Microscopic epidermal identification of yucca and agave for archaeological use.
The identification of epidermal fiber fragments is a common method employed by wildlife biologists and ecologists to determine the diet of animals. Epidermis has many distinct characteristics that allow for simple and concise identification to generic and many times specific levels, particularly in the grass family (Prat, 1948). Epidermis is also extremely resistant to digestion as evidenced by how most fragments pass untouched through the digestive system. Storr (1961) has observed that there is no digestion of the cuticle of annual plants or the epidermis of perennial plants when passed through the digestive system of herbivorous mammals. These qualities make the identification of epidermal fragments a ready determinant of animal diet.
Before the analysis of epidermal fragments, scientists used numerous techniques to determine diet. The main methods involved following the animal and directly observing dietary intake, analyzing pasture content before and after the animal had eaten, and killing the animal to observe digestive tract contents (Stewart, 1967). All of these methods have inherent problems; in particular, they are either extremely time consuming or interfere with the animal's natural habits.
The unique qualities of plant epidermis soon became a focal point in dietary studies. This technique first was used in a study of stomach contents of squirrels (Baumgartner and Martin, 1939), although the analysis of epidermis from feces also became prevalent. Fecal epidermal analysis is preferred as it is an accurate method of estimating diet that does not interfere with natural animal habits (Stewart, 1967). Epidermal fragments have been analyzed from the feces of a variety of animals, particularly those of herbivores (Croker, 1959; Storr, 1961; Zyznar and Urness, 1969; Stewart and Stewart, 1970; Paden et al., 1974; Hansen and Clark, 1977; Olsen and Hansen, 1977); for example, epidermal remains in feces have been analyzed from the rhinoceros, which is mainly herbivorous (Gyawali, 1986), the chuckwalla, an herbivorous reptile (Hansen, 1974), the red fox (Scott, 1941), and the crops of grasshoppers (Brusven and Mulkern, 1960). Epidermis also has been used to identify fossil plants (Harris, 1966), modern herbage content (Davies, 1959), and the diet of extinct herbivores (Iberall, 1972; Robbins et al., 1984).
With the prevalence of such techniques in the literature, it is surprising that epidermal identification has not been used extensively by archaeologists, particularly those concerned with prehistoric diets. Bohrer (1986) pointed out the scarcity of archaeological epidermal identifications. Coprolites, quids, and paleoethnobotanical material are all excellent sources of epidermal material, which mostly has gone unidentified. In many instances, particularly with coprolite and quid samples, fiber and epidermis make up the majority of the specimen. The identification of the epidermal fragments would greatly enhance our knowledge of that particular population and of prehistoric diet in general.
In archaeological literature, Bell and King (1944) initiated the identification of fiber types microscopically, although the focus was on fiber identification rather than distinctive epidermal fragments. They observed differences in vascular tissue, leaf cross-sections, and lumen diameters of agave (Agave), yucca (Yucca), beargrass (Nolina), and sotol (Dasylirion). Identifications of epidermal tissue of sotol, yucca, and agave (Agave lechugilla) also have been made in studies conducted on paleoethnobotanical material (Dering and Shafer, 1976), on stone tools to determine function (Shafer and Holloway, 1979; K. D. Sobolik and J. E. Dockall, unpublished data), and on woven fabric material (Korber-Grohne, 1988). Epidermis also has been identified from human coprolites. Stiger (1977) identified epidermal fragments from Anasizi coprolites from Mesa Verde, and Williams-Dean (1978) and Sobolik (1990, 1991) identified epidermis from hunter-gatherer coprolites from southwest Texas.
Different methods used in preparing epidermal specimens for analysis, and the unique features used in epidermal identification, are discussed in this paper. Plants analyzed include the common desertic taxa, yucca and agave. The analysis focussed on these succulents because the main purpose of this article is to show that microscopic epidermal identification is obtainable, and that plants within the same genus have similar characterics that allow for easy identification. These plants also were used extensively by prehistoric and historic North American Indians for a wide variety of purposes. Such uses included food, liquor, and weaving material for baskets, nets, sandals, and clothing. Distinguishing yucca and agave epidermis can be difficult with the unaided eye, particularly when the sample has passed through a digestive tract, or is too small or degraded to contain some of the more common surface morphology. It is hoped that archaeologists will use this method of epidermal identification, particularly for samples in which small plant fragments tend to remain unidentified.
Numerous methods have been used in preparing epidermal specimens for identification. These included examining the specimen without the aid of a microscope, peeling or scraping off the epidermal layer, mounting the epidermis on a microscope slide and staining the specimen, and examining the fragment in cross section (Prat, 1948). Other methods have involved first making a negative impression of a leaf, then making a positive transparent film of the impression and examining this film under the microscope (Shutak and Dayawon, 1966), as well as coating fingernail polish on the surface, peeling off the polish after it has dried, and examining this epidermal "fingerprint" under a microscope (Stoddard, 1965).
One of the simplest methods, however, involves scraping off the epidermis after first softening the plant fragment. Softening agents have included equal parts of glycerol, water, and alcohol (Prat, 1948), hot water (Iberall, 1972), a sodium hydroxide solution (Bell and King, 1944), and trisodium phosphate (Sobolik, 1991). After the epidermis has been removed it is mounted on a slide, with a liberal dose of glycerin, and examined under a microscope. Staining can be used; however, Zyznar and Urness (1969) and I have found that unstained material is the easiest to identify. Another method involves simply mounting a small portion of the epidermal sample on a stub and examining the surface morphology by means of a Scanning Electron Microscope. The later method was used for this analysis.
A comparative collection of yucca (Table 1) and agave (Table 2) epidermis was obtained from samples currated at the Tracy Herbarium, Texas A & M University. A wide variety of species in each genus was used in order to ascertain the reliability of each distinctive characteristic for the common genus. All of the species examined, however, could not be illustrated in this text due to space limitations.
After the sample was mounted, distinct characteristics of the epidermis were noted after examination with a Scanning Electron Microscope. Iberall (1972) has noted the basic points for identification of epidermis include cell arrangement, cell outline, presence or absence of trichomes, and the shape and size of the stoma. The main characteristics noted in this paper include the epidermal cell shape and arrangement, and the stoma and guard cell shape and arrangement. A few keys or identification manuals of epidermal fragments have been published, although they tend to refer mainly to identification of local or regional species, such as those in eastern Oregon (Schrumpf, 1968), Kenyan plains grasses (Steward, 1965), and basic grassland plants (Hansen, 1971).
After microscopically examining 70 different specimens of Agave and Yucca, it is apparent that the stoma and guard cell patterning of these two desert succulent genera are distinct. Laudermilk and Munz (1933) indicated through line drawings, however, that agave and yucca have similar stomatal complexes. The line drawings in that publication show that yucca stomatal complexes are similar to agave stomatal complexes presented here. However, none of the 48 yucca stomatal complexes examined for this paper resembled the yucca stomatal complexes drawn in Laudermilk and Munz (1933).
The characteristics used to describe the differences between yucca and agave are illustrated in Figure 1. In yucca, the stoma are circular or elliptic with two large, extended guard cells covering the stoma (Figs. 2A, 2B, 2D). Many times these guard cells touch or overlap with each other as they extend over the stoma (Figs. 2A, 2D). The edges of the guard cells, which extend over the stoma, may be larger in that area (Figs. 2A, 2B) or may remain the same width as the rest of the guard cell (Fig. 2D). The stoma also are surrounded by epidermal cells, or subsidiary cells (Tomlinson, 1974), which remain on the periphery (Figs. 2A, 2B, 2D). Stoma tend to appear frequently on yucca epidermis (Fig. 2A).
[FIGURE 1 OMITTED]
The stoma on agave epidermis are infrequent, with large cellular areas between each randomly placed stomatal complex (Figs. 4A, 4D). The stoma are usually elliptic in shape (Figs. 3C, 3D, 4A, 4C, 4D), with two large guard cells surrounding the stoma on either side (Figs. 3C, 3D, 4C). These guard cells protude from the surface (Figs. 3C, 3D, 4C). Four epidermal, or subsidiary cells, surround the guard cells; two at the polar ends and two at the lateral ends (Gentry and Sauck, 1978). If only the remaining epidermal cells are intact, the elliptic stoma will not retain the guard cells, and will blend in with the rest of the epidermal cells (Fig. 4A, 4D).
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The epidermal cell patterning is also distinctive in yucca and agave (Fig. 1). Yucca contains a short rectangular cell shape (Figs. 2A, 2B), although the rectangular shape may also extend into a semilozenge shape (Figs. 2C, 3A, 3B). The end walls in both the rectangular and semilozenge shaped cells are transverse (Fig. 1). The cell rows are well patterned and organized throughout the epidermis (Figs. 2A, 2B, 2C, 3A, 3B).
Agave contains epidermal cells which tend to be more elongated than the cells of yucca. Agave cells are lozenge-shaped (Fig. 1) and in most cases the cell end walls are oblique (Figs. 4A, 4B, 4D). The cell rows are not as patterned as those observed in yucca, and tend to be eratically organized throughout the epidermis (Figs. 4A, 4D).
Figures 3B and 4D illustrate the application of microscopic epidermal identification on archaeological samples. Figure 3B is a sample from an Archaic sandal excavated from Hinds Cave, Val Verde Co., Texas (Shafer and Bryant, 1977). The microscopic identification of the epidermis revealed that the sandal was at least partially made from yucca fiber. This sample was identified as yucca due to the epidermal cell shape and patterning. The cell shape is short and rectangular, with small extensions into a semilozenge shape. The cell rows are organized and evenly patterned. The above features conform to the characteristics of yucca (Fig. 1).
Figure 4D is an epidermal tissue taken from a coprolite sample. The coprolite was excavated from a latrine area of Baker Cave, Val Verde Co., Texas (Hester, 1983, 1986). This sample indicates that agave was part of the diet of the prehistoric occupants of that area. The epidermis was identified as agave due to the cell shape and patterning, and the stoma shape. The epidermal cell is lozenge-shaped with oblique cell ends, and the cell patterning is eratically organized throughout the sample. The stomata (indicated with arrows) are ovular, and the projecting guard cells have not been retained, probably due to the digestion process.
Identification of epidermal fiber from archaeological contexts is not only possible, but can provide information that previously has not been considered. Excepting a few studies (Dering and Shafer, 1976; Williams-Dean, 1978; Shafer and Holloway, 1979; Korber-Grohne, 1988; Sobolik, 1991), this unique source of information largely has been ignored. Due to the frequent identification of epidermal fragments in other sciences, archaeologists should be able to use epidermal analysis with ease in comparing reference material with that from archaeological samples.
[FIGURE 4 OMITTED]
Many archaeological samples are conductive to epidermal analysis, including coprolites, quids, and prehistoric botanical artifacts. Coprolites reflect the direct dietary intake of a specific individual (Bryant, 1974; Fry, 1985), and the analysis and identification of epidermis from such samples will yield the fiber type that was consumed. Quids, although not necessarily representing dietary intake, reflect individual choice of chewing material. The analysis of prehistoric botanical artifacts may indicate choice of construction material. Paleoethnobotanical material also reflects diet, although it also may represent natural or contaminated debris. The identification of epidermal fragments from such material thus must be considered carefully.
Archaeologists should strive to obtain the most information from their samples as possible. A major step in that direction is to follow the techniques that other sciences have developed or are developing. Other scientists from different fields have already greatly influenced archaeology. Examples include the physicist A. E. Douglass and the development of tree-ring dating (Fritts, 1976), the initiation and development of zooarchaeology by zoologists and paleontologists (Robison, 1978), and the improvement of coprolite research through the rehydration technique developed by botanists (Benninghoff, 1947) and zoologists (Van Cleave and Ross, 1947). The identification of epidermal fiber fragments is another example. Such techniques will improve the quality and quantity of information that can be gained through archaeological analyses.
TABLE 1. -- Yucca specimens analyzed. Yucca species Locality Date Collection no. Y. aloifolia Smith Co., Texas 26 October 1974 167502 Harden Co., Texas 12 September 1936 19560 Brazos Co., Texas 20 July 1947 54245 Brazos Co., Texas 20 August 1940 27798 Galveston Co., Texas 23 September 1974 142160 Y. arkansana Dallas Co., Texas 21 May 1938 31905 * Denton Co., Texas 22 May 1938 31996 Bexar Co., Texas 14 May 1948 56645 * Karnes Co., Texas 28 June 1952 876 Y. brevifolia San Bernadino, 8 October 1935 2509 * California Y. carnerosana Coahuila, Mexico 27 March 1981 160310 Y. baccata Clark Co., Nevada 10 May 1936 7054 Brewster Co., Texas 19 April 1942 57801 * Brewster Co., Texas 23 May 1925 11180 San Bernardino, 14 May 1935 6816 California Clark Co., Nevada 15 May 1938 7873 Y. constricta Refugio Co., Texas 13 July 1977 156475 Archer Co., Texas 11 May 1935 13095 McCullock Co., Texas 24 November 1941 38446 Brewster Co., Texas 1938 29938 San Saba Co., Texas 7 December 1941 40762 Y. elata Brewster Co., Texas 8 June 1937 57808 Brewster Co., Texas 15 April 1938 61306 * Presidio Co., Texas 11 June 1948 48024 Y. glauca Garza Co., Texas 8 December 1974 167515 Bell Co., Texas 13 May 1930 240 Borden Co., Texas 3 September 1938 30107 Y. louisianensis Brazos Co., Texas 14 June 1973 80 Brazos Co., Texas 18 May 1946 47288 Y. navajoa Webb Co., Texas 15 May 1963 8833 Y. pallida Galveston., Texas 16 May 1975 3618 Y. reverchoni Tarrant Co., Texas 24 May 1939 32102 Sutton Co., Texas 13 May 1939 31846 Uvalde Co., Texas 29 July 1958 83913 Y. rostrata Brewster Co., Texas 31 March 1940 61296 Brewster Co., Texas 7 December 1944 57794 Y. rupicola Coryell Co., Texas 11 June 1987 274 Uvalde Co., Texas 3 May 1933 6099 San Saba Co., Texas 6 December 1941 40758 Y. thompsoniana Brewster Co., Texas 10 December 1944 2000 Y. treculeana Jim Wells Co., Texas 21 March 1985 987 Karnes Co., Texas 29 March 1953 1218 Bexar Co., Texas 1 December 1941 38450 Brewster Co., Texas 24 February 1937 22000 Y. torreyi Brewster Co., Texas 27 January 1942 38609 Brewster Co., Texas 25 March 1937 62125 Jeff Davis Co., Texas 18 April 1930 1679 Uvalde Co., Texas 29 July 1958 38 All specimens curated at the Tracy Herbarium, Texas A & M University. * = SEM photographs of specimens included in the article. TABLE 2. -- Agave specimens analyzed. Agave species Locality Date Collection no. A. brevispica Dominican Republic -- 3273 A. intermixta Dominican Republic 30 March 1961 4490-A A. lechuguilla Val Verde Co., Texas 23 August 1988 KDS 1 Val Verde Co., Texas 4 December 1988 KDS 2 Val Verde Co., Texas 15 August 1990 KDS 3 * Terrell Co., Texas 18 August 1990 KDS 4 A. parrasana Coahuila, Mexico 7 June 1984 3273 A. scabra Brewster Co., Texas 2 June 1937 703 Brewster Co., Texas 28 June 1948 1917 Brewster Co., Texas 6 July 1927 11200 Terrell Co., Texas 18 August 1990 KDS 5 Jeff Davis Co., Texas 19 August 1990 KDS 6 A. tigrina Clark Co., Nevada 11 May 1936 7059 * San Saba Co., Texas 10 December 1941 40761 * A. virginica Louisiana 25 September 1957 80748 Brazos Co., Texas 21 July 1940 279 * Pontotoc Co., Oklahoma 15 June 1948 3094 Nacogdoches Co., Texas 25 July 1942 2056 Nacogdoches Co., Texas 25 July 1942 2057 Brazos Co., Texas 1 June 1946 47164 Brazos Co., Texas 29 June 1946 47745 Harrison Co., Texas 9 August 1950 57783 * KDS Collection no. = Author's collection. All other specimens curated at the Tracy Herbarium, Texas A & M University. * = SEM photographs of specimens included in the article.
I want to first and foremost thank Eleanora I. Robbins for the loan of her extensive epidermal article collection. I also want to acknowledge Harry J. Shafer, Vaughn M. Bryant, Jr., and D. Gentry Steele for reviewing this article, although I take full responsibility for all included data and interpretations relating to it. Thanks to Harry J. Shafer for the use of the Archaic sandal, and to Kenneth M. Brown for the use of the coprolite. Photographic processing was provided through a Graduate Student Research Grant received from the College of Liberal Arts at Texas A & M University. Those in charge of the Electron Microscopy Center at Texas A & M University provided use of their equipment. D. Gentry Steele and the Department of Anthropology at Texas A & M University allowed the use of the Photographic Processing Lab.
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KRISTIN D. SOBOLIK
Center for Archaeological Investigations, Southern Illinois University, Carbondale, Illinois 62901
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|Author:||Sobolik, Kristin D.|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 1992|
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