294 GENERAL PLANT PATHOLOGY 



caused by the acidity of raindrops serve repeatedly as the points of 

 entry of parasitic fungi, for there are many leaf spots and fruit spots 

 that show concentric rings of diseased tissue in the earUest lesions pro- 

 duced. A fungus, which is stimulated to growth by an acid condition 

 of the cell sap, would find ideal conditions for the commencement of 

 growth by entering areas influenced by acid raindrops. 



Traumatism. — Traumatism, or mechanic injury, may be of various 

 sorts and the effects are dependent upon the form and severity of the 

 injury. Mechanic injury to the plant usually takes the form of wounds, 

 which may be divided into natural and artificial. Natural wounds are 

 those which are produced on plants living in a state of nature, or in a 

 cultivated state in which other natural agents are concerned in their 

 production, man's activity not being considered. Insects and worms 

 may make burrows in the organs of plants. For example, bark boring 

 is accomphshed by species of beetles, so also are tunnels through the bark 

 and the wood. Pith flecks are minute brown specks, or patches, found 

 in the wood layers of trees. They consist of holes formed by boring 

 insects fiUed with dead parenchyma cells, or dead empty cells filled 

 with fungous material. Eroded and skeleton leaves, and shot-holes 

 in the leaf tissue are directly traceable to the work of cutting insects. 

 Frost cracks are longitudinal wounds produced by the rending action 

 of the frost on the bark and wood of the trees. Sometimes this takes 

 place with a loud report. The attempt on the part of the plant to 

 heal the crack generally produces a frost ridge. Rents made by light- 

 ning also occur. Strangulations arfe lesions formed by woody vines, by 

 telegraph wires, or the like pressing on the outer surface of stems which 

 grow about the compressing object and create additional pressure, so 

 that the compressed tissue dies. CaUus forms above the wounded 

 areas formed by compression. Large hailstones sometimes produce 

 bruises on the bark of young trees, as also the bombs shot out of vol- 

 canoes. The abrasion of a tree by the branch of a neighboring tree 

 rubbing against it or the cutting of large lateral roots in laying curb- 

 stones must be classed as wound phenomena. Wounds are also 

 formed by the teeth and horns of various mammals. Rodents, such 

 as mice, rats, beavers and squirrels, are responsible for wounds pro- 

 duced by gnawing with their chisel-shaped incisors. Bark is rubbed 

 off, or scratched by the horns and antlers of animals of the cow and 

 deer tribes. Wounds are formed by the breaking off of branches 



