FUNGI AND PLANT DISEASE 415 



since there is obviously no attack by a parasite. They may also 

 be called environmental, or physiological diseases, the latter 

 because their presence is manifest in the malfunctioning of the 

 plant. These diseases may be likened to upsets in. the normal 

 health of man himself, which are due not to disease organisms 

 but rather to too much or too little of some substance or some 

 factor in the environment. For example, sunlight is very bene- 

 ficial to the human body when taken in the proper amounts, 

 but may result in discomfort or illness when taken in too large 

 quantities; also, certain foods are very necessary in small 

 amounts, but cause indigestion when eaten in large portions. 

 Plants, as living organisms, respond similarly to deficiencies 

 or excesses in their surroundings. Thus a shortage of water may 

 result in a dwarfing of plant growth, and an excess of water, 

 causing a poorly aerated, waterlogged soil may produce a yellow, 

 unhealthy growth. High temperatures, accompanied by intense 

 sunlight, may cause death of the cambium and scorching of 

 leaves in full grown trees, as well as a heating of dark soil sur- 

 faces to the point where they actually kill young tender seedlings 

 by heat injury at the ground line. Low temperatures may 

 similarly kill parts of trees, and in young seedlings may also kill 

 indirectly by causing an injury known as "frost heave" which 

 is an upheaval of the soil due to the freezing of soil water, which 

 carries the young plants upward. When the soil thaws, it settles 

 back into place but often leaves the young, practically uprooted 

 tree seedlings to die 'for lack of moisture. Many chemical sub- 

 stances, like boron, are necessary in minute quantities for suc- 

 cessful plant growth, but in excessive amounts cause yellowing 

 and stunting of certain kinds of plants. The sprays and dusts 

 which are used to kill insects or fungi, when improperly mixed 

 or applied, or used in too high concentrations, cause injury to 

 the very plants they were supposed to protect. In cities which 

 have a municipal gas plant, leaks in the underground mains 

 may cause the injury or death of valuable shade trees. Such 

 injury may first be diagnosed as a parasitic disease, but careful 

 examination will reveal no parasite and will sometimes be an 

 indication of a developing gas leak. Similarly, trees along trolley 

 lines, where overhead wires are used as a power source, are some- 



