643 



Finally, the chloroplasts form groups, a dirty tea-green to a blackish green 

 in color, which assume a cord-like form because the cell collapses. These 

 content masses, which lie against a wall, bleach very quickly in sunshine 

 and cause the yellowish gray color of the burned place. The cell walls do 

 not lose their cellulose character, as is proved by testing them with chlor 

 zinc iodide. 



The healthy tissue begins at once to cut itself oft" from the injured 

 tissue by a cork zone {k) whereby the cells of the transitional zone (hr), 

 which have remained rich in contents, at first somewhat enlarged by an 

 undulation of their walls {h, z), show enlarged intercellular spaces and 

 gradually die. 



When the burned spot becomes somewhat older, it turns a deeper 

 brown, in which the epidermal cells, which have not collapsed {e), partici- 

 pate even up to the healthy tissue. The cork zone {k) is produced by a 



Fig-. 151. Cross-section through a sunburn spot in a leaf of Clivia nobilis. 



division and elongation of the mesophyll cells which have remained alive at 

 the edge of the burned place. The normal cells, back of these (/>) usually 

 remain somewhat poorer in chlorophyll. The callous appearance of the 

 peripheral zone (w) of the normal leaf part at the edge of the 'burned 

 place should be noted ; this is explained by the distention of the cells, which 

 develop the cork zone, and of the mesophyll {h) lying in front of them, 

 which had been injured but did not die at once. 



Sunburn Spots in Conservatories. 



Complaints of the occurrence of burned spots on the leaves of tender 

 plants in conservatories abound, especially in spring, and opinions as to 

 their production differ greatly. Sometimes bubbles in the glass are held 

 responsible for this. Sometimes, it is thought that the drops of water, 

 which remain on the upper surface of the leaf after the plants are sprinkled, 

 act as burning glasses or become so warm from the sunshine that they injure 



