CHARRED TUBERS : IDENTIFICATION
How to Recognise Root/Tuber material
Roots and tubers can be difficult to recognise since they may be confused with other remains - degraded pieces of wood charcoal, dung or damaged seeds. When preserved whole, roots and tubers can be easily differentiated from other remains, however, root and tuber remains are often only fragments. Due to their size, large organs such as tap roots or rhizomes are rarely preserved whole, whereas small corms, tubers, bulbs and roots can survive with their overall morphology and surface textures undamaged which makes them easier to recognise and identify. Since ‘root and tuber’ is a collective term for several different types of underground plant organs both the gross morphology and internal anatomy varies widely from tap roots with secondary tissue, to tubers composed of balls of parenchyma with scattered vascular bundles, and to bulbs with tightly packed scale leaves.
Roots and tubers can be recognised by their overall morphology, if preserved, and through their internal tissue structure. If surface features are present then root and tubers can be distinguished from other remains by surface projections of vascular tissue (rootlets), warts, abscission scars, scale leaf scars, and root nodes. Fragments of root and tuber tissue are often rounded and composed of spherical or isodiametric parenchyma cells. These cells can be differentiated from wood tissues since wood is composed primarily of elongated and rectangular cells. Fibres, tracheids and vessels, found in wood, are rectangular and elongated where as parenchyma is rounded. Tap roots, like wood, are a product of secondary growth, however, unlike wood the central core of xylem is surrounded by substantial and retained phloem band. When charred, tap roots will produce radially orientated cavities in the xylem, which can then fragment into thin wedges whereas the phloem will fragment tangentially (Hather 2000).
In monocot tuberous organs, vascular bundles are scattered through a matrix of parenchyma cells. Seeds are also composed of rounded parenchyma cells and can sometimes be mistaken for a tuber especially if the epidermal layer is lacking. These balls of cells can be distinguished from tubers since seeds lack vascular tissue and are composed of undifferentiated parenchyma cells.
Dung pellets from sheep/goats are often rounded with a pointed projection and can be mistaken for carbonised tubers. These can be distinguished from tubers since they lack surface scars from rootlets and rhizome attachments. Also dung tissue, unlike roots and tubers, has no internal anatomical structure and is instead composed of fragmented plant tissue.
Guidelines for recognising tubers/root fragments archaeologically (Hather 1988)
- Fragments are often rounded due to surface erosion. The remains of rootlets may leave spikes of vascular tissue or rounded scars on the surface;
- The cells are spherical or more or less isodiametric as opposed to being elongated;
- Tissues are composed of cells with distinct organisation and each cell has a specific relationship with those around it;
- Some tissues are dull in texture but have small dense reflective regions;
- There are often either regular or irregular patterns of cavities.
Identification of Roots and Tubers
Root and tuber fragments can be preserved in different forms; whole (more likely for smaller organs), fragments with surface features or fragments without surface features. How you identify the taxa present, and which characters are diagnostically important, varies according to the condition of the material in the sample. Where root and tubers are preserved whole or with surface tissue, identification can begin with the morphological and external features. Identification based on gross morphology and external features can be confirmed by examining the internal anatomy under high magnification. If preserved in fragmented form the internal anatomy becomes important for identification. While all characteristics are diagnostic to a certain level the arrangement of root and tuber vascular anatomy, according to Hather, is often the commonest, most well preserved and taxonomically unique feature useful for identification (Hather 2000 pp72-73). When preparing samples for examination at high magnification it is important to try to section the tissue to include as many internal features as possible to help with identification.
It is important to remember however, that not every piece of charred tissue can be identified to species. If diagnostic features are not present or if it does not correspond to material in the root and tuber reference collection, a fragment, no matter how well preserved, may not be identifiable to species or genus. Nevertheless, even broad taxonomic groupings of indeterminate charred parenchyma can provide valuable information on past plant economies from the tissue type. The origin of a fragment, whether it be from a tap root, aquatic rhizome or deeply subterranean tuber, can be useful for interpreting plant collection strategies. At a basic level, being able to differentiate between a fragment of dicotyledon or monocotyledon plant tissue will both give an indication of the type of plant preserved and provide a starting point for higher level identification. In order to identify a piece of charred parenchyma it is necessary to base identifications on charred modern reference material. Research into archaeological roots and tubers is relatively new and therefore there are few identification manuals available, the key references being Hather 2000 ‘Archaeological Parenchyma’ and Hather 1993 ‘An Archaeological Guide to Root and Tuber Identification’ which focuses primarily on internal anatomy.
Once you have identified material likely to be root and tuber remains the next stage of identification is to group similar remains with each other. This can be performed under a low powered microscope with good lighting. Fragments can be placed in a small petrie dish filled with sand or salt to view every angle. Parenchyma can be grouped according to texture, surface characteristics, internal anatomy, shape and other shared features. It is important to examine several fragments from the same group under high power to ensure valid taxonomic groupings and homogeneity within groups as well as to identify the species present.
Due to the nature of the tissues, small tubers and roots are more likely to survive whole or fragmented with surface features which may make them identifiable based on external characteristics and morphology (Hather 2000). When examining these types of remains overall organ shape and size can be useful for identification. Other potentially useful characteristics include: the surface topography (smooth/ridged/warty), the surface cell pattern, the presence/absence of leaf scale scars, presence/absence of adventitious roots, the distribution and density of adventitious roots, the presence/absence of a stem attachment and the positioning, size and number of root attachments. When comparing samples to specimens in a reference collection these are the type of features that can help with identification. For identification purposes no single character should be used in isolation, identification should always be based on the combination of features. Due to the charring process some but not all surface features will be preserved. The level of preservation varies according to species and is dependent on charring conditions.
To confirm identifications made on external and morphological features or to identify fragments of internal tissue, the cellular anatomy should be examined. While on a general level some features of the internal anatomy are visible under low power, such as the distribution and size of parenchyma cells, the arrangement of vascular tissue and the position of the endodermis, high power is needed for more detailed examination and accurate identification of roots and tubers. Hather’s ‘Archaeological Parenchyma’ (2000) is a detailed study of root and tuber anatomy, describing the types of cells that form vegetative storage organs.
A high power reflected light microscope, the type used for wood charcoal work, can be used for more detailed investigations of the internal anatomy. This will also help ensure that no non-parenchymatous material has been misidentified as root and tuber parenchyma. A clean, freshly fractured surface is needed for viewing the cellular anatomy. To make a new surface, apply pressure with a scalpel or razor blade to an edge of the fragment. Do not cut or saw through the charcoal. Instead use light pressure to encourage the charred tissue to break itself. As the fragment begins to fracture, withdraw the blade. Another technique is to snap the fragment using your fingers. When examining the internal anatomy take note of the vascular tissues since these are often of high diagnostic value.
One problem with reflected light microscopy is that it has a narrow depth of field which can make root and tuber examination difficult. Root and tuber parenchyma breaks irregularly to make an undulating surface, unlike wood charcoal which fractures evenly to form a flat surface. An uneven surface means that at high magnifications not all the features will be in focus in the same plane.
Scanning Electron Microscopy (SEM) can be used to examine and photograph root and tuber fragments at high magnification with excellent resolution. When selecting pieces for the SEM choose representative samples from within the sorted groupings of parenchyma. It is useful to check the fragments under a reflected light microscope before using the SEM to eliminate all the non-root and tuber material. A clean fresh surface is needed for viewing under the SEM. Fragments can be attached to a SEM stub by carbon adhesive double sided tape or carbon cement. Carbon cement is useful for affixing large or oddly shaped fragments. It is important to ensure that there is good contact between then charred tissue and the carbon mount to stop ‘charging’ of the sample. If the conductivity is poor between the sample and the mount then the electron beam will not be earthed and can damage the specimen as well as obscure the image. To increase conductivity the tissue is Sputter coated with gold, silver or platinum. When Sputter coating, make sure the sample is clean otherwise dust and broken fragments can prevent the tissue being completely coated and lead to charging. While SEMs produce high resolution images at great magnification, they can be expensive and time consuming to use. Good sample preparation and a targeted fragment selection plan are essential for efficient research. (Hather 2000)
Under high and low magnification you should describe and, if possible, photograph your parenchyma types. Until identified to an appropriate taxonomic level you can name parenchyma groups with a numerical or alphabetical naming system, such as Type 1, Type 2, Type 3.... Detailed descriptions of your root and tuber types may provide clues for plant habitat and thereby collection strategies and help with identification at a later stage. See Holden et al. (1995) for an example of this approach. For levels of confidence in plant taxonomy see Charcoal Levels Identification.
Tips for Identifying Roots and Tubers
- Group charred tissues according to shared characteristics.
- Examine several specimens from within each parenchyma group under high power to both check group homogeneity and identify the taxa present.
- Check all identifications with a charred modern reference collection
In order to identify archaeobotanical material it is often necessary to check your identifications against a reference collection. For roots and tubers it is preferable to examine modern material which has been charred since charring affects both overall morphology and internal anatomy. If a charred reference collection is not available then material held in a local herbarium can be consulted.
Description of some common Roots and Tubers found archaeologically in Europe and the Near East
Hather, J. G. (1988) The Anatomical and Morphological Interpretation and Identification of Charred Vegetative Parenchymatous Plant Tissues. Unpublished Ph.D. Thesis: London University.
Hather, J. G. (1991) 'The identification of charred archaeological remains of vegetative parenchymatous tissue.' Journal of Archaeological Science 18: 661 - 675.
Hather, J. G. (1993) An archaebotanical guide to root and tuber identification: Europe and south Asia. Oxbow Monograph 28. Oxbow, Oxford.
Hather, J. G. (2000) Archaeological parenchyma London, Archetype Publications.
Holden, T. Hather, J., and Watson, J. (1995) 'Mesolithic plant exploitation at the Roc del Migdia, Catalonia,' Journal of Archaeological Science, 22, 769-778.