shows the evaporating power of 

 the air as determined by Livingston's standardized porous cup at- 

 mometers, which were operated near the plants. These data have 

 been reduced to that of the standard atmometer."' 



A glance at the graphs shows that at the beginning the trans- 

 piration of the rusted plants is somewhat higher than the controls. 



