There are about twenty species, but the number cannot be very accurately defined - several, usually regarded as distinct, being probably merely variable forms of the same type, and the ease with which the trees intercross has led to the appearance of many hybrids.
Usually they are twinned on a prism plane, M, producing pentagonal stellate groups of five crystals; twinning on the plain g, in which the crystals intercross at angles of nearly 60°, is less common.
As these varieties intercross with each 1 See drawings made to scale by Mr Worthington Smith in the Gardener's Chronicle (25th December 1886).
19 other, the presumption is that they, like the species of Aegilops, which also intercross with wheat, may have all originated from one common stock.
De Vilmorin's experiments have shown that all the varieties will intercross, and that even such a distinct form as the Polish is no exception.