1913.] PUBLIC DOCUMENT — No. 31. 71 



and this would appear to prove that burning takes place through 

 the stoniata. In other cases burning appears in more or less 

 large, irregular spots scattered over the leaf, which was the case 

 in these experiments. The margins of the leaves of cucumbers 

 at times become injured from excessive transpiration, the thin 

 and apparently less developed tissue farthest from the water 

 supply being most suscej)tible. From the nature of many of the 

 burnings resulting from fumigants in these experiments, it does 

 not appear that the condition of the stomata has anything to 

 do with the susceptibility to burning, but is due rather to the 

 condition of the tissue. The experiments bear out this conclu- 

 sion, it being found that the plants grown under conditions 

 calculated to develop a less resistant tissue are the ones which 

 burn most severely, and that any condition which has a ten- 

 dency to develop immature tissue favors injury from fumi- 

 gants. Other factors, such as the continued use of too high 

 temperatures, and probably in some cases certain fertilizers 

 and stimulated growth from various causes such as pruning, 

 growing in sterilized soil, electrical stimulation, etc., bring 

 about the formation of tissue susceptible to burning ; and while 

 burning may be associated in some cases with the condition 

 of the stomata and with the process of photosynthesis, the de- 

 velopment of the cuticle and texture of the tissue in general 

 plays an important part. In plants growing under more or 

 less dense shade IMr. Thomas noticed, as anticipated, that the 

 epidermis was more poorly developed than that of those plants 

 growing under better light conditions, and that the hairs on the 

 leaves of the well-lighted plants were much larger and more 

 numerous than those on the leaves of the poorly lighted plants. 

 Ecologically, hairs are for the purpose of modifying the effect 

 of light and transpiration, but whether their presence has any 

 bearing on susceptibility to burning was not learned. The dif- 

 ference in the number of stomata on the upper surface of the 

 leaves of plants growing under varying moisture conditions 

 is evidently an adaptation to transpiration. 



As regards the various factors associated with burning, such 

 as light, atmospheric moisture, temperature, etc., we are in 

 need of more experimental evidence relating to the specific role 



