V] MINERAL DEPOSITS SIMULATING PLANTS. 105 



differences in the form of the pinnules and in the shape and 

 extent of the lamina, to which a specific value is assigned, are 

 no doubt in many cases merely the expression either of 

 differences in the state of the leaves at the time of fossilisation 

 or of the different conditions under which they became em- 

 bedded. Differential decay and disorganisation of plant tissues 

 are factors of considerable importance with regard to the fossil- 

 isation of plants. As Lindley' and later writers have suggested, 

 the absence or comparative scarcity of certain forms of plants 

 from a particular fossil flora may in some cases be due to their 

 rapid decay and non-preservation as fossils ; it does not neces- 

 sarily mean that such plants were unrepresented in the 

 vegetation of that period. The decayed rhizomes of the 

 Bracken fern often seen hanging from the roadside banks 

 on a heath or moorland, and consisting of flat dark coloured 

 bands of resistant sclerenchjrma in a loose sheath of the hard 

 shrivelled tissue, are in striking contrast to the perfect stem. 

 A rotting Palm stem is gradually reduced to a loose stringy 

 mass consisting of vascular strands of which the connecting 

 parenchymatous tissue has been entirely removed. It must 

 frequently have happened that detached vascular bundles or 

 strands and plates of hard strengthening tissue have been pre- 

 served as fossils and mistaken for complete portions of plants. 



Apart from the necessity of keeping in view the possible 

 differences in form due to the state of the plant fragments at 

 the time of preservation, and the marked contrast between the 

 same species preserved in different kinds of rock, there are 

 numerous sources of error which belong to an entirely different 

 category. The so-called moss-agates and the well-known 

 dendritic markings of black oxide of manganese, are among 

 the better known instances of purely inorganic structures 

 simulating plant forms. 



An interesting example of this striking similarity between 

 a purely mineral deposit and the external form of a plant is 

 afforded by some specimens originally described as impressions 

 of the oldest known fern. The frontispiece to a well-known 

 work on fossil plants, Le monde des plantes avant l' apparition de 



1 Lindley and Hutton (31) vol. iii. p. 4. See also Sohenk (88) p. 202. 



