Geology Reference
In-Depth Information
open but lacking cell structure (Fig.
3.3d, g
), mixed open and closed (Fig.
3.4
),
closed layered (Fig.
3.3c, e
) to open layered (Fig.
3.3b
) organisations. In the last
case it is remarkable that the fossil leaf has survived as an integral structure at all.
Only the gross morphology (Fig.
3.2a, b
, c, d) of the entire specimens enables
recognition of these fossils as leaves. If fragments became dispersed in sediments
their origin as higher plant tissue would be obscured, the fragments would be classifi able
as amorphous or structured kerogen.
Surface Structures
In modern leaves a cuticle is present on the surface. Depending on the leaf surface
(abaxial/adaxial) the cuticle will be a continuous layer sometimes perforated by
stomatal apertures. The pattern of the underlying epidermal cells can usually be seen
at the surface in SEM study. Figure
3.5a
is representative of all of the surfaces (both
sides) of all the fossil leaves. There is no continuous surface layer and no pattern of
underlying surface cells. The surface structure consists of discontinuous, overlapping
and interlocking, multidirectional thread to sheet-like organic matter. There is no
resemblance to a leaf cuticle. The origin of this surface organisation is not understood.
Possibilities could include (i) major modifi cation (decomposition, dissolution, melting)
of the cuticle rendering it unrecogniseable, (ii) an usually unseen expression of the
microfi brillar layering of the epidermal cell walls or (iii) an externally sourced layer
of microbially (bacterial or algal) derived material. In any case, this surface layer
represents an extremely small percentage (<1 %) of the fossil leaf organic matter.
In modern leaves, the cuticle surface is sometimes obscured by a covering of
surface hairs or waxes. However, the fossil surface structure is quite unlike modern
leaf hairs or waxes. The possibility that waxes or hairs are covering and obscuring
the surface is also excluded by the fact that all surfaces of all taxa show essentially
the same structure whilst hairs and waxes differ in morphology between taxa and
occur in different proportions on opposite leaf surfaces. The surface organisation
could possibly result from major modifi cations (dissolution or melting and repre-
cipitation) of surface waxes but in this scenario a cuticle should still be present
beneath the surface and this is not the case as shown by TEM sections.
Much of the surface area of all specimens is covered in impressions of diatoms,
recognisable by their shape and also by impressions of the rows of small pores
(striae) in their frustules. Their presence confi rms that the leaf surface we are studying
is the surface into which diatoms were pressed by compaction as sediment built
up above the leaves. This shows that we have not removed any surface layers
accidentally during collecting or preparation.
The cuticle would be recognised in section as a discrete, single, continuous outer
layer usually with a distinct electron density (often lighter than underlying cell walls).
Such a layer can be recognised on the lower surface of the modern
Quercusrobur
(Fig.
3.3a
) in spite of the low magnifi cation of the image. None of the fossil leaves
show such a layer. The surfaces are discontinuous in all cases. Although there are
layers of differing electron density, electron lucent layers are discontinuous, often
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