Showing papers in "American Journal of Botany in 1976"

Productivity and composition of a baldcypress-water tupelo site and a bottomland hardwood site in a louisiana swamp'

Abstract: hardwood site (BLH) and 500 g dry wt/m2 for a baldcypress-water tupelo site (CT). Litter-fall was 574 g dry wt/m2/yr for BLH and 620 g dry wt/m2/yr for CT. Harvest samples within the two plots yielded 200 g dry wt/m2 and 20 g dry wt/m2 for BLH and CT, respectively. Minimum net primary production was calculated as the sum of the three: 1574 g dry wt/m2/ yr for BLH and 1140 g dry wt/m2/yr for CT. Maximum estimates of herbaceous production and insect consumption were made by using values from the literature. Estimated total net primary productivity was 1733 g dry wt/m2/yr for BLH and 1516 g dry wt/m2/yr for CT. Tree composition was determined by the point-centered quarter method. Relative frequency, relative density, absolute density, relative dominance, and importance value (IV) were calculated for the tree species along each transect. In the bottomland hardwood area many woody species exist with Acer rubrum var. drummondii (IV = 23.9) and Nyssa aquatica (IV = 18.4) the most dominant. In the baldcypress-water tupelo area, fewer woody species exist and Taxodium distichum (IV - 39.2) and N. aquatica (IV = 37.6) dominated. Comparison of productivity data from several southeastern swamps indicate that flowing water regimes tend to result in the highest swamp forest productivity.

The absorptive capacities of bromeliad trichomes

Abstract: Tritiated glycine and leucine and microautoradiographic techniques were employed to determine whether various bromeliad species produce foliar trichomes capable of absorbing these two solutes from solutions placed on intact leaf surfaces. All trichomes borne by members of subfamily Tillandsioideae, whether on the leaf blade or sheath, accumulated both amino acids in substantial quantities. Trichomes located on the leaf sheaths of tank-forming members of Bromelioideae absorbed much smaller quantities while trichomes located on the blades of the same tank-producing bromelioids, as well as those of nontank-forming members of Pitcairnioideae and Bromelioideae, took up little or none at all. The evolution of the bromeliad trichome is considered in the light of these results. AMONG FAMILIES of flowering plants, Bromeliaceae is noteworthy for its diverse habitat preferences and for the ability of many of its species to flourish in arid, nutrient-deficient environments (Pittendrigh, 1948; Benzing, 1973). Numerous bromeliads are epiphytic, some living in dry forests while others are mesic or xeric terrestrials. Epiphytic species adapted to very xeric sites have been designated "atmospherics" because they characteristically lack the tank leaves featured by "cistern" bromeliads and have either nonabsorptive holdfast roots or none at all. As such, atmospherics must extract all required mineral nutrients directly from rainfall and other highly dilute solutions when these fluids pass over the shoot during periods of precipitation. It has been recognized for many years that the foliar trichome is the major organ of moisture absorption in species belonging to the largely epiphytic subfamily Tillandsioideae-atmospheric Tillandsia usneoides, for example (Haberlandt, 1914). The assertion has been made, but never tested by appropriate modern analyses, that this epidermal structure is involved in solute accumulation as well. The involvement of bromelioid and pitcairnioid (members of ecologically less specialized subfamilies Bromelioideae and Pitcairnioideae, respectively) trichomes in either moisture or salt absorption has received even less attention. Recent studies on the uptake of various mineral nutrients by bromeliad leaves representing all three subfamilies demonstrated that foliar permeabilities were high in some species and much ' Received for publication 25 August 1975. Much of this effort was supported by National Science Foundation grants to the first author and a National Science Foundation Undergraduate Research Participation grant to the Oberlin College Departments of Biology and Chemistry. lower in others (Benzing and Burt, 1970). These differences were associated with the type of trichome present and with its frequency of distribution over the leaf surface, with habitat preference and, in tank species, with the location of the trichomes on the leaf (blade or sheath). Foliar uptake was most pronounced in atmospheric species. In the following investigation, microautoradiographic analysis by use of tritiated leucine and glycine was employed to determine which, if any, of the foliar trichomes of selected members of all three subfamilies and the major ecological types within Bromeliaceae could be major sites of solute accumulation by these species. Although free amino acids are not abundant in foliar leachates (Tukey, 1970), they are present and therefore may be at least minor sources of nitrogen. In any case, tritiated amino acids can be employed to demonstrate absorption by bromeliad trichomes of a solute from solutions situated on the intact leaf surface. MATERIALS AND METHODS-All tissues were obtained from material grown in the Oberlin College greenhouse. Reference specimens are on deposit in the Oberlin College Herbarium. Leaves were excised in midmorning and for 0.5 hr were maintained in aerated distilled water. In some cases samples were killed by plunging them in boiling water just before the treatment solution was applied. After an appropriate pretreatment, samples designated to receive a one-half hour or longer contact with a particular treatment solution were placed in an apparatus previously utilized which permitted contact of the desired leaf surface with 10 ml of treatment solution (Benzing, 1970). All treatment solutions contained either L-leucine4,5-3H or glycine-2-3H at 10 Ac/ml, 10-5 M CaC12

Pollination and fruit-set in asclepias: a reappraisal

Abstract: Experimental pollinations of Asclepias tuberosa L., the first for this species and second largescale effort for the genus, reveal trends toward local population differentiation. The species possesses a low level of self-compatibility, the first documented report for the genus. Analysis of flowers visited by natural pollinators in populations across the eastern half of the United States demonstrated a linear relationship between pollinia removal and pollinia insertion. Clarifications in older literature regarding the specificity of the pollination mechanism, effective levels of pollination in natural populations, and the performance of experimental manipulations are made, and it is concluded that fruit-set in Asclepias is regulated by the interaction of both mechanical aspects of pollination and physiological aspects of fertilization and fruit development. A model incorporating these restraints is seen to predict fairly well the observed fruit to flower ratio of about 1:100 in natural populations. WITH MORE RECENT attempts to quantify and explain pollination biology in evolutionary terms (e.g., Faegri and van der Pijl, 1966; Heinrich and Raven, 1972; Proctor and Yeo, 1972), there has been a resurgence of interest in the reproductive biology of asclepiads. The reproductive biology of the Asclepiadaceae, or milkweed family, is rivalled only by that of the Orchidaceae in its floral complexity (Fig. 1). The five antisepalous stamens have undergone extreme modification; the anther connectives extend upward to partially overlap the stigmatic head. In addition, coronal extensions (hoods) containing arching structures (horns) retain the nectar secreted by nectaries located inside the stigmatic chamber within the flower (Galil and Zeroni, 1965). The stigmatic surfaces have been shifted laterally and are enclosed by the closely adjoining anther wings. The pollen sacs (pollinia) of two adjacent anthers are joined by translator arms to a black body (corpusculum or gland) located just above the slit of the stigmatic chamber formed by the projecting anther wings. Pollinia removal is effected when a groove in the corpusculum catches on a hair of an insect leg. When the insect flies to another flower on a different plant the pollinium lodges in the chamber, thus effecting cross-pollination. Quantitative studies of the reproductive biology I Received for publication 26 June 1975. I wish to thank Janis Antonovics for supplying encouragement, ideas, and criticism. Gordon Whitaker provided able assistance in the field during the re-collection of the transect. Henry M. Wilbur and Mary F. Willson suggested improvements in an earlier draft of the manuscript. The line drawings were executed by Karen Teramura. Funds for use of the Duke University Phytotron were supplied by NSF Grant GB-28950 and for travel by an NSF Graduate Fellowship. of the Asclepiadaceae were first attempted in the 1940's when commercial use of species of Asclepias was proposed (Fischer, 1941; Whiting, 1943; Stevens, 1945; Moore, 1946, 1947; Sparrow and Pearson, 1948). These early studies raised many questions regarding reproductive biology, but no general agreement could be reached on even such basic matters as the precise mechanism of pollinia removal and insertion, the relative importance of mechanical versus physiological interspecific isolating mechanisms, the possibility of self-compatibility, and factors limiting fruit-set to levels of 1 %. Floral morphology (Safwat, 1962; Galil and Zeroni, 1965, 1969), specificity of pollinia transfer (Macior, 1965), and temporal and spatial aspects of the floral display (Willson and Rathcke, 1974) have all been re-examined. Life history characteristics (Wilbur, 1976) and interspecific variation in ovule and pollen production (Bell, 1974) have also been analyzed recently. This paper reports detailed studies on pollination, breeding system, and fruit-set in Asclepias tuberosa L., the common roadside "butterflyweed." This species was studied previously by Woodson (1947, 1953, 1962, 1964). MATERIALS AND METHODS-In July, 1973, twenty flowering individuals were taken from each of five populations located more or less equidistantly along Woodson's (1947, 1962) 1200-mile west-to-east transect. The populations were therefore separated by average distances of about 300 miles. The plants were cut back to the rootcrown, established in pots in a controlled environment greenhouse and grown under natural light in a temperature regime of 23 C day and 20 C night. The plants flowered after two months, and all

Reproductive isolation of triticum boeoticum and triticum urartu and the origin of the tetraploid wheats

Abstract: A B ST RA C T The diploid wheats Triticum boeoticum and T. urartu are sympatric with one another throughout the geographic range of the wild tetraploids. Reciprocal crosses between ecogeographic types within each diploid species gave viable seed, but interspecific crosses consistently gave viable seed only when T. boeoticum was the female parent. Apparently urartu cytoplasm in combination with the boeoticum genome resulted in nonviable seed. The endosperm failed to develop normally despite regular endosperm fertilization. The F1 plants obtained were completely self sterile although they showed regular intergenomic pairing (711) at meiosis. Presumably the accumulation of cryptic differences between the two closely related genomes under reproductive isolation accounts for this sterility. The same accumulated cryptic differences could largely account for the preferential diploid pairing in the tetrapolid wheats which presumably were derived from such hybrids by chromosome doubling. The behavior of reciprocal crosses between the diploids and tetraploids suggested that T. boeoticum contributed the cytoplasm to both of the wild tetraploid species. THE EVOLUTION of closely related sympatric species is contingent on their reproductive isolation. In plants, sterility of the F1 hybrid between such species opens the door to amphiploidy and to evolution at the polyploid level based on more or less autonomous and divergent genomes. These factors seem to have played vital roles in speciation within the diploid wheats and in the origin of the tetraploids. The wild tetraploid Triticum dicoccoides K6rn Schweinf., from which early agricultural tribes domesticated the emmer wheats, is endemic primarily to the western arc of the Fertile Crescent from the upper Jordan Valley to southeastern Anatolia (Johnson, 1975). Sporadic collections have been reported from Iraq and Iran (Dagan and Zohary, 1970; Tanaka and Ishii, 1973). The closely related tetraploid T. araraticum Jakubz., ancestral to the cultivated timopheevii wheats, is dispersed from Transcaucasia along the eastern arc of the Fertile Crescent into Iran (Fig. 1). The wild diploid T. boeoticum Boiss. (AA), long recognized as the A-genome parent of both the emmer (AeAeBeBe) and timopheevii (AtAtBt Bt) wheats, occurs throughout the range of the tetraploids, but its geographic distribution is much greater, extending across Anatolia from Transcaucasia to Greece. For many years the B-genome parent of the

Effects of simulated acid rain on phaseolus vulgaris L. (fabaceae)

Abstract: A B S T R A C T Experiments were performed to determine the effects of simulated acid rain on Phaseolus vulgaris L. At pH values below 3, plants exhibited a failure to attain normal height, had necrotic and wrinkled leaves, excessive and adventitious budding, and premature abscission of primary leaves. Histologically, leaves had smaller cells, less intercellular space, and smaller starch granules within the chloroplasts. Respiration rates of the treated plants increased only slightly at low pH values. Apparent rates of photosynthesis, however, increased dramatically. Both carbohydrate production and root biomass were reduced by low pH treatments, and application of Congo red indicator to the acid-treated leaf tissue showed that the cell contents were acidified to a pH of below 4.0.

Plantlets from petal segments, petal epidermis, and shoot tips of the periclinal chimera, chrysanthemum morifolium ‘indianapolis'

Abstract: A B S T R A C T Cultivated chrysanthemums, especially the greenhouse series of 'Indianapolis' cultivars, are probably periclinal chimeras for flower color. Therefore, in vitro propagation of chrysanthemum, which has recently been described, might produce plants not true to type. To test this, plantlets were generated from cultures of petal segments, petal epidermis, and shoot tips; these plantlets were grown to flowering to determine whether chrysanthemums with two genetically different chimeral layers in the petals are stable in tissue culture. Layer I displaced layer II in the formation of new meristematic areas in shoot tip and petal culture, showing that such chimeras are unstable in culture. Many more abnormal morphological types were exhibited by the plants which were regenerated from petal cultures rather than those from shoot tip cultures. Abnormalities included quilled and incised petal forms, as well as lack of anthocyanin pigmentation, characteristics which may not be attributable to the rearrangement of chimeral layers. Paramutation, true mutation, and environmenal effects are offered as possible explanations for this phenomenon. CULTIVARS of Chrysanthemum morifolium are among the most important flowers in commercial floriculture. Many of them are thought to be periclinal chimeras involving flower color since whole series or "families" of cultivars have arisen

Reactions of alkaloid and histochemical indicators in laticifers and specialized parenchyma cells of catharanthus roseus (apocynaceae)

Abstract: A B S T R A C T Chromic acid, iodine-potassium iodide and Dragendorff reagent were employed to identify reactive cells that may indicate sites of alkaloid accumulation in fresh tissues and latex of C. roseus. Laticifers in all parts of the mature plant and certain parenchyma cells in the cortex and pith regions accuLmulated reaction products of these alkaloid indicators. These same cells showed primary fluorescence, accumulated vital dyes and lipid indicators in excess of other cells, and exhibited a more intense nadi reaction than other cells. Tests on fresh tissues are interpreted to indicate possible qualitative and quantitative differences in alkaloid content between subterranean and aerial portions of the plant and between mature and immature tissues. These studies showed that' reactive products are unevenly distributed in cells and organs of the plant and can be microscopically detected only in laticifers and specialized parenchyma cells.

Initiation and development of root nodules of casuarina (casuarinaceae)

Abstract: A B S T R A C T Roots of seedlings of the "beefwood" tree, Casuarina cunninghamiana Miq. grown in nitrogenfree nutrient solution were inoculated with a suspension prepared from crushed root nodules taken from mature plants. Marked deformation of root hairs was evident but no infection threads were observed in root hairs. The mode of infection remains undetermined. Root nodules were initiated within three weeks and thereafter numerous upward-growing nodule roots developed from each nodule. Nodules in this symbiotic nitrogen-fixing plant resulted from an infection caused by an unidentified actinomycete-like soil microorganism. Anatomical analysis of nodule formation showed that nodules are the result of repeated endogenous lateral root initiations, one placed upon another in a complexly branched and truncated root system. The endophyte-infected cortical tissues derived from successive root primordia form the swollen nodular mass. Nodule roots develop from nodule lobes after escaping from the initial inhibitory effects of the endophyte. Included is a discussion of the anatomical similarities between nodules of Casuarina which produce nodule roots and those of Alnus which form coralloid nodules usually lacking nodule roots. CASUARINA (Casuarinaceae) is one of the genera of non-leguminous angiosperms whose species are capable of forming root nodules in response to invasion from the soil by an actinomycete-like organism. The genus comprises some 45 species, most of them trees inhabiting the Old World tropics and Australia, but present as introductions in many other areas, including Florida and California. Evidence that the nodules are nitrogenfixing has been provided by Aldrich-Blake (1932), Mowry (1933) and Bond (1957a, b). The root nodules of Casuarina are of complex morphology, comparable to those of Alnus, Myrica and several other genera. From a single infection point in a root there can be formed eventually a spherical mass several cm in diameter composed of close-packed, radiating nodule lobes. In Casuarina, as well as in Myrica and Comptonia, fairly normal, uninfected roots ("noduleroots") emerge from the tips of the swollen noidule lobes. The initiation, development and structure of nodules on roots of nitrogen-fixing non-leguminous angiosperms have been reviewed by Bond

Dispersal ecology of carex pedunculata (cyperaceae), a new north american myrmecochore

Abstract: Carex pedunculata is the first North American species of the Cyperaceae that is identified as a myrmecochore. Many morphological and phenological features of this species and its breeding system are interpreted as adaptive for seed dispersal by ants. In laboratory tests, workers of the ant species Aphaenogaster rudis carry the diaspores to the nest, eat the elaiosomes, carry larvae to the elaiosomes to feed, and deposit diaspores whose elaiosomes have been eaten with other nest debris. The achenes then germinate. Achenes will also germinate without any handling by ants. Workers will also transport diaspores with uneaten elaiosomes when the nest is disturbed. Greenhouse tests show that seedling growth is greatly inhibited if a diaspore remains near the parent plant and cohort seedlings. Field studies of natural populations identify rotting logs (the location of ant nests) as forest floor microsites for colonization of C. pedunculata and other myrmecochores. Ant nesting behavior may pattern much of the herb stratum. This species is self-compatible, and single seeds may start successful new populations. Three processes contribute to population growth: vegetative growth, germination of untransported diaspores, and germination of ant-transported diaspores. THE GENUS CAREX has had a remarkable evolutionary radiation, with hundreds of species recognized in America (Mackenzie, 1935; Hermann, 1974) and Eurasia (Kuikenthal, 1909). Despite this, little is known about the distribution or ecology of most species, perhaps because the superficial vegetative similarities of these sedges make identification difficult. As part of a study of the comparative popula

Morphology and anatomy of floral and extrafloral nectaries in campsis (bignoniaceae)

Abstract: The genus Campsis (Bignoniaceae), with one New World and one Old World species, is unusual among temperate plants in having five distinct nectary sites. Multiple nectaries occur at all four of the extrafloral sites (petiole, calyx, corolla, fruit), representing an advanced strategy for ant attraction. The morphology and anatomy of the extrafloral nectaries in both species are uniform for the petioles, calyces, and young fruits; those on the outer corolla lobes are of slightly different forms. The generalized structure consists of one layer of basal cells, and a oneto two-layered secretory cup. Because of their small size, there is no vascular tissue in them. The large, vascularized (phloem only) floral nectary is an annular structure subtend-

Allelopathy in the first stages of secondary succession on the piedmont of new jersey

Abstract: A B S T R A C T Ambrosia artemisiifolia (ragweed) is a dominant species in the first year of old field succession but rarely persists for more than two years. Ragweed and Raphanus raphanistrum (wild radish), also an early invader, failed to become re-established in plots cleared of second stage perennial vegetation (dominated by Aster pilosus), despite the large number of seeds of these primary invaders present in the soil. Edaphic experiments revealed that this pattern of succession was not due to mineral or physical properties of the soil. Field soil from the second stage of succession inhibited the growth and germination of ragweed and wild radish while soil from the first stage had no effect. Inhibitory volatile materials from ragweed were not detected. However, root exudate of ragweed, and shoot extracts of ragweed and aster inhibited the germination and growth of early invaders of abandoned fields. These results indicate that the vegetational change from the first to the second stage of succession may be mediated at least partially by an allelopathic response. Chromatography and bioassay techniques revealed the inhibitory compounds to be phenolic acids, including caffeic and chlorogenic acids. AMBROSIA ARTEMISIIFOLIA (ragweed) and Raphanus raphanistrum (wild radish) have been reported to be the pioneer invaders in old fields that have been abandoned after spring plowing on the piedmont of New Jersey (Bard, 1952; Fleet, 1972; Cocking, 1973). These authors also indicate that the plants in this first stage of succession remain dominant for the first year only and appear only sporadically after that. The investigators characterize the second stage of succession as being dominated by Aster pilosus (aster) with quite a diverse group of subordinate species, Hieracium pratense (hawkweed) being typical of these. The observations and the allelopathic effects described by a multitude of authors, es

Volatile growth inhibitors produced by salvia leucophylla: effects on seedling root tip ultrastructure

Abstract: Volatile monoterpenes emanating from leaves of Salvia leucophylla (Labiatae) are responsible for anatomical and physiological changes occurring in herb seedlings which were exposed to the vapors Examination of treated root tip cells from Cucumis sativus by electron microscopy revealed widespread systemic disturbances These included the accumulation of globules which appear to be lipid in nature in the cytoplasm, a drastic reduction in the number of a variety of intact organelles including mitochondria, and a disruption of membranes surrounding nuclei, mitochondria, and dictyosomes These effects, coupled with the effects of environmental stress, may account for the failure of herb seedlings to survive in the areas adjacent to Salvia thickets

The isolation and culture in vitro of the quiescent center of zea mays

Abstract: A B S T R A C T Using 3-day-old seedling roots of Zea mays L., cv. Kelvedon 33, it was possible to remove the root cap by a simple surgical manipulation without damage to the root proper. By a further small cut, the quiescent center (QC) itself was isolated. This double-convex lens-shaped tissue piece 100 X 250,um is composed of 1000-1500 cells representing only 0.25 mm3 in volume. The explant was demonstrated unequivocally by 3H-thymidine incorporation before excision and then by autoradiography to be composed of the specific cells usually designated the quiescent center. Using sterile techniques, the QC's were placed on nutrient agar slants and allowed to grow in culture. Of a number of nutrient media tested, only a medium supplemented with organic nitrogen components, ind6leacetic acid, kinetin and inorganic nutrients plus sucrose (S2M + K -2,4-D) was effective in eliciting development. Thirty to 40 percent of the 150 isolated QC's grown on this medium formed elongated roots, up to 2 cm in length in 3-4 weeks. Roots developing on agar medium showed in their proximal portion a vascular pattern with 5-6 metaxylem elements or variations of this pattern, but as the root elongated, the vascular pattern was progressively reduced in complexity at the more distal end to a small central group of metaxylem elements. When agar-grown roots were transferred after one week in culture to a liquid nutrient medium of the same composition, the initially reduced vascular pattern evident in the proximal tissues became progressively more complex in the distal portion of the root and after 2 cm of elongation, showed an essentially normal primary vascular tissue pattern characteristic of the seedling root.

Cytokinin- and gibberellic acid-induced effects on the structure and metabolism of shoot apical meristems in opuntia polyacantha (cactaceae)

Abstract: The dormant axillary buds of Opluntia polyacanithla can be activated by either cytokinins or gibberellic acid. Under the influence of benzylaminopurine (BAP), the axillary bud meristem increases greatly in size and becomes mitotically active. The primordia produced by the meristem develop as normal photosynthetic leaves. Gibberellic acid (GA) also causes the meristem to become mitotically active, but the meristem does not increase in size. The primordia produced under the influence of GA develop as normal cactus spines. Leaf-producing meristems and spine-producing meristems have the same zonation, despite the differences in size. The meristems are composed of a uniseriate tunica, a central mother cell zone, peripheral zone, and a pith rib meristem. The mitotic activity of each of the zones in the leaf-producing meristem differs significantly from the mitotic activity of the corresponding zones in the spineproducing meristeni.

The pollination biology of tilia

Abstract: A B S T R A C T An anthecological study of Tilia in the Great Plains and New England was undertaken for both native and introduced species. The floral bracts are postulated as being at least as important in pollinator attraction as they are in fruit dispersal. A characteristic sweet odor is always present, but becomes stronger at dusk. Flowers are protandrous with anthers opening for the first time late in the afternoon and releasing pollen abundantly for about 24 hr. Initial nectar production coincides with stigma receptivity which begins late in the afternoon, but on the second day a flower is open. Sixty-six species of insects in 29 families were collected. Bees and flies are the most common diurnal visitors and moths are the primary nocturnal visitors. Samples of pollen taken from the insects indicate a relatively high constancy. Experimental tests show that Tilia is not apomictic or self compatible. Anemophily plays a secondary role to entomophily in pollination. Nocturnal pollinators are slighly less effective than diurnal pollinators in effecting fruit set. Although nocturnal pollinators are favored by the syndrome of floral characteristics, it is concluded that Tilia is a generalist in regard to pollination. It is proposed that the lack of clear morphological differences between Tilia species leads to pol

Morphology, fine structure and ontogeny of the stinging emergence of tragia ramosa and t. saxicola (euphorbiaceae)

Abstract: A B S T R A C T The ontogeny and ultrastructure of Tragia ramosa and T. saxicola are described. The stinging emergence of T. ramosa and T. saxicola consists of a central stinging cell and three lateral cells. The stinging cell possesses a compound crystal in the apical region which is held in place by cell wall extensions. The stinging cell cytoplasm is characterized by a large central vacuole which contains a proteinaceous substance as determined histochemically. Upon contact, the stinging cell wall is pushed back over the crystal, exposing it to penetrate an individual. This stinging mechanism is unique among stinging emergences. The stinging cell is subepidermal in origin whereas the three lateral cells are epidermal in origin. The morphology, ultrastructure and ontogeny of the stinging emergence of T. ramosa and T. saxicola appear to

Racial evolution in Eleusine coracana ssp. coracana (Finger Millet)

Abstract: The study was based on 150 collections of domesticated E. coracana from diverse localities in Africa and India. Considerable variation in vegetative, floral and seed morphology was observed. Three ecogeographical races were identified: (1) an African highland race which is cultivated in the East African highlands, (2) a lowland race which is grown in the lowlands of Africa and south India and (3) an Indian race with its centre of distribution in north-east India. In addition to these basic races, an Indian highland type was identified. The African highland race is the most primitive and is the progenitor of the lowland race. The latter race was subsequently introduced to southern India where a secondary centre of diversity became established. The Indian race originated from the lowland race, while the north Indian highland type could be derived from either or both of the two basic races in India. The study indicated that natural selection has played a major role in the evolution of the crop