GENERAL CONSIDERATION OF PLANT DISEASES 295 



under the tearing action of the wind, or by the breaking action of the 

 weight of the ice and the snow of winter. The repair of wounds 

 will be discussed with the consideration of the pathologic anatomy 

 of plants, which will form a separate chapter of this treatise. 



Artificial wounds are due to the influence of man. The ploughing, 

 discing, harrowing and cultivation of the soil frequently abrade roots, 

 break them off, or seriously wound them. Limbs are broken off and 

 bark removed by farm implements. Knife and axe wounds are easily 

 recognized by their sharp character, where the cut may have been 

 made vertically, obliquely, or horizontally. The stripping off of 

 pieces of bark opens up the inner tissues of the stem to the attack of 

 the agents of disintegration and decay. The removal of twigs and 

 branches in the ordinary operations of pruning opens up wounds, some- 

 times of a gaping character. The ringing, girdling, or scarification 

 of trees for various purposes, if not properly performed, opens up 

 wounds, so do nails, or spikes driven into the tree for various purposes 

 and the placing of electric cables and telegraph wires along our streets 

 and roads results in the removal of tree tops. The habit of cutting 

 initial letters and monograms in smooth^barked trees, such as the 

 beech, or the removal of sheets of birch bark, opens up wounds of vari- 

 ous menace to the health of the tree. Injuries due to man-created 

 environment may be of a thousand and one kinds too numerous for 

 even a brief mention. 



Animate Agents of Disease. — These may be divided into two 

 groups, namely, animal and plant. Many animals are responsible for 

 the production of wounds and the destruction of plant parts. Man, 

 cattle, herbivorous animals, rodents (mice, rats, squirrels, rabbits), and 

 birds do great injury to plants by their horns, teeth, claws and beaks 

 (woodpeckers). Among the invertebrates are to be included the in- 

 sects, mites and worms. Certain nematode worms attack the roots of 

 a large variety of plants and produce galls of characteristic form and 

 appearance. Phylloxera, an hemipterous insect, winters on the roots 

 of the grape, mostly as a young wingless form. Wingless individuals 

 then abandon the roots and crawl up the stems to the leaves, where they 

 form galls. Formerly introduced into Europe, it was very destructive 

 to European grape vines until it was found that it could be controlled 

 by grafting the European vine on the roots of American varieties. 

 Insects injurious to plants may be roughly divided into two groups: 



