734 



changed. If the middle part of the leaf is injured, the edges curl up like 

 the sides of a boat. 



Attention should be called in passing to the fact that in many roses 

 (for example, Rosa turbinata), a similar discoloration appears in the late 

 autumn. In this rose, for example, I found that the older leaves, still 

 hanging on the stems, had become dully spotted with black without any 

 previous red coloration; this arose from the contraction and browning of 

 the contents of the epidermal cells. These cells, however, retained their 

 natural turgidity and height, but began to collapse after having been afifected 

 by asphalt fumes. In this the contents of the mesophyll also retain their 

 normal consistency and position for some time, while, in the autumn colora- 

 tion, they contract at once and change into uniform masses, at first green, 

 but later turning brown. Under the microscope parasitic blackening 

 (Asteroma radiosum, etc.) can be distinguished easily from asphalt cor- 

 rosion. 



Before I began my experiments, Alten and Jannicke^ had already 

 described the blackening of roses and strawberries caused by the action of 

 asphalt fumes. They considered the iron which was proved present in these 

 fumes to be the actual injurious factor since it combined with the tannic 

 acid of the cells and they supported this theory by experiments in which 

 they produced black spots, corresponding to those in asphalt injuries, b)' 

 sprinkling the leaves with ferrous chlorid and ferric sulphate. Ferric 

 chlorid did not have this effect. 



I could not obtain this result and observers who have sprayed with iron 

 solution as a means of overcoming chlorosis and icterus do not report any 

 blackening. 



In the strawberry leaf illustrated in Fig. i68, 2 (a cultivated form of 

 Fragaria chilcnsis), only a partial blackening of the upper side is found at 

 g because only this part of the leaf had lain free ; otherwise the phenomena 

 were similar to those in roses, the curling of tlic leaf edges, the ])artial dry- 

 ing of the leaf serrations, etc. 



In Fig. 168 5 we see a leaf of Ampelopsis quinquefolia a few weeks after 

 it had been acted upon by tar fumes from a factory making electric light 

 carbons. The less diseased leaves were found to be still green but not out- 

 spread; the edges were curled up like bowls and the inside of the blade 

 wrinkled by the outpushing of some of the tissue lying between the finer 

 ramifications of the veins. At times small places with a cork colored upper 

 surface were found near the midrib. With more extensive injury, these 

 places were always present and passed over partially into blight spots which 

 became dry and ultimately united. Finally, each leaf may show very regular 

 markings due to the, drying of the intercostal fields. (Fig. 168 jj.) These 

 dry places often break away, due to the rubbing of the leaves against one 

 another, thus producing a lattice-like perforation (Fig. 168 3/). 



1 Alien, H., und Jannicke, W. Eine Schadig-ung von Rosenblattern durch 

 Asphaltdampfe. Rel". Zeitschr. f. Pflanzenkrankh. 1891, p. 156 und 1892, p. 33. 



