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 as that of the silver-fir, while others are rather breakable, such as the wood of the olive. Again some are without knots, 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 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. Boxwood and ebony are heavy because the grain is close, and oak because it contains mineral matter. In like manner the other peculiarities too can in some way be referred to the essential character.

VI. Again there are differences in the 'core': in the first place according as plants have any or have none, as some say 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 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