THE RUST MITE. Ill 



Is Rust a Fungus or an Exudation of Gum f The term 

 "rust" is very indefinitely applied to a great variety of 

 plant diseases, some of which are clearly due to the pres- 

 ence of fungi, and others are considered pathological con- 

 ditions of the plant, attributable to, for the most part, un- 

 known or conjectural conditions of soil or climate. 



A good example of the first class is found in the com- 

 mon and very destructive rust of the fig. Any one who 

 will take the trouble to examine with a good glass the 

 brown discoloration upon the surface of the leaves, may 

 easily detect the sacs or asci of the fungus, filled to burst- 

 ing with the spores, or pouring them out upon the surface. 



Nothing of this kind is seen upon the leaves or rusted 

 fruit of the orange. A microscopic examination of the 

 fruit rind reveals no forms of fungus, but shows the oil 

 cells to be more or less completely emptied of their con- 

 tents, and the outer layers, the epithelial cells, clogged 

 with brownish resin, or entirely broken up and divided by 

 fissures, which permit evaporation of the fluids from the 

 underlying cells. The rind of rusted fruit, therefore, 

 shrinks and toughens, and loses by evaporation or oxida- 

 tion the greater part of its essential oil. 



Reasons for Considering it the Work of a Mite. If we ex- 

 amine critically with a hand lens of considerable magnify- 

 ing power the surface of a rusted orange, we will find here 

 and there in the depressions, groups of minute white fila- 

 ments adhering closely to the rind. Carefully transferring 

 one of these filaments to the stage of a compound micro- 

 scope, and applying a power of several hundred diameters, 

 the character of the object is clearly shown. It is the cast 

 skin of an insect. 



If the examination chance to be made in winter, when 

 the fruit is ripe, the number of these exuviae will not be 

 strikingly great ; but if made in autumn or late summer, 



