492 



BOTANICAL GAZETTE [june 



In conclusion it may be said that the amount of injury follows 

 the stomatal movement rather closely. A fumigation at 5 : 30 p.m. 

 on a dark day was about equivalent in injury to one at 8:30 p.m. 

 on a bright day. 



CHEMICAL FACTORS 



Water. — It was early observed that there were rather wide 

 differences in the resistance of tomato plants grown under various 

 conditions. It was found that when plants grew rather slowly, with 

 a high chlorophyll content per unit area, they were very resistant 

 to the hydrocyanic acid. Plants growing rapidly, with a low 

 chlorophyll content per unit area, were very susceptible to injury 

 from the hydrocyanic acid. Thus, judging solely by intensity of 

 color, it was possible to select from a large group of plants 2 lots 

 differing widely in their ability to withstand injury. Variations in 

 water supply seemed to be the underlying cause of these differences, 

 although other conditions will produce similar characters. To test 

 this the following experiment was conducted : Twenty-four plants 

 in vigorous growing condition were selected and divided into 2 lots. 

 Lot I was watered only enough to keep growing, while lot 2 was 

 watered abundantly. After 10 days, plants from lot i, now dark 

 green, were fumigated and found very resistant. Plants from lot 2, 

 which were light green, were very easily injured by fumigation. An 

 exhaustive chemical analysis of these plants was not made, but 

 preliminary tests revealed one significant fact: the resistant plants 

 (lot i) had a greatly increased carbohydrate content. The reducing 

 sugars claimed attention as being the most reactive of these sub- 

 stances, and also the ones most concerned in plant respiration. 

 Determinations of the reducing sugar content of the leaves of lots 

 I and 2 gave the following results (the samples were taken at 5:15 

 P.M.): lot I, non-resistant leaves, 0.108 per cent calculated as 

 dextrose per unit weight of green tissue; lot 2, resistant leaves, 

 0.57 per cent dextrose per unit weight of green tissue. Thus the 

 resistant plants had much more reducing sugar. The actual 

 amount is not large in either case, but the relative difference is 

 great, lot 2 being 5 times as rich in reducing sugars as lot i. The 

 dry weight of the plants of lot 2 averaged little more than i . 5 times 

 the dry weight of lot i. 



