484 Forestry Quarterly 



any adequate reason for the plant's deterioration. This is in part 

 due to the less organized structure of plants as compared to 

 animals. The plant (tree) is not dependent on any particular 

 organ or parts for the maintenance of its organization. Conse- 

 quently, it is often difficult to distinguish, owing to the poorly 

 defined line between health and disease, whether a given tree is 

 producing the best possible growth under favorable conditions 

 or is laboring against internal disorders or unbalanced external 

 factors. The leaves of a tree turn yellow, a very common indi- 

 cation of a diseased condition. In order to correctly interpret 

 the cause of the yellow appearance it is evident all accompany- 

 ing circumstances must be taken into consideration, and it is often 

 necessary to render the verdict, as does the physician in human 

 ills, "death from a complication of diseases." In regard to 

 physical forces as influencing plant growth, it is not possible to 

 assign chief place to any one of them, partly because experimental 

 proof in actual plant life is difficult to obtain. These circum- 

 stances indicate a field in which much is to be done and expected 

 from the science of chemistry, soil study, and plant physiology. 

 It is impossible to expect a complete solution of the problems 

 when the varied influences of living nature are held in mind. 

 By means of the microscope more and more is known of 

 the world of micro-organisms and their great influence on plant 

 development. The study of parasitic life bids fair to lend an 

 entirely different trend to research in plant propagation. In con- 

 sideration of the earlier view that numerous natural processes in 

 plant life, both normal and otherwise, hinged completely on 

 chemical characters, we now know them to be directly referable 

 to the influence of specific organisms. In this connection, some 

 epoch-making discoveries have been made. Probably the greatest 

 of these is the work of Pasteur and Koch in discovering the 

 causes of fermentation and of contagious diseases. Oxidation 

 and deorganization induced by bacteria resulting in the formation 

 of nitrogen in the soil are other important advances. Numerous 

 lower animals and plants, fungi in particular, owing to the early 

 predominating idea that they were in nowise connected with 

 plant disease, through the researches of De Bary, were shown to 

 have a true parasitic nature and were the cause of many epidemic 

 diseases among plants. The recognition of these facts initiated 

 a very active study along the line of plant diseases with very im- 



