The phenomenon has been observed in later days, though Basil may be at fault in his account of the cause.  When pines have been cleared away in North American forests young oaklings have sprung up.  The acorn lay long hid, unable to contend against the pine, but, when once the ground was clear, it sprouted.  This upgrowth of a new kind of tree had been accounted for partly by the burial of germs by jays, rooks, and some quadrupeds; partly by the theory of De Candolle and Liebig that roots expel certain substances which, though unfavourable to the vitality of the plant excreting them, are capable of supporting others.  So, on the pine pressure being removed, the hidden seeds sprout in a kind of vegetable manure.  cf. Sir Charles Lvell’s Travels in the United States and Rough’s Elements of Forestry, p. 19.