Weaver: effects of rusts upon transpiration 403 



preceding experiment (where only a small amount of rusted area 

 was involved) and the two exceptions to the general results shown 

 in the table, the writer is of the opinion that further experiments 

 will reveal a higher water loss from rusted than from healthy carna- 

 tions where they are grown under similar conditions. 



Discussion and Conclusions 



A review of the preceding data shows that without exception, 

 the presence of the cereal rusts even in very small amounts has a 

 decidedly accelerating influence upon the rate of transpiration. The 

 increase in transpiration occurs about the time the pustules break 

 through the epidermis and evidently continues for a long period of 

 time. Further, a quantitative relation exists between the extent of 

 the pustular area and the increase in the transpiration rate. This 

 was evident, with one exception, in every case where the area of the 

 pustules was measured, and indeed the relationship was so close 

 that an increase of 0.2 to 0.3 per cent in the pustular area was regis- 

 tered by a definite increase in the transpiration rate. After most of 

 the pustules were well broken out, any increase or dcrease in the 

 accelerated transpiration rate was not apparent. It is possible that 

 the older pustules permitted less water to escape while the initial 

 accelerated transpiration rate was maintained by the appearance of 

 new pustules. In most cases when more severe environmental con- 

 ditions as regards transpiration were brought to bear upon a battery 

 of cereals, the rusted plants gave a correspondingly greater rate of 

 transpiration over the normal than before or after such a period 

 of stress. 



Although a close relation exists between the amount of excess 

 water loss and the extent of the pustular area, still the writer does 

 not hold that the cause of accelerated transpiration is due wholly 

 to the torn epidermis. As is well known, whitish areas occur about 

 the rust pustules. Even if the mesophyll cells in these regions do 

 remain alive for a long time in susceptible hosts, as shoAvn by 

 Stakman^° in the case of certain cereals, and by Ward" in Bromiis 

 sp., still their transpiration rate might be greatly altered. When 

 these leaf cells become half filled with haustoria, as is frequently the 

 case at the time of pustule formation, it is hard to conceive how they 

 would function normally. 



The increase in transpiration rate reported by Blodgett^ for 



