An

Introduction
 


 
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
Early Cretaceous conifer leaf under photographed under UV-
light. Only the cuticle is preserved. Note the anticlinal
walls, and the stoma on the lower leaf surface.
 
Epidermal 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.
 
Pteridosperm cuticles 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.
  
 
Odontopteris minor-zeilleri cuticle showing a clear differentiation into costal and intercostal fields (Upper Carboniferous)
 
Very typical features are the so-called stomata 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 guard cells
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. 
 
 
Blanzyopteris praedentata (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 (Upper Carboniferous). Right below: large multicellular glandular trichomes of Autunia conferta (Lower Permian).
 
 
 
 
Papillae. left: Quadrocladus with stomatal complexes with strongly cutinized subsidiary cells and numerous papillae (Upper Permian); upper row, middle: Autunia conferta (Lower Permian); upper row, right: Peltaspermum martinsii (Upper Permian); middle row: Lescuropteris genuina (Upper Carboniferous); lower row: Peltaspermum martinsii with sunken stomata partly covered by papillae (Upper Permian) 
 
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.
 
 

Lescuropteris genuina 
from the Upper Carboniferous
Applications 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.
Ecological interpretations
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 with: