62 Diagnosing of Diseases 



foliage is greatly increased, but finally, after a year or more, 

 a disproportion of supi)ly and demand arises, and, as the 

 lower branches appropriate a greater share of the water 

 and minerals from the soil, the upper ones are starved and 

 die. 



We will see later that electric discharges may also cause 

 this stagheaded condition. 



Although water is the most imjjortant contribution of 

 the soil to tree growth, — most soils containing sufficient 

 quantities of the needful mineral nutritive elements — 

 yet, sometimes a deficiency of the latter in available form 

 may account for sick!}- a{)pearance of the foliage. Hence 

 the chemical constituents of the soil should also be exam- 

 ined. 



If the premature yellowing of the leaves and other abnor- 

 mal appearances do not find any other explanation, they 

 may be due to deficiency in iron, magnesium, potash, lime, 

 phosphoric acid or nitrogen, and the application of appro- 

 priate fertilizers will correct the e\'il. 



The deficiency most likely to occur is in nitrogen, of which 

 trees require a relatively large^ amount. The absence of 

 nitrogen in sufficient quantity is indicated by the etiolation 

 of the foliage, which assumes a yellowish or even a whitish 

 appearance. 



It will have appeared from the foregoing statement that, 

 while the foliage gives sure indications that something is 

 wrong, it is not so easy to decide what is wrong. Different 

 causes may produce the same abnormal appearance and 

 the same behavior in fohage and twigs; just as the paleness 

 of the human patient may be due to various causes. 



The abnormality may be primarily due not to local, but 

 to more remote causes, w^hich affect the whole physiology 

 of the tree, and often the secondary evils, which, to the 



