INSECTS, DISEASES, AND INJURIES 139 



natural food by the application of artificial fertilizers. The 

 drainage in such places may 'be such that trees will suffer 

 from lack of water under certain conditions. This must also 

 be readjusted. Air is another essential for growth and health 

 of trees. This applies as much to the root system as to the 

 aerial portions, for trees are often killed by root suffocation 

 due to the banking up of soil around their butts during grading 

 operations or to the tight packing of the soil. It is not always 

 possible to loosen the soil around trees, but doing so will 

 often add to their vigor and health by conserving the soil- 

 moisture and by permitting the circulation of air among their 

 root systems. Certain other factors, such as too high or too 

 low a temperature, sudden changes in temperature, the 

 presence of smoke or noxious fumes in the air, the penetration 

 of illuminating gases into the soil from leaky mains, lightning, 

 and ordinary mechanical injuries, play an important role in 

 impairing the health of trees outside of the forest. 



A brief introductory statement concerning the fungi as 

 causal factors in disease will probably help the layman better 

 to appreciate such diseases in the following discussion. The 

 fungi are, for the most part, microscopic plants which possess 

 no green parts and are, therefore, unable to manufacture any 

 food materials for themselves. For these materials they depend 

 largely on the higher plants on or in which they live. The 

 various tissues of the plants are attacked and certain parts of 

 them converted into food for the fungi. During these processes 

 conditions arise which are injurious to the higher plants and 

 which are designated by the term disease, while the plant 

 affected is called the host. Sooner or later, changes consequent 

 on the diseased condition become apparent. Such changes, 

 which may take the form of swellings, wilting, variations in 

 color, death of parts or of the whole plant, dwarfing, and the 



