Question 6
Why are the stock and the scion taken from two different plants?
The right answer is the one marked above.
The stock supplies roots and hardiness while the scion supplies the fruit that is wanted. The stock is chosen for what happens below ground, the scion for what happens above it, and one plant then does both.
Giving the stock the fruit and the scion the roots swaps them. Requiring two different species has the rule inside out — the two parts must be of the same species or of related species. That a single plant cannot produce both roots and fruit is plainly untrue of every ungrafted tree.