88 THE PRESERVATION OF PLANTS AS FOSSILS. [CH. 



Gycadeoidea gigantea Sew.^ a magnificent Cycadean stem from 

 Portland recently added to the British Museum collection ; in 

 the cell Ay 1, the nucleus is fairly distinct and in 2 and 4 the 

 contracted cell-contents is clearly seen. Other interesting 

 examples of fossil nuclei are seen in a Lyginodendron leaf 

 figured by Williamson and Scott in a recent Memoir on that 

 genus ^ Each mesophyll cell contains a single dark nucleus. 

 The mineralisation of the most delicate tissues and the 

 preservation of the various forms of cell-contents are now 

 generally admitted by those at all conversant with the pos- 

 sibilities of plant petrifaction. If we consider what these facts 

 mean — the microscopic investigation of not only the finest 

 framework but even the very life-substance of Palaeozoic 

 plants — we feel that the aeons since the days when these 

 plants lived have been well-nigh obliterated. 



Occasionally the plant tissues have assumed a black and 

 somewhat ragged appearance, giving the impression of charred 

 wood. A section of a recent burnt piece of wood resembles very 

 closely some of the fossil twigs from the coal seam nodules. It 

 is possible that in such cases we have portions of mineralised 

 tissues which were first burnt in a forest fire or by lightning 

 and then infiltrated with a petrifying solution. An example of 

 one of these black petrified plants is shown in fig. 74 B. Chap. x. 

 In many of the fossil plants there are distinct traces of fungus 

 or bacterial ravages, and occasionally the section of a piece of 

 mineralised wood shows circular spaces or canals which have the 

 appearance of being the work of some wood-eating animal, and 

 small oval bodies sometimes occur in such spaces which may 

 be the coprolites of the xylophagous intruder. (Fig. 24, p. 107.) 



It is well known to geologists that during the Permian and 

 Carboniferous periods the southern portion of Scotland was the 

 scene of widespread volcanic activity. Forests were overwhelmed 

 by lava-streams or showers of ash, and in some districts tree 

 stems and broken plant fragments became sealed up in a volcanic 

 matrix. Laggan Bay in the north-east corner of the Isle of 

 Arran, and Petticur a short distance from Burntisland on the 

 north shore of the Firth of Forth, are two localities where 

 1 Seward (97). ^ Williamson and Scott (96) PI. xxiv. fig. 16. 



