these ribs (intercostal fields), suffer earliest and most extensively from 

 frost, sunburn, etc. With the action of the acids in smoke, however, the 

 boundaries between the dead and healthy tissues are as sharp as usual, while 

 with the action of atmospheric factors they are less distinct because of the 

 many transitional stages. 



The appearance of the injury in decidedly smoky districts also differs 

 because, besides sulfurous acid others/ such as sulfuric acid, hydrochloric 

 acid, hydrofluoric acid, etc., become eff'ective. The action of these acids 

 strongly soluble in water (hygrophilous) is restricted, however, to the 

 immediate surroundings of the centre of production, where they act at any 

 rate much more intensively and kill the tissue rapidly, while sulfurous acid, 

 distributed in a gaseous form over wide districts, is usually breathed in by 

 the plant slowly but permanently. The former effect, appearing rapidly and 

 eating into the tissue, is distinguished as "acute" from the phenomenon of a 

 slow poisoning which is termed "chronic injury from smoke." Of course, the 

 latter must have made itself felt inside the plant before the external char- 

 acteristics appeared. The chlorophyll apparatus is changed (as has been 

 proved by Wislicenus^ with the spectroscope and by Sorauer- with the 

 microscope) even if the plants still appear perfectly normal. In this case an 

 "invisible injury from smoke" is spoken of. Naturally such disturbances 

 can be averted very easily and the plant, as has been found, is in a position 

 to cure itself after the cessation of a weaker action of smoke. 



Such cases will also occur in forestry if changes in local conditions take 

 place which divert a stream of smoke or dilute it to the point of uninjurious- 

 ness. Wislicenus^, to whom we owe recent especially thorough, conscien- 

 tious investigations, states that the point of uninjuriousness is 0.0005 P^^ 

 cent, of the volume. 



He emphasizes the fact that, aside from the extreme individual differ- 

 ence in sensitiveness, the stage of development of the plant is of decisive 

 significance. The time when the new leaves and needles tmfold is the most 

 critical ; the plants suft'er most then, because the cuticular covering of the 

 epidermis is still insufficiently developed. The above-mentioned influence 

 of light, which promotes injury and was observed by v. Schroder and Hartig, 

 has been tested experimentally by Wislicenus*, who found that, visible 

 injuries did not appear in young spruces in the dark and in winter, although 

 an increase of the sulfur content could be proved. Ramann and Sorauer^ 

 have also observed that the amount of demonstrable sulfur in an organ is 

 not determinative for the degree of injury and Count zu Leiningen'' calls 



1 Wislicenus, Resistenz dor Fichte g-cg-on saure Rauchgase bei ruhender und 

 tatiger Assimilation. Tharandter Forstl. Jahrbiicher 1S9S, Sept. 



~ Sorauer, P., u Ramann, E. Sogenannte unsichtbare Rauchbeschadigungen 

 Bot. Centralbl. 1899, Vol. LXXX. See also Brizi in Zeitsch. f. Pflanzenkrankh. 1901, 

 p. 160. 



3 Wislicenus, H. Massnahmen gegen die Ausbreitung von Hlittenrauchschaden 

 im Walde. Referat 5 der Sekton VIII d. Internat. landw. Kongresses in Wein 1907. 



4 Tharandter Forstl. Jahrbucher 1898, p. 152. 



5 Loc. cit. 



6 Graf zu Leiningen, W., Licht und Schattenblatter der Buche. Naturwiss. Z. f. 

 Landw. u Forstw. III. Jahrg. Part 5. 



