266 Dr. DaAvusENY on the Degree of Selection exercised by Plants, &c. 
But in all these instances the poisonous quality of the substance was evinced 
by the more or less rapid decay of the plant that had imbibed it; whereas, 
where nitrate of strontian was employed, the functions of life appeared to go 
on for a considerable time without material obstruction. 
Upon the whole, then, I see nothing, so far as experiments have yet gone, 
to invalidate the conclusion, to which the preceding facts appear to lead, that 
the roots of plants do, to a certain extent at least, possess a power of selection, 
and that the earthy constituents which form the basis of their solid parts are 
determined as to quality by some primary law of nature, although their amount 
may depend upon the more or less abundant supply of the principles presented 
to them from without. 
