292 NINETEENTH REPORT. 



glued to the outside of the tube so that the zero point was opposite the 

 bubble. Three such potometers were set up. As water evaporated from 

 the leaf, the bubble moved forward, the distance traversed being used 

 on the scale. By this means the relative transpiration was determined. 

 After three hours the distance traversed by the bubble was recorded. The 

 diseased spots were then coated with anhydrous lanolin on their upper 

 and lower surfaces in order to prevent the evaporation of water from 

 these parts. After three hours the reading was again taken. 

 The following table gives the results : 



Table 3. 

 Transpiration of diseased leaves with spots covered and uncovered. 



Hourly readings of the temperature were taken. These ranged from 

 19° to 20° C. 



While this experiment was conducted upon a small scale, yet from the 

 fact that the relative transpiration of the leaves with the spots uncovered 

 checked so closely with that from the leaves with the spots covered, it is 

 fairly certain that the disease spots, at least when old and dried, do not 

 transpire. 



No other physiological studies were made upon the diseased host. The 

 damage to the clover plant is for the most part brought about by the 

 direct injury to the leaf surface. The profound unbalancing of root and 

 top leads to rapid growth. The new leaves are again subjected to the 

 same conditions. Thus so long as the weather favors, the plant is being 

 pushed to extra leaf production, and each crop of leaves becomes more 

 severely attacked. The badly diseased leaves cannot perform the 

 photosynthetic functions, so that the growth takes place at the expense 

 of tlie cell reserves. It is extremely likely that the young diseased spots 

 transjjire more than healthy leaf surface, as was found by Levin, but 

 in the case of clover these spots dry out so quickly that excessive water 

 loss does not continue long enough to be very important. We may con- 

 clude, therefore, that the chief characteristic of the clover plant in 

 disease is the unbalanced metabolism, and that the injury comes from 

 the constant depletion of the cell reserves. 



