642 



appeared striped with white, since the parts of each intercostal field farthest 

 from the ribs, conducting water, showed dead tissue. The white stripes 

 were broadest at the edge of the leaf and dwindled gradually toward the 

 midrib so that it was clearly evident that the burning of the leaf occurred 

 earliest and strongest in those regions which lay farthest away from the 

 water conducting system of the large vascular bundles. 



The epidermis did not seem essentially changed in the white places, 

 but the palisade parenchyma which no longer had chloroplasts was greatly 

 changed, while a transitional zone toward the healthy tissue, provided with 

 large chlorophyll bodies arranged along the walls, showed a content still 

 green but cloudy. In tissue, which had become white, the cell walls of 

 which had remained clear, glycerin contracted only a small amount of the 

 contents so that it was necessary to conclude that in this short time a large 

 part of the contents had been used up in respiration. In the places most 

 greatly injured, the epidermis was raised here and there, like blisters, from 

 the flesh of the leaf (hum blisters) and the destruction of the chlorophyll 

 had extended even to the under side of the leaf. After some weeks it was 

 possible to observe a regeneration of the chloroplasts* in the burned leaves 

 in the above-mentioned transitional zones. Thus, a healing process had 

 taken place exactly as after slight injuries from frost. The presence of 

 mycelium could now be demonstrated beneath the burn blisters in which 

 part of the epidermal cells seemed to have collapsed. 



Rowlee^ observed a collapse of the epidermal cells even after an 8 hour 

 exposure to electric arc light which acted on the leaves of heliotrope at a 

 distance of one metre; other plants (for example Ficiis elastica), under 

 similar conditions, remained unchanged. 



In fleshy, long-lived leaves, the healthy tissue is separated from the 

 burned tissue by a cork zone, as is shown in the subjoined illustration of a 

 Clivia leaf injured in August from sunburn. It is easy to observe that the 

 position of the leaf determines the place of production of the burned spot, 

 since only those places, perpendicular to the source of heat, turned a yellow- 

 ish gray and collapsed. On the following day the burned spot was per- 

 fectly brown and brittle. The youngest leaves were uninjured. The 

 boundary between dead and living tissue becomes sharp, as soon as the 

 burned spot extends through the whole thickness of the leaf. If, however, 

 only the upper side of the leaf is injured, a faded, transitional zone is found. 

 In this, the chloroplasts turn the color of verdigris, while the remaining cell 

 contents show a yellow green. Therefore, there may occur here first of all 

 the disappearance of the xanthophyll, while the cyanophyll remains com- 

 bined in the chloroplasts. Thus, the contours of the mass of chlorophyll 

 grains, which at first refracted the light equally strongly, become less sharp 

 and a large amount of very fine granules give it a sandy consistency. 



1 Rowlee, W., Effect of electric light upon the tissues of leaves. .Just's bot. 

 Jahresber, 1900. II, p. 287. 



