found that the sulfurous acid is taken up nf)t only by the stoniata but 

 uniformly by the entire uijper surface of the leaf. He found that just as 

 much gas was taken up by the upper side, free from stomata, as by the 

 underside which abounds in these respiratory organs only the action of the 

 gas which had penetrated the underside was much more rapid and energetic. 

 This is explained by the fact that sulfurous acid is greedily absorbed by 

 water and oxidizes easily in contact with it. Now, since the loss of water 

 from the leaf into the air takes place especially through the porous under- 

 side which abounds in stomata, the action of the gas manifests itself so much 

 the more here. If the water in the niicellar interstices of the cell-walls is 

 combined with the acid in greater amounts than can be supplied to the 

 walls, they become deficient in water and finally dry up, thereby losing their 

 capacity to conduct water. 



Thus only those cell bodies will remain well supi)lied with water and 

 will retain their normal color, which lie directly against the rapidly conduct- 

 ing tissue of the vascular bundles while the dry part, lying between the 

 vascular bundles (the leaf veins) takes on a faded, brownish color. This 

 phenomenon of bright green venation in a faded leaf mass has been taken 

 as a characteristic point for recognizing leaf poisoning from sulfurous acid. 

 Hartig^ maintained that the red coloration of the guard cells of the stomata 

 in conifers is a positive characteristic of injury due to acid. This statement, 

 however, was immediately refuted by other observers. Wieler- and 

 Sorauer^ have proved that slow death, under the influence of light and with 

 the action of very different factors causes a red coloration. Directly in 

 connection with this characteristic, apparent to the naked eye, is the 

 decreased water evaporation from poisoned leaves, as found by v. Schroder 

 in weighing experiments. The amount of the transpiration may be used, 

 however, as the expression of the amount of production and thus it may be 

 concluded here that the leaf-assimilation is less. The general effect of the 

 poisoning on the plant body will, therefore, resemble permature defoliation 

 and, in fact, the action sets in the more quickly the greater the amount of 

 sulfurous acid present, the drier the air, the higher the temperature and the 

 stronger the illumination, which are the factors inciting the leaf to more 

 intensive activity. Because^ of this fact, which has been determined experi- 

 mentally, the supposition that the smoke from smelting works and from 

 hard coal will act less vigorously at night than during the day is pertinent, 

 and we will find later that it is confirmed. 



Caution is necessary, however, when forming one's judgment from the 

 characteristic of green venation and dried middle fields. Almost all injurious 

 atmospheric effects express themselves in such a way that the parts of a leaf 

 lying furthest from the water-conducting ribs, namely, the fields between 



1 Hartig-, Rob. tJbcr die Kinwirkungr des Hiitten- und Steinkohlenrauches aiif 

 die Gesundheit der Nadelholzbaume. Miinchcn 1S96, Rieg-er'sche Biicbliandl. 



- Wieler, tjber iinsichtl)are Rauchscluidcn bei Nadelbaumen. Zeitsclirift I'tir 

 Forst. VI Jasdwesen 1897, Sept. 



3 Sorauer, P. Ubcr die Rotfiirbung: von SpaltcJffnungcn bei Picca. Notizbl. 

 d. Bot. Gart. Berlin 1S96, No. 16. 



