Forest Sanitation 483 



nificance in the proper care of field and garden crops and has 

 been of great service in the management of forest growth as 

 well. It hastened a correct understanding of the biological and 

 chemical relations of the humus covering of the forest floor to 

 tree growth. 



In the course of its progress the investigations in purely chemi- 

 cal processes in plant life led to some error in understanding 

 the significance of plant protection. When it was possible to 

 bring a plant to its best development in an artificially prepared 

 medium containing known quantities of the substances required 

 in growth, it was easy to make the mistake of assuming that the 

 natural vigor of the plant depends entirely on the presence of 

 any required element in suitable quantity. While it is true that 

 plants are weakened and suffer a loss in vigor when the proper 

 amount of food substances are withheld, as every tiller of the 

 soil well knows, every indication of disease cannot be referred 

 to this cause alone. Often soils become "tired," not through lack 

 of the necessary elements of growth, but through the presence of 

 parasitic organisms which exert an influence on continued de- 

 velopment. 



The knowledge of the activities of parasitic plants and animals 

 and the great results made possible through research in chemistry 

 are incomplete and would not explain all phenomena of plant 

 growth if other natural factors were not recognized as playing 

 an important role in the problems of growth. It is not necessary 

 to think alone of the meteorological conditions as exercising in 

 relation to the chemical composition of the soil great influences, 

 since a number of other physical forces are at work, and have a 

 great bearing on the state of health and development of the plant. 

 Physiological research alone is not sufficient to explain why it is 

 a plant responds so differently to different kinds of soil or why 

 a plant in a particular soil, notwithstanding an ample supply of 

 the necessary food substances, refuses to develop favorably. 

 Since moisture conditions, temperature and air content of the soil 

 vary according to its physical nature, it seems likely that physical 

 factors may be in many cases responsible for fluctuations in 

 plant growth. To make, then, a correct diagnosis of symptoms in 

 diseased plants is often very difficult. A study of the causes in- 

 ducing disease often leads even the experienced observer into a 

 tangle from which he is unable to extricate himself or to give 



