[Sidenote: DECORTICATED STEMS.] The fact that plants usually occur in detached fragments, and that they have often been sorted by water, and that portions of the same plant have been embedded in sediment considerable distances apart, is a constant source of difficulty. Deciduous leaves, cones, or angiospermous flowers, and other portions of a plant which become naturally separated from the parent tree, are met with as detached specimens, and it is comparatively seldom that we have the necessary data for reuniting the isolated members. As the result of the partial decay and separation of portions of the same stem or branch, the wood and bark may be separately preserved. Darwin[143] describes how the bark often falls from _Eucalyptus_ trees, and hangs in long shreds, which swing about in the wind, and give to the woods a desolate and untidy appearance. In the passage already quoted from the narrative of the voyage of the Challenger, illustrations are afforded of the manner in which detached portions of plants are likely to be preserved in a fossil state. The epidermal layer of a leaf or the surface tissues of a twig may be detached from the underlying tissues and separately preserved[144]. It is exceedingly common for a stem to be partially decorticated before preservation, and the appearance presented by a cast or impression of the surface of a woody cylinder, and by the same stem with a part or the whole of its cortex intact is strikingly different. The late Prof. Balfour[145] draws attention to this source of error in his text-book of palaeobotany, and gives figures illustrating the different appearance presented by a branch of _Araucaria imbricata_ Pav. when seen with its bark intact and more or less decorticated. Specimens that are now recognised as casts of stems from which the cortex had been more or less completely removed before preservation, were originally described under distinct generic names, such as _Bergeria_, _Knorria_ and others. These are now known to be imperfect examples of Sigillarian or Lepidodendroid plants. Grand’Eury[146] quotes the bark of _Lepidodendron Veltheimianum_ Presl. as a fossil which has been described under twenty-eight specific names, and placed in several genera.