62 EXPERIMENT STATION. [Jan. 



is termed " topburii," a certain textnre being required to do this. 

 Even a difference of a few degrees in the night temperature 

 exerts marked influence on the texture of the tissue. Lettuce 

 plants which are grown under a night temperature of 45° F. 

 differ greatlj from those grown at a temperature of 50° F., the 

 structure of the plant being modified for each degree of increase 

 or decrease in the temperature. With the lower night temper- 

 ature there is less likelihood of burning than with the higher. 

 If a night temperature of 50° F. is maintained during cloudy 

 weather, and on a following bright, sunshiny day the tempera- 

 ture is allowed to reach 75 or 80° F., topburn is likely to 

 occur. On the other hand, a night temperature of 40° F., 

 during cloudy periods followed by a relatively high day tem- 

 perature, is not likely to produce any burning. 



Light has a marked accelerating effect on transpiration, as 

 the stomata of the leaves open in bright sunshine, and the loss of 

 water from the foliage under these conditions is large. In the 

 blossom end rot of tomatoes, a disease caused primarily by 

 heavy drainage of water from the fruit during its development, 

 we have noted a difference of 35 per cent, in the amount of rot, 

 caused by the shading of the tomato j)lants by other plants, 

 reducing the transpiration. 



Practical growers who have had occasion to fumigate green- 

 houses to destroy aphis, white fly and thrips have observed that 

 their plants will burn slightly at one time and at other times 

 not; for instance, hydrocyanic acid gas, tobacco leaves or con- 

 centrated solutions of tobacco, and other fumigants sometimes 

 cause injury to plants and at other times produce no burning. 

 This is also true of spraying mixtures, whether used in the 

 greenhouse or field. 



The experiments given here are designed to show some of the 

 causes underlying burning from fumigation, and were made by 

 Mr. F. L. Thomas during his course as a graduate student. 

 The plants selected were cucumbers, as they are susceptible to 

 burning from various causes, and are also affected greatly by 

 variations in light intensity. The plants were grown under five 

 cloth screens, ranging in grade from mosquito netting to rather 

 finely woven cotton material, each covering a space 2 feet wide, 

 6 feet long and 2 feet high on a long bench in the greenhouse. 



