Quercus robur has twenty-eight varieties, all of which, excepting six, are clustered round three subspecies, namely Q. pedunculata , sessiliflora and pubescens . The forms which connect these three subspecies are comparatively rare; and, as Asa Gray again remarks, if these connecting forms which are now rare were to become totally extinct the three subspecies would hold exactly the same relation to each other as do the four or five provisionally admitted species which closely surround the typical Quercus robur