472 



CITRUS FRUITS AND THEIR CULTURE. 



transpiration and the assimilation of food. Beneath the 

 warts on the opposite side of the leaf, there is often a 

 well-marked, conical depression, corresponding to the 

 elevation on which the excrescence is situated. Under the 

 warts on the fruit there is an abnormal thickening of 

 the tissue, resulting in the formation of somewhat conical 

 elevations. Thus the corky portions are lifted above the 

 normal level of the rind. The warts are at first yellowish, 

 then grayish, becoming dusky in appearance as the dis- 

 ease advances, until they become almost black, and event- 

 ually crack and open. When the excrescences are iso- 

 lated they are minute and of the shape of a cone or trun- 

 cated cone. Generally, however, they are confluent, pre- 

 senting the appearance of a bark-like substance firmly at- 

 tached to the 

 epidermis. 



The specific 

 cause of the dis- 

 ease is a minute 

 parasitic fun- 

 gus, a species 

 ofOladosporium 

 described b y 

 Prof. F. Lam- 

 son- Scribner in 

 1886, probably 



Fig. 100. Spores and spore-bearing parts of scab identical With 

 fungus (Cladosporium elegans penzig). (j f>lf>nans Pen- 

 B, spores. A, spore-bearing parts. 



zig. The spores 



are very small, smoky in color, and usually one or two, 

 though sometimes three-celled They are borne on brown- 



