100 DIFFICULTIES AND SOURCES OF ERROR. [CH. 



the finest veins are clearly marked, there is no trace of internal 

 structure. It is, however, frequently the case that a knowledge 

 of the internal structure of a particular plant enables us to 

 interpret certain features in a structureless cast which could 

 not be understood without the help of histological facts. A 

 particularly interesting example of anatomical knowledge 

 affording a key to apparently abnormal peculiarities in a 

 specimen preserved by incrustation, is afforded by the fructi- 

 fication of the genus Sphenophyllum. Some few years ago 

 Williamson described in detail the structure of a fossil strobilus 

 {i.e. cone) from the Coal-Measures, but owing to the isolated 

 occurrence of the specimens he was unable to determine the 

 plant to which the strobilus belonged. On re-examining some 

 strobili of Sphenophyllum, preserved by incrustation, in the 

 light of Williamson's descriptions, Zeiller was able to explain 

 certain features in his specimens which had hitherto been a 

 puzzle, and he demonstrated that Williamson's cone was that of 

 a Sphenophyllum. Similar examples might be quoted, but 

 enough has been said to emphasize the importance of dealing 

 as far as possible with both petrifactions and incrustations. 

 The facts derived from a study of a plant in one form of preser- 

 vation may enable us to interpret or to amplify the data 

 afforded by specimens preserved in another form. 



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^ 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 

 1 Darwin, (90) p. 416. 



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