62 Diagnosing of Diseases 
foliage is greatly increased, but finally, after a year or more, 
a disproportion of supply 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 important 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 sickly appearance 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 evil. 
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 foliage 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, which affect the whole physiology 
of the tree, and often the secondary evils, which, to the 
