Sir James George Frazer (1854–1941).  The Golden Bough.  1922.

§ 2. Osiris a Tree-spirit
BUT Osiris was more than a spirit of the corn; he was also a tree-spirit, and this may perhaps have been his primitive character, since the worship of trees is naturally older in the history of religion than the worship of the cereals. The character of Osiris as a tree-spirit was represented very graphically in a ceremony described by Firmicus Maternus. A pine-tree having been cut down, the centre was hollowed out, and with the wood thus excavated an image of Osiris was made, which was then buried like a corpse in the hollow of the tree. It is hard to imagine how the conception of a tree as tenanted by a personal being could be more plainly expressed. The image of Osiris thus made was kept for a year and then burned, exactly as was done with the image of Attis which was attached to the pine-tree. The ceremony of cutting the tree, as described by Firmicus Maternus, appears to be alluded to by Plutarch. It was probably the ritual counterpart of the mythical discovery of the body of Osiris enclosed in the erica-tree. In the hall of Osiris at Denderah the coffin containing the hawk-headed mummy of the god is clearly depicted as enclosed within a tree, apparently a conifer, the trunk and branches of which are seen above and below the coffin. The scene thus corresponds closely both to the myth and to the ceremony described by Firmicus Maternus.   1
  It accords with the character of Osiris as a tree-spirit that his worshippers were forbidden to injure fruit-trees, and with his character as a god of vegetation in general that they were not allowed to stop up wells of water, which are so important for the irrigation of hot southern lands. According to one legend, he taught men to train the vine to poles, to prune its superfluous foliage, and to extract the juice of the grape. In the papyrus of Nebseni, written about 1550 B.C., Osiris is depicted sitting in a shrine, from the roof of which hang clusters of grapes; and in the papyrus of the royal scribe Nekht we see the god enthroned in front of a pool, from the banks of which a luxuriant vine, with many bunches of grapes, grows towards the green face of the seated deity. The ivy was sacred to him, and was called his plant because it is always green.   2