|The cuticle is a
non-cellular protective layer covering the outer cell layer (epidermis)
of the green, aerial parts of land plants. Cuticles protect plants
against dessication, UV radiation and various kinds of physical, chemical
and (micro)biological agents. Moreover, the cuticle also provides
some support. In fact, the cuticle which protects the underlying
tissues has basically the same function as our own skin. In several
groups of plants cuticles are very resistant and they have a high fossilization
potential; only few groups do not generally have highly resistant cuticles
(lycopods, Equisetophytes and ferns).
Section through a
leaf of a living plant showing the epidermis and the cuticle (stained red).
Note the stoma with the substomatal chamber
conifer leaf under photographed under UV-
light. Only the cuticle
is preserved. Note the anticlinal
walls, and the stoma
on the lower leaf surface.
cells usually do not exactly fit together, but there are small voids between
the individual cells. These voids between individual epidermal cells
are filled up by cuticle plugs which are termed anticlinal walls.
The cuticle forms a perfect natural cast of the epidermis and the
anticlinal wall reflect the cell pattern of the epidermis. Each plant
species has it own specific epidermal pattern. Therefore cuticles
can be used for identiying plant remains. They are so to say the
plant's "fingerprint ". Only small pieces of cuticle suffice for
a justified identification. Only in some cases cuticles of closely
related species within the same genus are so similar that a they cannot
be differentiated. Cuticles not only help to identify and classify fossil
plant species, they are also a valuable source for further information,
such as gross morphology, ecology and climate.
from the Upper Carboniferous (left) and Upper Permian (right)
clearly showing the relief
of the anticlinal walls
|The form and arrangement of epidermal
cells, differences between upper and lower leaf cuticles are some of the
important characters. The illustration below shows a cuticle with
a clear differentiation. The epidermal cells of the pinna axis are
more or less rectangular and elongated, arranged in longitudinal rows.
The cells overlying the veins are very similar but less cutinized.
The cells in the areas between the veins are smaller, polygonal and
here the so-called stomata occur.
minor-zeilleri cuticle showing a clear differentiation into costal and
intercostal fields (Upper Carboniferous)
|Very typical features are the so-called
which serve for gass exchange, i.e. the uptake of carbondioxide and release
of oxygen. Stomata consist of an opening or stomatal pore, and two
kidney-shaped guard cells. The guard cells are used for opening and
closing the stomatal pore, in order to regulate the evapotranspiration
and gass exchange. Early land plants and several angiosperms
have such simple stomata. In many gymnosperms stomata and surrounded
by cells that are differently shaped from the normal epidermal cells.
Usually stomata are surrounded by one ring of neighbouring cells, occasionally
a second ring of encircling cells occurs. The stoma together with
the neighbouring (and encircling) cells is then called a stomatal complex
or stomatal apparatus. Stomata are often more common on the lower
leaf surfaces; not rarely they are completely restricted to lower leaf
surfaces; being in the shade reduces the risk of excessive water loss.
For the same reason stomata may also be sunken and stomatal pores may be
covered by overhanging papillae (see below). Not only the shape of
the stomata is typical, but also their distribution (e.g., concentrated,
randomly, in rows) and the orientation are useful diagnostic characters.
Leaf cuticle of
Symplocos hallensis (Eocene) with three stomata
each consisting of two
Axial cuticle of
the Early Devonian land plant Aglaophyton major with stoma consisting
of two guard cells
Leaf cuticle of the
Early Permian pteridosperm Autunia conferta
with stomatal complexes
|Other typical features
include hairs and papillae. Papillae are small thickenings of the
cuticle, which may be hollow or solid. Papillae may occur on normal
epidermal cells and on neighbouring (and encircling) cells of stomatal
complexes; other they are restricted to the latter. In such cases papillae
may partly cover the stomatal pores. Larger structures are classified
as hairs or trichomes. These can be unicellular to multicellular
and have various functions, varying from extra protection againts desiccation
to protection against arthropods. Many trichomes are glandular and
have a secretory funtion.
(left) with numerous multicellular hairs, and Ortiseia leonardii
(right) with two stomatal
complexes (subsidiary and encircling cells!) and numerous hair bases
|Trichomes. Left above:
Blanzyopteris praedentata axis with numerous multicellular trichomes
(Upper Carboniferous). Middle above: short glandular trichome
(foreground) and two longer multicellular trichomes (background) of B.
praedentata (Upper Carboniferous). Right above: three
peltate glandular trichomes of Barthelopteris germarii (Upper Carboniferous).Left
below: several multicellular hairs of B. praedentata
Carboniferous). Right below: large multicellular glandular
trichomes of Autunia conferta (Lower Permian).
|Papillae. left: Quadrocladus
stomatal complexes with strongly cutinized subsidiary cells and numerous
papillae (Upper Permian); upper row, middle: Autunia conferta
Permian); upper row, right: Peltaspermum martinsii (Upper Permian);
row: Lescuropteris genuina (Upper Carboniferous); lower row:
Peltaspermum martinsii with sunken stomata partly covered by papillae
|Cuticles are among
the most resistant parts of the plant, and in compression floras they are
often the only part of the plant that still provides some information on
cellular structures. Because of the uniqueness of epidermal cell
patterns, and their high fossilisation potential the study of cuticles
hase become become a standard method in palaeobotany. The first
cuticle studies from the mid 19th century were mainly taxonomically oriented.
Nowadays, many studies focus on whole plant reconstructions and palaeoecology.
In the last decade stomata densities and indices have become widely used
proxies in palaeoclimatological studies. Several applications of
cuticulare studies are listed below.
from the Upper Carboniferous
of cuticular studies in palaeobotany
||Identifcation and classification
of fossil plant remains on the basis of biological criteria
In palaeobotany leaf remains are usually
classified according to their outline and venation in form-genera and form-species.
However, this is a very artificial system because such leaf shapes are
not necessarily typical for s specific group of plants. Cuticular
analysis provides a sound basis for biological classification.
||Correlation of isolated organs
Foliage and fructifications are rarely
found in organic connection. With the help of cuticular analysis
dispersed organs can easily be correlated.
||Reconstructions of fossil plants
Some features are rarely or never found
as compression fossils but can easily be recognized in bulk-macerated samples.
Good examples are various types of climbing hooks and tendrils.
Cuticles can show a number of features
which can provide helpful information on the palaeocology.
||Palaeoclimatic studies and estimations
of past carbondioxide concentrations
Cuticles can also given improtant information
on the palaeoclimate. Stomata densities and indexes are popular proxies
for carbondioxide concentrations.
Further pages on cuticle studies will soon be available. These will deal
methods to study fossil cuticles
Late Palaeozoic conifer cuticles
Late Carboniferous climbing pteridosperms:
Lescuropteris, Blanzyopteris, Pseudomariopteris