A thing is deemed to be affixed to land when it is attached to it by roots, as in the case of trees, vines, or shrubs; or imbedded in it, as in the case of walls; or permanently resting upon it, as in the case of buildings; or permanently attached to what is thus permanent, as by means of cement, plaster, nails, bolts, or screws; except that for the purposes of sale, emblements, industrial growing crops and things attached to or forming part of the land, which are agreed to be severed before sale or under the contract of sale, shall be treated as goods and be governed by the provisions of the title of this code regulating the sales of goods.