380 MlxXNESOTA BOTANICAL STUDIES 



the healthy specimen evaporated 23 cc. of water under parallel 

 conditions. 



"Thus the brancii with the rusted leaves absorbed nearly twice 

 as much water as the healthy branch, and yet failed "to remain un- 

 wilted. The rust covered the lower surface of nearly all of the 

 leaflets almost completely, and the extra demand for water thus im- 

 posed upon the plant was equivalent to doubling the leaf surface, as 

 indicated by the volume of water transpired." 



Dr. Harry B. Humphrey, Pathologist in Charge of Cereal Dis- 

 ease Investigations, of . the Bureau of Plant Industry, has kindly 

 furnished me with the data on an experiment which he performed 

 in the physiological laboratory of the University of Minnesota in 

 1906. He compared the rate of transpiration of rusted and non- 

 rusted wheat plants. He used two paraffined wire baskets, each 

 containing four wheat plants. The baskets were effectually sealed 

 so that water loss could take place from the plants only. After the 

 first two leaves of each of the four plants in one container were 

 badly rusted, as a result of an inoculation with uredospores of 

 Puccinia rubigo-vera, the loss of weight from the two baskets was 

 determined for a period of 22 hours, a part of which time the plants 

 were in full sunshine out of doors. The loss in grams per unit area 

 was calculated and it was found that the rust-infected plants trans- 

 pired 38 per cent more water than the healthy plants of the same 

 size and age. The area of the rust pustules was less than one per 

 cent of the trans])iring surface. 



Reed and Cooley" have carried out a series of experiments upon 

 the transpiration of apple leaves afifected with the cedar rust fungus, 

 Gymnosporanginm Jiiniperi-virginianac Schw. The work was car- 

 ried out on leaves and twigs on the trees in their normal position. 

 Small branches of similar leaf area were enclosed in glass cylinders 

 and the amount of transpired water was measured by the increase 

 in weight of calcium chloride, which absorbed the exhaled moisture 

 as it was continuously drawn from the glass cylinder through the 

 calcium chloride tube. Fifty-two determinations were made upon 

 healthy and diseased leaves of York Imperial and twenty-six upon 

 the leaves of Ben Davis. Each experiment extended over a period 

 of approximately two hours, and experiments were made at various 

 periods of the day from July 9 to August 20, 1911, and July 26 to 

 August 23, in 1912. They found that the unit transpiration of the 

 diseased leaves of both varieties of apples was in the majority of 



