A dried Bracken frond lying on the ground in the Autumn presents a very different appearance as regards the form of the ultimate segments of the frond to that of a freshly cut leaf. In the former the edges of the pinnules are strongly recurved, and their shape is considerably altered. Immersed in water for some time fern fronds or other leaves undergo maceration, and the more delicate lamina of the leaf rots away much more rapidly than the scaffolding of veins. Among fossil fern fronds 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 embedded. Differential decay and disorganisation of plant tissues are factors of considerable importance with regard to the fossilisation of plants. As Lindley[151] 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 necessarily 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 sclerenchyma 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 preserved as fossils and mistaken for complete portions of plants. [Sidenote: MINERAL DEPOSITS SIMULATING 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.