the morphology and domestication of pearl millet

The Morpho logy and Domes t i ca t i on of Pearl Millet 1

JERE BRUNKEN 2, J. M. J. DE W E T AND J. R. HARLAN 3

INTRODUCTION

Among the many grasses which man has succeeded in domesticating, pearl millet (Pennise tum amer icanum (L.) Leeke) is one of the most tolerant to conditions of drought. Throughout most of its distribution, mean annual rainfall varies between only 250 and 800 mm. As such, pearl millet is well adapted to moisture conditions which few other grain crops would even survive. A native of the Old World tropics, it reaches its greatest importance in the dry sahel zone which stretches across sub-Saharan Africa and in the semiarid regions of northwestern India. Throughout its distribution, pearl millet serves a multitude of traditional societies which would otherwise be hard pressed for their sustenance.

The present study was initiated in order to fill some of the wide gaps which currently exist in our knowledge of the biology of pearl millet. Summarized here are the results of investigations into: (1) the present-day biological structure of the crop in terms of the composition of its primary gene pool; (2) the characteristics and distributions of the basic races in pearl millet; and (3) the time, place and events which surrounded its domestication.

THE PRIMARY GENE POOL

In this paper, we are concerned only with those morphological and adaptive units which together comprise the primary gene pool (PGP) of pearl millet. As defined by

i The research reported here was supported by NSF Grant GB-33802 and the University of Illinois Agricul- tural Experiment station. Submitted for publication Oc- tober 24, 1975; accepted for publication March 31, 1976.

2 Department of Botany. Ohio State University, Co- lumbus, Ohio 43210.

3 Crop Evolution Laboratory, Department of Agron- omy, University of Illinois, Urbana, Illinois.

Harlan and de Wet (1971), the PGP of a crop includes all plants, both cultivated and spon- taneous, with which it freely hybridizes to produce fully fertile offspring. The PGP cor- responds, therefore, to the biological species of the crop in question. Although they may be morphologically quite dissimilar, the components of the PGP nevertheless form a single reproductive unit.

As in many other sexual, diploid crops, the PGP of pearl millet is divided into three morphologically distinct units: (1) wild plants which exhibit no cultivated characteristics and are not dependent upon man for their survival; (2) weedy plants having intermediate morphology which occur exclusively in association with pearl millet cultivation and are indirectly dependent upon man; and (3) cultivated pearl millets whose survival is directly linked to man's agricultural activities. These groups represent the three adaptive strategies which were taken by various members of the PGP in response to the selective pressures which the species has en- countered under domestication. Brunken (1975) has treated each of the three groups (Figs. 1-4) as subspecies of Pennisetum americanum .

The Wild Progenitor Pennisetum americanum subsp, monodii

(Malre) Brunken includes all wild plants which are capable of hybridizing with pearl millet to produce fully fertile offspring. Until recently, these plants were divided among two species, Pennisetum violaceum (Lam.) L. Rich. and Pennisetumfallax (Fig. and de- Not.) Stapf and Hubb., both native to the West African sahel. Recent genetic studies by Bilquez and Lecomte (1969) and Brunken (1975) have demonstrated, however, that neither of these two species is reproductively isolated from pearl millet. Hybrids between

ECONOMIC BOTANY $1: 163-174. April-June 1977. 163

Figs. 1-4. Examples of the three morphological variants included in the primary gene pool of pearl millet. Left: The wild progenitor. Middle: The intermediate, weedy type. Right: Cultivated pearl millet. 1. Inflorescences. • V3. 2. Involucres. • 2V2. 3. Spikelets. • 6.4. Caryopses. • 7.

pearl millet and members of both P. faUax and P. violaceum are uniformly vigorous and fully fertile. The union of these two species into a single subspecies of P. americanum was necessitated in order to reflect the close evolutionary relationship between these taxa and pearl millet. From the initial domestication of pearl millet to the present, a period of several millenia, the members of subsp. monodii have retained both their wild morphologies and their adaptations to a natural

environment. As such, the subspecies repre- sents the wild progenitor of pearl millet.

Morphology and distribution. Members of subsp, monodii are generally less than 1 m tall but may exceed 2 m under ideal moisture conditions. They are distinguished from weedy and cultivated pearl millets by the ab- sence of an involucral stalk, seed size (less than 1 mm deep), seed shape (elliptic), fertile lemma length (more than 5 mm) and inflorescence length (mostly less than 15 cm). The

Figs. 5-10. Distributions in Africa of the various components of the primary gene pool of pearl millet. 5. The wild progenitor--P, americanum subsp, monodii. 6. Shibras---P. americanum subsp, stenostachyum. 7-10. The four basic races of pearl millet P. americanum subsp, americanum. 7. Race typhoides. 8. Race nigritarum. 9. Race globosum. 10. Race leonis.

164 ECONOMIC BOTANY

B R U N K E N E T AL.: MORPHOLOGY AND DOMESTICATION OF PEARL M I L L E T 165

subspecies is found only in the sahel zone of West Africa from Senegal to central Sudan and in the central highlands of the central Sahara (Fig. 5). A natural colonizer, it be- comes locally common in disturbed sites such as seasonally dry stream beds, roadsides, abandoned fields and human habitations.

Shibra

In addition to the cultigen and its wild progenitor, the PGP of pearl millet includes a large number of spontaneously occur- ring, weedy plants which mimic the crop in their vegetative and floral morphologies. Throughout much of West Africa, these mimetic weeds are quite common in fields of pearl millet and are colloquially called shibra. Prior to maturation, shibras are difficult to distinguish from the associated race of pearl millet. Those races having, for in- stance, extremely long inflorescences or elongated terminal bristles have weeds with identical characteristics. Following seed set, however, they disperse their seeds naturally through callus formation as in subsp. monodii. Clayton (1972) has recently divided all such intermediates among two species, Pennisetum stenostachyum (Klotzsch) Stapf and Hubb. and Pennisetum dalzielii Stapf and Hubb. according to their degrees of bristle pubescence. Neither of these species, however, are reproductively isolated from pearl millet. In view of the fact that they occupy a wholly distinct adaptive niche within the PGP, shibras are treated as P. americanum subsp, stenostacgyum (Klotzsch) Brunken. Since it is difficult to distinguish on an individual plant basis between shibras and recent hybrids involving pearl millet and subsp, monodii, all plants having intermediate morphologies are included in subsp. stenostachyum.

Morphology and distribution. As mimet- ics, shibras closely resemble pearl millets in gross morphological characters such as leaf shape and the dimensions of the inflorescence. In more detailed characters such as seed size (1-2 mm deep), seed shape (obovate to elliptic) and fertile lemma length (1.5-6 ram), they are quite variable and generally intermediate between pearl millet and the wild progenitor. In Africa, shibras are found throughout much of the area of pearl millet

cultivation (Fig. 6). They do not appear as frequently in the more mesic zones to which pearl millet has become adapted since domestication. Mimetic weeds have not been reported in the pearl millet fields of India.

Pearl Millet Pennisetum americanum subsp, ameri-

canum includes all of the cultivated races of pearl millet. Known in India under the name bajra, in much of Arab Africa as dukhn and in West Africa under a variety of names such as maiwa, gero and sanyo , subspecies americanum is by far the largest and most variable component in the PGP of pearl millet. In terms of annual production, pearl millet is the sixth most important cereal crop in the world. Among the millets, it is second only to sorghum. This is in spite of the fact that nearly all pearl millet is grown under primitive agricultural conditions. It has only been within the last twenty years that plant breed- ers, primarily in India and West Africa, have made serious attempts at developing im- proved varieties of pearl millet suitable for modern agricultural methods.

At present, the world collection of pearl millet is very small compared to the wide variety of land races available within the crop. The size of the world collection and the lack of a central germ plasm depository have greatly limited the genetic base of past breeding programs. There is currently an attempt under way at the International Crop Re- search Institute for the Semi-arid Tropics (ICRISAT) in Hyderbad, India, to centralize and expand the world collection of pearl millet. The development of such a collection is well in order in view of recent events which have significantly eroded the germ plasm resources within the crop. The five-year drought which has plagued the sahel of West Africa has caused many traditional farmers to abandon their lands resulting in the loss of many local races of pearl millet. This, coupled with the introduction of modern agriculture into areas formerly under pearl millet cultivation, has significantly narrowed the genetic base of the crop.

Morphology and distribution. The ab- sence of callus formation at maturity is the single, critical character which separated subspecies americanum from monodii and

166 ECONOMIC BOTANY

stenostachyum. According to Bilquez and Lecomte (1969), the lack of callus formation in pearl millet is controlled by three recessive genes which appear to work in an additive fashion. Cultivated pearl millets which pos- sess one, two or all three of the recessive loci vary in the toughness of their inflorescences from weakly to strongly non-disarficulating. Besides the lack of callus formation, evolution under domestication has led to remarka- ble increases in inflorescence size (up to 2 m long in some races) and seed size (often more than 3 mm deep) and a decrease in the length of the floral bracts (less than 4 ram). Pearl millet is most important as a cereal crop in the semiarid tropics of the Old World. In Af- rica it occurs from Senegal to Ethiopia and from North Africa to South Africa (Figs. 7-10). Pearl millet is a major crop in the semiarid portions of northwestern India. It occurs less frequently in the southern portions of the Arabian peninsula and Spain and has recently been introduced to the southeastern United States. The greatest portion of its variability lies in the sahel zone of West Africa.

The Taxonomy of pearl millet. The modern taxonomic history of pearl millet began in 1753 when Linnaeus included elements of the cultigen in two different species, Panicum glaucum and P. americanum. In the 200 years following Species Plantarum, well over 100 species have been described to include all or part of the variation in pearl millet. As recently as 1934, Stapf and Hubbard recog- nized 13 such species. Although taxonomists are now in general agreement that all pearl millets belong to a single species, considerable confusion persists over its proper species name. Unfortunately, two of the most commonly used names, Pennisetum typhoides (Burm.) Stapf and Hubb. and P. glaucum (L.) R. Br., are invalid. The former is a later synonym for the crop and the latter is based upon a specimen of foxtail (Setaria). Pen- nisetum americanum (L.) Leeke, an incon- gruous name for an Old World crop, is based upon an early description of a pearl millet by Clusius (1602).

lntraspecific hybridization. Wherever the three subspecies of P. americanum come into contact, the possibility of hybridization is very great. Hybridization is facilitated by the

protogynous nature of the species and the relative abundance of pearl millet pollen throughout the flowering season. Extensive hybrid swarms have been inspected by us in western Sudan, northern Nigeria and espe- cially western Senegal. They occur most commonly at the margins of actively cultivated fields of pearl millet. Although no data are available, the swarms appear to persist for several years. Owing to the abundance of its pollen, backcrossing is predominantly to the cultivated parent. Hybridization between pearl millet and its wild progenitor has probably been a common occurrence following their divergence during domestication and may have allowed genetic exchange with local races of subsp, monodii as the cultivation of pearl millet spread.

Origin of shibras. Although they are intermediate in morphology, true shibras are not the products of recent or transient hybridization. Artificial plantings of shibra populations do not segregate, indicating that the intermediate genotype has been sta- bilized. The origin and continued survival of shibras is tied directly to man's weeding activities. Following domestication, past and present pearl millet growers have endeav- ored to insure the success of their crops through the selective elimination of their natural competitors, the weeds. In West Af- rica, one of the chief competitors of pearl millet has always been its own wild progenitor, a natural colonizer. In removing all spon- taneous plants from his field, man created a niche for those individuals which resembled the crop but retained the capacity for natural seed dispersal. Such selection over many years, coupled with frequent hybridization between pearl millet and subsp, monodii, eventually led to the stabilization of the intermediate genotype. Morphological variation within shibras today is nearly as great as that in pearl millet. In view of their variability and the high frequency of hybrid swarms, the origin of subsp, stenostachyum may be polyphyletic.

After a pearl millet field is abandoned and the dominant role of man in determining the composition of the population is removed, shibras are quickly eliminated, generally within three years. Thus shibras have apparently sacrificed the ability to compete with

BRUNKEN ET AL,: MORPHOLOGY AND DOMESTICATION OF PEARL MILLET 167

nonmimetic natural colonizers and, indeed, have become dependent upon the weeding activities of man for their survival.

THE RACES OF PEARL MILLET

One of the primary objectives of the present study was to define the patterns of racial variation within present-day pearl millets. To that end, the world collection of over 200 accessions of pearl millet which is located at the Crop Evolution Laboratory, University of Illinois, was scored for a number of floral characters including involucral stalk length, bristle number and length, fertile lemma length and shape, and caryopsis size and shape. Only those accessions which appeared to represent unimproved, local pearl millets were included in the study.

In very few instances were any two collec- tions from widely separated geographic regions morphologically identical. Consider- able variation was found even in different plants within the same field. Pearl millet is, therefore, apparently composed of a myriad of highly variable and locally restricted races. Indeed, sample size appeared to be the limiting factor in the amount of variation observed. In looking for general trends within the characters scored, we found that seed shape followed the most consistent geographic pattern. Four basic seed shapes were found in the world collection. The racial classification outlined below is based primarily upon the characteristics of the seed. It is emphasized that not all pearl millets fit neatly into one of these four basic races. Con- siderable intermediacy does exist, but in each case it is between two of the basic races. For the purpose of clarity, such intermediates have been excluded from the distribution maps. It is also noted that intraplant variation in seed morphology occurs depending upon seed density and position within the inflorescence. In most cases, such variations are in size and not shape.

Race typhoides. The caryopses (Fig. 11) in this race are obovate in both frontal and profile views. They are also obtuse and terete in cross section. Seed size is extremely variable being 2.5-5.5 mm long, 1.5-3 mm wide and 1.2-2.4 mm deep (dorsal-ventral measure- ment). The grains are occasionally shorter than and enclosed by the floral bracts.

Typhoides is highly variable in bristle number, pubescence of the bristles (almost glabrous in some), number of spikelets per involucre (1 to 9) and pubescence of the floral bracts (villous to glabrous).

It is not surprising that the most morphologically variable of the four races is also the most widely distributed (Fig. 7). Typhoides occurs from Senegal to Ethiopia, from North Africa to South Africa and is the only basic race of pearl millet found outside the continent of Africa.

Members of typhoides exhibit two distinctive types of inflorescence. In most plants, the pseudo-spike is many times longer than wide, giving it a cylindrical, candle-like ap- pearance. Less often, the inflorescence is only 2 to 4 times longer than wide, producing a short, almost elliptic shape. Variation in inflorescence shape does not appear to follow any geographic pattern. The ball-headed type has been collected from North Africa to Tanganyika in Africa and was the type introduced to Spain by the Moors.

In view of its diversity and wide distribution, typhoides is probably the most primitive of the four basic races described here. Local forms of this race often exhibit characters usually associated with the wild progenitor. These include small grain size, glabrous floral bracts which are also lanceolate in shape, and relatively small inflorescences. One such form is commonly cultivated in northern Southwest Africa in the Etoshapan area. The presence of a primitive form of pearl millet is surprising in view of the late arrival of agriculture to the area. These primitive pearl millets may represent rem- nants of migrational events early in the history of the crop. During and following their migration from northern Africa, these primitive forms may not have been subjected to the intense cultural selection which occurs in areas of high density pearl millet cultivation and have thus retained many of their primitive characteristics.

Race nigritarum. The caryopsis (Fig. 12) here is obovate as in the previous race but extremely angular in cross section with between three and six facets per grain. The apex of the grain is usually truncate and often tinged with purple. The dimensions of the grain are 3-5 mm long, 1.7-2.5 mm wide

168 ECONOMIC BOTANY

Figs. 11-14. Scanning electron micrographs of the caryopses in the four basic races of pearl millet, x 20. 11. Race typhoides, 12. Race nigritarum. 13. Race globosum. 14. Race leonis.

and 1.5-2.2 mm deep. The mature grain is generally longer than and protrudes beyond the floral bracts. The inflorescence is uniformly candle-like.

The center of distribution for nigritarum is from western Sudan to northern Nigeria (Fig. 8). It is infrequently cultivated as far west as Senegal. In view of its distribution,

B R U N K E N E T AL.: MORPHOLOGY AND DOMESTICATION OF PEARL M I L L E T 169

nigritarum is probably native to the eastern sahel and may represent the original race of pearl millet in that area. Isolated occurrences in the western sahel are probably the result of recent migrational events.

Race globosum. As its name implies, the caryopsis in race globosum (Fig. 13) is characteristically spherical with each of its dimensions being approximately equal. The important dimension, however, is depth as measured from the apex of the embryo to the back of the grain. In globosum, depth of the grain always exceeds 2.4 mm. The grain is otherwise terete and obtuse. Inflorescence shape is candle-like.

Globosum has been collected from central Upper Volta to western Sudan. It is most common from central Nigeria to Niger (Fig. 9). Although it appears to reach its greatest importance in the central sahel, globosum does not appear to have a well-defined center of distribution. It may be that this large seeded race has experienced a great degree of migration in recent times.

Race leonis. The fourth and smallest race of pearl millet is characterized by an acute, oblanceolate, terete caryopsis (Fig. 14). Un- like the previous three races in which the profile and frontal views of the grain are quite different, the grain of leonis appears nearly oblanceolate from all lateral perspec- tives. The grains are from 3.8-6.3 mm long and 1.9-2.5 mm wide and deep. The most distinctive character of the leonis grain is its acute apex which is terminated by the rem- nants of the stylar base. At maturity, approximately one-third of the grain protrudes beyond the floral bracts. The inflorescence in leonis is candle-like.

Although it occurs infrequently as far north as Senegal and southern Mauritania, leonis is virtually indigenous to Sierra Leone (Fig. 10). The greatly elongated characteris- tic of the grain in leonis may be an adapta- tion to the relatively high rainfall regimes of Sierra Leone. The extended apex protruding beyond the floral bracts increases the surface area which is exposed to the air, thus facilitating the drying of the grain under high humidity conditions.

It has been argued that the patterns of racial variation observed within morphologically diverse crops are the result of several

independent domestications. Such a possibility has been suggested by Snowden (1926) for sorghum, by Mangelsdorf (1974) for maize and by Port~res (1962) for pearl millet. Al- though independent domestications cannot be ruled out, it appears that migrational events, possibly very early in the history of pearl millet, followed by a combination of geographic and ethnographic isolation are re- sponsible for the present-day pattern of variation in seed morphology of the crop. During the past several centuries these patterns have been somewhat obscured by the relatively intense migration brought on by cultural changes following the arrival in Africa of first the Arabs and more recently the Euro- peans. Migration has been mediated by man both directly through attempts to improve local races of pearl millet and indirectly through increased trade between previously isolated sectors of the native community.

THE DOMESTICATION OF

PEARL MILLET

References to pearl millet are absent from the early literature of the Near East, Egypt, Greece and Rome. There are no biblical references to the crop, nor is it mentioned in the works of Pliny, Dioscorides or Theophrastus. Pearl millet has not been reported among the multitude of plant remains and illustrations found in Egyptian ruins. No early Sanskrit word can be traced directly to the crop. Possibly the earliest mention of pearl millet is by Al-Idrisi (1154), an Arab scholar, in an account of his travels in North Africa and Spain. In describing the region of Abyssinia (the Upper Nile), he mentions that two types of millet, durra and dokhn, are commonly cultivated. The first name refers to cultivated sorghum and the second to pearl millet. Both of these crops are grown under these same names in the Upper Nile today. The earliest mention of pearl millet in western literature is attributed to Leo Africanus, a 16th century Moorish slave in the service of Pope Leo X for whom he wrote an account of Africa north of the forest zone (Pory, 1600). In describing the kingdom of Gualata, an area to the west of Timbuktu, he states that there was "some quantity of mil-seed (sorghum or pearl millet), and a great store of a round and white kind of pulse (probably

170 ECONOMIC BOTANY

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Vigna unguiculata L.), the like whereof I never saw in Europe." Later he records that in Guber far to the southeast of Timbuktu was an "abundance of rice (Oryza glaberrima Steud.) and of certain other grain and pulse, the like whereof I never say in Italie. But I think it groweth in some places of Spain." According to Colmeiro (1885), pearl millet was introduced to the south of Spain from North Africa after the Moorish invasion in the 8th century. Although the above account

has been used as evidence of pre-Columbian maize in Africa (Sauer, 1952), the descrip- tions in question are more reasonably inter- preted as pearl millet.

Thus early literature tells us little more than that pearl millet occurred in the semiarid tropics of Africa before 1100 A.D. The bulk of the evidence concerning the origin of pearl millet comes from the interpreta- tion of present-day distributions of the crop and its wild ancestor. On the basis of mor-

B R U N K E N E T A L . : M O R P H O L O G Y A N D D O M E S T I C A T I O N O F P E A R L M I L L E T 1 7 1

phological diversity, Koernicke and Werner (1885) concluded that pearl millet was origi- nally native to Africa. Although their state- ment has never been seriously questioned there has been considerable controversy over the specific region of Africa and the time period in which domestication took place. Three wholly different hypotheses have been put forward.

In his classic work on the origins of crops, Vavilov (1949/1950) placed pearl millet in the Ethiopian center of domestication. The highlands of Ethiopia are today an agriculturally diverse region in which crops from many parts of the world are grown. Two lines of evidence argue against an Ethiopian origin for pearl millet. First, the wild progenitor (P ennisetum americanum subsp, monodii) is rare east of Sudan and has never been collected in the Ethiopian highlands. The wild progenitor is adapted to the sandy, semiarid conditions of the sahel and very likely would have been absent from the high rainfall and high altitude environment of the Ethiopian highlands. Secondly, the pearl millets of Ethiopia lack sufficient morphological diversity. Today pearl millet is a minor crop in Ethiopia and is probably the product of post-domestication introduction.

A second theory as to the origin of pearl millet was proposed by Murdock (1959). Using primarily linguistic evidence, he postu- lated that pearl millet was one of several West African crops domesticated by the Mande people near the headwaters of the Niger River between 4000 and 5000 B.C. Murdock's general hypothesis has been the center of considerable controversy since its publication (Baker, 1962; Wrigley, 1969). During the period suggested by Murdock, the headwaters of the Niger probably exhib- ited a climax, tropical rain forest type of vegetation. It is highly unlikely, therefore, that a dryland crop such as pearl millet would have been domesticated there.

The greatest morphological diversity in pearl millet occurs today in West Africa south of the Sahara Desert and north of the forest zone. The wild progenitor also occurs in the drier, northern portion of this zone. Taking these facts into consideration, Harlan (1971) suggested a third center of origin for the crop in a diffuse belt stretching from

western Sudan to Senegal. On the basis of present-day distributions, the sahel zone of West Africa does appear to be the original home of pearl millet. A question remains, however, whether climatic conditions and plant distributions have remained un- changed since domestication took place. A central problem in unravelling the origin of pearl millet is, therefore, the determination of the distribution of its wild ancestor at the time of domestication.

Rainfall is the primary environmental factor which has influenced the vegetational patterns of northern Africa during the Quaternary period (Butzer, 1966). During the past 30,000 years, the region currently enclosed by the Sahara has experienced a series of wet and dry fluctuations which have drastically altered its flora and fauna. The Sahara is presently in one of the longest and driest phases in its history. The last wet phase occurred between 5000 and 3000 B.C. From analyses of pollen core samples, the climate and vegetation of the Sahara during this 2,000 year period was Mediterranean. Throughout most of the last wet phase, the semidesert periphery of the Sahara was shifted 100 to 200 kilometers inward. The central highlands of the Sahara were mesic islands surrounded by drier lowlands.

Clark (1962) has suggested that cereal cultivation spread to the central highlands of the Sahara in the 5th millenium B.C. from the Near East probably by way of Egypt and that the knowledge of cereal cultivation was distributed throughout the Sahara by 3000 B.C. This stimulus from the east involved the introduction of Near Eastern cereals such as wheat and barley which were adapted to the winter rainfall patterns which dominated the Sahara at the time. Even today these two cereals are marginally cultivated in the central highlands and oases of the Sahara. Clark (1964) has also suggested that as the most recent dry phase began and the rainfall patterns shifted from winter to summer, these Mediterranean crops became ill-adapted, drastically decreasing their production. Clark surmised that this series of events forced the inhabitants of the central Sahara, many of whom had by this time become dependent upon cereal cultivation, to experiment with local grasses.

172 ECONOMIC BOTANY

All available evidence points to Pen- nisetum americanum subsp, monodii as the nearest wild relative of pearl millet. Indeed, the subspecies has probably changed very little since the time of domestication. Today subspecies monodii is restricted in distribution to the southern margins and central highlands of the Sahara Desert where annual rainfall varies between 250 and 800 mm. In view of the rainfall patterns which existed at the end of the last Saharan wet phase, the distribution of subspecies monodii was probably much north of what it is today. Prior to domestication, subspecies monodii was probably one of the wild grasses adapted to the dry foothills of the southern margins of the Saharan highlands. As a natural colonizer, it may have been a common weed near human habitations, occupying disturbed sites such as the margins of cultivated fields. As such, P. americanum subsp, monodii would have been a natural choice for local farmers who were forced to turn to local wild grasses when their poorly adapted, Mediterranean crops failed to survive. Corre- lation of present-day morphology and distribution in P. americanum with what is known of the climatic and cultural history of the Sa- hara suggests, therefore, that pearl millet was domesticated along the southern margins of the central highlands at the beginning of the present dry phase, probably between 3000 and 2000 B.C.

The available archaeological evidence neither disputes nor confirms the above con- clusion. Davies (1968) reported a pearl millet from Ntereso in northern Ghana at 1250 B.C. and suggested that it was introduced to the area by invaders from the north. Munson (1975) has uncovered pottery impressions of pearl millet in the Dhar Tichitt region of southeastern Mauritania dated 1100-1000 B.C. Upon recent examination, we have found that spikelets of what are unmistak- ably subspecies monodii are also a common component of these impressions. Archaeolog- ical remains in the form of carbonized grains indicate that pearl millet reached the north- west coast of India at Gujerat by 1000 B.C. (Rao, et al., 1963). The culfigen may have reached southern Africa by the 8th or 9th century A.D. as evidenced by a single grain uncovered in the Ngonde region of northern

Malawi (Robinson, 1966). Two grains of grass pollen from the Ameki site in the Hog- gar region of the central Sahara dated 6100 to 4500 B.C. were identified by Camps (1969) as belonging to pearl millet. In view of the highly uniform morphology of nearly all grass pollen (Wodehouse, 1935) the validity of this identification, which was based upon observations using light microscopy, is extremely tenuous at best.

Faced with the wide diversity found in pearl millet today, Port~res (1962) concluded that the crop is a product of multiple domestications. Whether or not pearl millet has a single or multiple centers of origin is difficult to determine without a virtually complete archaeological record. Post-domesficational differentiation through adaptive radiation, possibly facilitated by introgression with local populations of the wild subspecies, appears to be a more likely explanation for the extensive diversity found within the crop today. Once domestication had occurred, pearl millet was probably quickly introduced throughout the drought plagued regions of the Sahara. As the desert continued to expand, the distribution of pearl millet ex- panded proportionately. It was also introduced through human migration and trade to semiarid, tropical regions beyond the Saha- ra. As the distribution of pearl millet continued to expand, selection by man within a myriad of cultural contexts and natural selection to local environmental conditions led to the wide morphological diversity apparent in the crop today.

LITERATURE CITED

Al-Idrisi. 1154. Description de l'Afrique et de l'Es- pagne. (Transl. by R. P. A. Dozy and M. J. de Goeje. 1969). Oriental Press, Amsterdam.

Baker, H. G. 1962. Comments on the thesis that there was a major centre of plant domestication near the headwaters of the river Niger. J. Mr. Hist. 3:229-233.

Biiquez, A. F., and J. Lecomte. 1969. Relations entre mils sauvages et mils cultives: etude l'hybride Pen- nisetum typhoides Stapf & Hubb. X P. violaceum (Lain.) L. Rich. Agron. Trop. 24:249-257.

Brunken, J. N. 1975. Biosystematic studies in Pen- nisetum L. Rich. (Gramineae). Ph.D. Dissertation, University of Illinois, Urbana. 124 pp.

Butzer, K. W. 1966. Climatic changes in the arid zone of Africa during early mid-Holocene times. In World Climate from 8000 to 0 B.C. Royal Metrological Inst., London.

BRUNKEN ET AL.: MORPHOLOGY AND DOMESTICATION OF PEARL MILLET 173

Camps, G. 1969. Ameki: n6olithique ancien du Hog- gar. M~moire du C.R.A.P.E. 10. Algiers.

Clark, J. D. 1962. The spread of food production in sub-Saharan Africa. J. Afr. Hist. 3:211-228.

�9 1964. The prehistoric origins of African culture. J. Afr. Hist. 5:161-183.

Clayton, W.D. 1972. Gramineae. 101.Pennisetum. In F. N. Hepper (ed.), Flora of West Tropical Africa, ed. 2, Vol. 3, pt. 2. Crown Agents, London.

Clusius, C. 1602. Rariorum plantarum historia, pp. 215-216.

Colmeiro, M. 1885. Enumeracio y revision de las plan- tas de la peninsula hispano-lusitano e islas Balearas. Madrid�9

Davies. O. 1968. The origins of agriculture in West Africa. Curr. Anthro. 9:479-482.

Harlan, J. R. 1971. Agricultural origins: Centers and non-centers. Science 14:468-474.

, and J. M. J. de Wet. 1971. Toward a rational classification of cultivated plants. Taxon. 20:509-517.

Koernicke, F., and H. Werner. 1885. Handbuch des Getreidesbaues. Verlag von Paul Paery, Berlin.

Mangelsdorf, P. C. 1974. Corn: Its origin, evolution and improvement. Harvard Univ. Press, Cambridge.

Murdock, G. P. 1959. Africa--Its people and their cultural history. McGraw-Hill, N.Y.

Munson, P.J. 1975. Archaeological data on the origins of cultivation in the southwestern Sahara and its im- plications for West Africa. In Harlan, J. R., J. M. J. de

Wet and A. Stemler (eds.), The origins of African plant domestication. Mouton Press, The Hague.

Portbres, R. 1962. Berceaux agricoles primaires sur le continent Africain. J. African Hist. 3:195-210.

Pory, J. 1600. The history and description of Africa- written by Leo Africanus. R. Brown (ed.), Hakluyt Society Publ. No. 92. 1896.

Ran, S. R., B. B. Lal, B. Nath, S. S. Ghosh and K. Lal. 1963. Excavation at Rangpur and other explo- rations in Gujerat. Bull. Arch. Surv. India 18/19:5- 207.

Robinson, K. R. 1966. A preliminary report on the recent archaeology of Ngonde, Northern Malawi. J. Aft. Hist. 7:169-188.

Sauer, C. 1952. Seeds, spades, hearths and herds. The domestication of animals and foodstuffs. MIT Press, Cambridge.

Snowden, J. D. 1926. The cultivated races of Sor- ghum. Allard and Son Ltd., London.

Stapf, O., and C. E. Hubbard. 1934. Pennisetum. In D. Praln (ed.), The flora of tropical Africa. Vol. 9. Crown Agents, London.

Vavilov, N. I. 1949/1950. The origin, variation, im- munity and breeding of cultivated plants. Chronica Botanica 13:1-366.

Wodehouse, P. G. 1935. Pollen grains. McGraw-Hill, N.Y.

Wrigley, C. 1960. Speculations on the economic pre- ~history of Africa. J. Afr. Hist. 1:189-203.

Book Reviews (continued from p. 162)

by A. Kjaer, The Technical Universi ty of Denmark , Lyngby; 9) "Lipids in the Cruciferae," by L-A. Appelqvist, Royal Vet- er inary College, Stockholm; 10) "The Seed Protein Contents of Some Cruciferae," by A. J. Finlayson, Prairie Regional Labora tory , Nat ional Research Council of Canada , Sas- katoon; 11) "Volatile F lavour Compounds of the Cruciferae," by A. J. MacLeod, Queen Elizabeth College, Universi ty of London; and 12) "The Biology and Chemist ry of the Cruciferae---General Conclusions," by V. H. Heywood , Universi ty of Reading.

Each contr ibution is supported by a list of references. There are a general subject index (pp. 342-349) and a species index (pp. 350- 355). Quick and economical product ion was accomplished by photo offset printing f rom well-prepared typescript. The 32 figures and six plates include botanical line drawings, diagrams, and electron micrographs; there are, in addition, 49 tables. The book is sub- stanfially and at tractively cloth bound and jacketed.

J .F .M.

Aquat ic Plants o f Australia. Helen I. As- ton. 368 pp. illus. Melbourne Universi ty Press, Melbourne, 1973. $36.00.

This book, "the first a t tempt to produce a complete flora of the native and natural ized aquatic ferns and flowering plants of Austra- lia," is quite reminiscent of Mason 's A Flora of the Marshes of California. Produced in about the same format with the same obvious care, and having similarly excellent illustrations (done by Ms. Aston herself), the book is an admirable one indeed.

The " In t roduct ion" briefly discusses " W h a t is an Aquat ic Species?" and "Climate and Phys iography" of Australia. A glossary defines about 300 terms. Appendix 1, "Water Hyac in th (Eichhornia crassipes)," presents a good general account of this species and of its arrival and spread in Australia. Appendix 2, "Sea-grasses," is a list of the 12 genera of aquatic angiosperms recorded for Australia. Appendix 3, "Distr ibution Char t , " records distribution of Australia 's aquatic plants by state. The bibl iography is a "selected guide

Book Reviews (continued on p. 179)

174 ECONOMIC BOTANY

the morphology and domestication of pearl millet

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