I've seen trees where the largest single component of the mass comes from butterflies. They alit on the branches. Water for most trees' mass mostly comes up out of the ground, though.
On the larger point, testing learning is hard in principle in that you are usually testing theory. Practice is the basis of theory and much harder to test.
But even then, writing a good test to test theory is very hard. Most objective test questions on university tests are terrible and only slightly, if at all, more likely to reward the testee for knowing the theory.
On the larger point, testing learning is hard in principle in that you are usually testing theory. Practice is the basis of theory and much harder to test.
But even then, writing a good test to test theory is very hard. Most objective test questions on university tests are terrible and only slightly, if at all, more likely to reward the testee for knowing the theory.