SHADE TREES. 231 



symptoms even in the same species. Burning from gases in general is 

 affected by light, soil and air moisture, and the age of the foliage consti- 

 tutes a factor, as probably does the condition of the stomata or breathing 

 pores of the leaves, which vary in number from 800 to 170,000 per square 

 inch of leaf surface. 



Some recent European experiments show that burning from gases is 

 intimately associated with sunlight, a fact long recognized by American 

 gardeners in connection with the fumigation of greenhouses. Fogs and 

 mists are conducive to burning. As is well known, they have a tendency 

 to drive gases downwards, imprisoning them, as it were, and preventing 

 their diffusion. Burning even with the same concentration of gas is more 

 severe in moist than in dry air. Southern exposures are the most favor- 

 able to burning from gas, as are the exposed tops of trees, where the light 

 conditions are more intense, and it has been demonstrated that burning 

 is associated with the assimilative activity of the leaf, which is at its 

 maximum during bright sunlight. Hence, a plant in sunlight will show 

 discoloration or burning at a much less degree of concentration of the gas 

 than during cloudiness or darkness, and the proportions of sulfur dioxide 

 in the atmosphere must be considerably greater to produce the same 

 effects under poor light conditions than during sunlight. 



As the stomata or breathing pores are open during bright sunlight and 

 closed during dull days and darkness, these organs would appear to have 

 some influence as regards burning. However, experiments have shown 

 that the stomata or breathing pores of the leaves, at least in some cases, 

 close immediately when exposed to various gases, and in this way they 

 may prevent severe injury to a certain extent. The age of the leaf is 

 very important as regards susceptibility to burning, the younger leaves 

 not being so susceptible to burning as the older ones. This is shown by 

 injury from illuminating gas in greenhouses. This gas affects the older 

 foliage, while the younger leaves remain normal or unaffected with small 

 dosages. This may be explained in two ways, i.e., that the stomata of 

 the older leaves which are injured are more or less inactive, whereas on 

 the younger ones they are more active. Moreover, the assimilative 

 processes more nearly approach their maximum condition in the well- 

 developed or older leaves than in the younger ones; or, in other words, 

 carbon assimilation is undoubtedly more active during June and July 

 than during April and May in some species, and as burning is associated 

 with the assimilative activity of the foliage, burning may naturally be 

 expected to occur more severely to older leaves than younger ones. The 

 probability of the inactivity of the leaf stomata constituting a factor in 

 susceptibility to burning from gases is borne out by the fact that some 

 species which possess thick and tough leaves appear to be the most sus- 

 ceptible to burning, and the inability on the part of the stomata to respond 

 to external influences may be an important factor underlying injury from 

 gases. The condition of the atmosphere is often extremely variable even 

 in the same locations, and any gas would be variable in its concentration, 



