as that of the silver-fir, while others are rather breakable,[1] such as the wood of the olive. Again some are without knots,[2] as the stems of elder, others have knots, as those of fir and silver-fir.
Now such differences also must be ascribed to the essential character of the plant: for the reason why the wood of silver-fir is easily split is that the grain is straight, while the reason why olive-wood is easily broken[3] is that it is crooked and hard. Lime wood and some other woods on the other hand are easily bent because their sap is viscid.[4] Boxwood and ebony are heavy because the grain is close, and oak because it contains mineral matter.[5] In like manner the other peculiarities too can in some way be referred to the essential character.
Further 'special' differences.
VI. Again there are differences in the 'core': in the first place according as plants have any or have none, as some say[6] is the case with elder among other things; and in the second place there are differences between those which have it, since in different plants it is respectively fleshy, woody, or membranous; fleshy, as in vine fig apple pomegranate elder ferula; woody, as in Aleppo pine silver-fir fir; in the last-named[7] especially so, because it is resinous. Harder again and closer than these is the core of dog-wood kermes-oak oak laburnum mulberry ebony nettle-tree.
The cores in themselves also differ in colour; for that of ebony and oak is black, and in fact in the oak it is called 'oak-black'; and in all these the core is harder and more brittle than the ordinary
