■^] DECORTICATED STEMS. 



101 



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 ^ 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'" 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 Arauoaria imhri- 

 cata 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* quotes the bark of Lepi- 

 dodendron Veltheimianum Presl. as a fossil which has been 

 described under twenty-eight specific names, and placed in 

 several genera. 



Since the microscopical examination of fossil plant-anatomy 

 was rendered possible, a more correct interpretation of decorti- 

 cated and incomplete specimens has been considerably facilitated. 

 The examination of tangential sections taken at different levels 

 in the cortex of such a plant as Lepidodendron brings out the 

 distribution of thin and thick-walled tissue. Regularly placed 

 prominences on such a stem as the Knorria shown in fig. 23 are 

 due to the existence in the origiaal stem of spirally disposed 

 areas of thin-walled and less resistant tissue; as decay pro- 

 ceeded, the thinner cells would be the first to disappear, and 

 depressions would thus be formed in the surrounding thicker 

 walled and stronger tissue. If the stem became embedded in 

 mud or sand before the more resistant tissue had time to decay, 

 but after the removal of the thin-walled cells, the surrounding 



1 Solms-Laubach (91) p. 9. ' Balfour (72) p. 5. 



3 Grand'Eury (77) Pt. i., p. 3. 



