104 THE EXTENSION OF THE PRINCIPLE. 
I will content myself with specifying a few of 
the more important instances in which the new 
method will be found of practical utility : — 
1. Observations, strictly comparative, can now 
he made on the effects of different soils, manures, 
&c., in cases divided into several compartments, 
each compartment being filled with different soils, 
but the same plants. 
2. To determine the powers possessed by plants, 
of absorbing and selecting various substances by 
their roots. 
3. To ascertain the existence and nature of the 
excretions from the roots, the deleterious charac- 
ters of which, if they exist, being rendered very 
problematical by the fact of plants in a state of 
nature occupying the same situation for ages.* 
4. To show the effects of poisons upon plants. 
5. To test the influence of light in protecting 
plants from the effects of low temperatures. This 
has already been proved by the same species sur- 
viving in the light, but dying in the dark portions 
of a closed case. 
In the severe winter which occurred many 
years ago, the noble plant of Araucaria excelsa, in 
the Pinetum at Dropmore, was killed. I believe 
* Drummond states that he has no doubt that many of the Swan 
River Orchidacece , of the genera Thelimytra , Diuris , &c. have con- 
tinued to flourish in half a square inch of earth for ages. 
