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Asteraceae
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NCBI
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Fibers reinforce hydrostatic skeletons: sunflowers
Hydrostatic structures found in sunflowers and other many other organisms serve various functions but almost always use helical fibers as reinforcement.
"With few exceptions, nature uses the second arrangement of fibers for her internally pressurized, water-filled cylinders. These structures (often termed 'hydrostatic skeletons' or 'hydroskeletons' as well as 'hydrostats') have helical reinforcing fibers. And this particular arrangement is no rare or once-evolved thing. It occurs in the stems of young herbaceous (nonwoody) plants such as sunflowers; it provides a wrapping for flatworms (platyhelminths and nemerteans), roundworms (nematodes), and segmented worms (annelids); it stiffens the body wall of sea anemones; it determines the response to muscle contraction of the outer mantle of squids; and it's a major functional component of shark skin. The material of the fibers varies widely, the functions of these hydroskeletons are even more diverse, but the wrapping is almost always helical." (Vogel 2003: 409)
Learn more about this functional adaptation.
- Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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Herbs, suffrutices, shrubs or (rarely) climbers or trees. Leaves alternate, less often opposite. Stipules 0, but false stipules occur in a few species (for example: Vernonia myriantha and Senecio deltoideus) . Flowers small (florets), aggregated into heads (capitula) and simulating single larger flowers and surrounded by a calyx-like involucre of one or more series of bracts (phyllaries). Receptacle of the head expanded, with or without receptacular scales or bristles each subtending a floret. Florets all similar sexually (head homogamous) or central and marginal florets differing (head heterogamous) and then the central florets usually bisexual or rarely male, the outer female or rarely neuter. Calyx never typically herbaceous but represented by a pappus of numerous simple or feathery (plumose) hairs, or a smaller number of membranous scales, teeth or bristles, or by a continuous membranous ring; sometimes 0.
Corolla composed of (3-)5 united petals fused into a tube below and with a distal limb consisting of either: (1) 5 actinomorphic lobes or teeth (tubular florets); or (2) a unilateral strap-shaped limb (ray) 0-3(-4)-dentate at the apex; or (3) a unilateral strap-shaped limb (ligule), 5-dentate at the apex.
Stamens 5, borne on the corolla-tube; anthers usually fused into a cylinder around the style. Ovary inferior, 1-celled, with 1 basal ovule; style single below but branching above into 2 stigmatic arms. Fruit an achene, crowned by the pappus, sometimes with a slender beak interposed between them.
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Rights holder/Author | Mark Hyde, Bart Wursten, Petra Ballings, Flora of Zimbabwe |
Source | http://www.zimbabweflora.co.zw/speciesdata/family.php?family_id=26 |
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The fossil record of Asteraceae is for the most part composed of pollen deposits and fruits. Few pollen records exist for the Eocene but the pollen of Asteraceae becomes increasingly common in samples dated to the Oligocene/Miocene (Graham, 1996). These data show the increasing importance of the family in most biomes of the world from the mid to late Oligocene to present. Pollen samples from Paleocene-Eocene deposits in southwestern Africa have been attributed to Mutisieae (Zavada and De Villiers, 2000; De Villiers and Cadman, 2001) or to a Dicoma-like taxon and recently dated to the mid-Eocene (Scott et al., 2006). Asteraceae pollen dated to the Eocene has also been found in Egypt (Kedves, 1971), China (Song et al., 1999) and North America (Texas Gulf coast, Elsik and Yancey, 2000).
Fossil pollen records have been used recently to shed light on the time of origin of Asteraceae. Dated and properly identified, pollen deposits can be used to place a minimum age for the origin of a particular clade. Using the rate of mutation of the gene rbcL and a fossil calibration, Bremer and Gustafsson (1997) concluded the family to have originated at least 38 Ma. A similar approach using the ndhF and rbcL genes was used by Kim et al. (2005) to date the origin of molecular rearrangements in the chloroplast genome of the Asteraceae. Their study concluded that the family originated in the mid Eocene (42-47 Ma).
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Rights holder/Author | Jose L. Panero, Bonnie S. Crozier, Tree of Life web project |
Source | http://tolweb.org/Asteraceae/20780 |
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The initial classification of the family was produced by Cassini (1819a,b) who grouped genera into tribes. His tribal concepts have been refined by taxonomists subsequently and are still the main category above the genus level used to classify sunflowers. The grouping of these tribes into subfamilies is a relatively recent academic pursuit in Asteraceae systematics that began with Carlquist (1976) and Wagenitz (1976) defining the concepts of subfamilies Cichorioideae and Asteroideae. The morphological characteristics used to circumscribe these two groups are mostly based on discontinuities in corolla, anther, and style morphology. According to Bremer (1994), the Asteroideae are characterized (with some exceptions) by having true ray florets, disc corollas with short lobes, caveate pollen, stigmatic surfaces of style branches separated into two marginal lines sometimes confluent at apices, and a distinctive secondary chemistry. These morphological characteristics are rarely seen in Cichorioideae.
With the incorporation of results from molecular phylogenetic studies the classification of the Asteraceae has changed relatively quickly, mainly through the recognition of monophyletic groups traditionally included in Cichorioideae. With the work of Jansen and Palmer (1987), Bremer (1994) recognized three subfamilies (Asteroideae, Barnadesioideae, and Cichorioideae) and 17 tribes. Thorne and Reveal (2007) recognized the same subfamily groups using the possibly earlier name Carduoideae for the Cichorioideae. They expanded the number of tribes to 25, recognizing the three new tribes identified by molecular analysis named in Baldwin et al. (2002) but maintained a polypheletic Heliantheae. Jeffrey (2007) accepted the tribal groupings of Panero and Funk (2002), ultimately recognizing 24 tribes and grouping them into five subfamilies. He recognized a monophyletic Barnadesioideae, a monophyletic Asteroideae, and split the grade of clades between these two groups (Cichorioideae of Bremer (1994) or Carduoideae of Thorne and Reveal (2007)) into 3 subfamilies: Mutisioideae, Carduoideae, and Cichorioideae, each shown to be polyphyletic (Panero and Funk, 2002, 2008). As the capacity to expand both taxon and character sampling in molecular studies has grown, providing more resolution and certainty in phylogenetic analyses, more major lineages of the family have been identified. Increased sampling, particularly sampling of taxa considered anomalous in the family (uncertain tribal position, Bremer, 1994), resulted in the discovery that the genera Corymbium, Gymnarrhena, and Hecastocleis represent monotypic lineages sister to major clades and provided evidence for more lineages than previously recognized. The classification of Panero and Funk (2008), shown in the tree above, recognizes 12 strictly monophyletic subfamilies.
The classification of the Asteraceae is dominated by the large subfamily Asteroideae that contains more than 70% of the species of the family. The three main lineages within Asteroideae found by molecular studies (Kim and Jansen, 1995, Panero and Funk, 2008) have been recognized at the supertribe level recently (Robinson, 2004, 2005) as Asterodae, Helianthodae, and Senecionodae. Other large subfamilies include Carduoideae and Cichorioideae each with more than 2000 species. All other subfamilies each contain less than 1000 species with Gymnarrhenoideae and Hecastocleidoideae containing only one species each.
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Rights holder/Author | Jose L. Panero, Bonnie S. Crozier, Tree of Life web project |
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The family is characterized by having a capitulum or head, an inferior, unilocular ovary with one ovule, and with few exceptions fused anthers surrounding the style. The capitulum (capitula plural) is a specialized indeterminate inflorescence that can contain 1 to hundreds of individual flowers (florets). The flowering sequence in the capitulum is nearly always from the outside to the center, that is, centripetal (Harris, 1995). The florets sit on the disc or receptacle, an expanded shoot that can be flat, concave, convex, or rarely columnar. The disc and florets are surrounded by bracts or leaf-like structures called phyllaries and collectively forming an involucre. The phyllaries can be arranged in one row and be of equivalent length or can be unequal in length. Most sunflowers have involucres with several series of phyllaries. In these sunflowers the phyllaries can be subequal with all phyllaries of equivalent length or imbricate. Involucres composed of imbricate phyllaries are the most common condition in the family and exemplified by the artichoke (Cynara cardunculus) in which multiple series of phyllaries overlap each other. In some species the outermost phyllaries sometimes resemble leaves. The receptacle can be naked or sometimes have bract-like structures called paleae, scales or hairs surrounding each floret. The involucre can have different shapes ranging from tubular to hemispheric.
There are six types of corollas present in the Asteraceae two of which are actinomorphic and the other four are zygomorphic (Bremer, 1994). Actinomorphic corollas are composed of five equivalent lobes and normally termed disc corollas (as they occupy most of the disc area). They have five lobes and when viewed from above, the reflexed lobes of the actinomorphic corolla resemble a five point star. Disc corollas may have fewer lobes with four being the most common departure, although corollas with three lobes are also seen. Tubular corollas are narrow actinomorphic corollas, mostly lacking stamens. Zygomorphic corollas are mostly confined to the first row of florets in the capitulum, although some species may have several rows of zygomorphic corollas. Bilabiate corollas are generally present only in several genera belonging to the earliest divergences of the family. The bilabiate corolla has a 3+2 arrangement of lobes with the 3-lobe lamina facing towards the outside and the 2-lobe lamina to the center of the capitulum. Sometimes the 2 lobes are separate and coiled. The pseudobilabiate corolla has a 4+1 arrangement. The ray floret is present in several tribes of the subfamilies Cichorioideae and Asteroideae and consists of a lamina that terminates in 2-3 lobes. Some members of tribe Arctotideae have a ray floret that terminates in 4 lobes. Ligulate corollas have 5 lobes.
The capitula of sunflowers can contain florets with corollas of the same morphology or a combination of two or sometimes three types of corollas. In discoid capitula all florets have actinomorphic corollas and can be either bisexual and fertile or functionally staminate or pistillate. In radiate capitula the peripheral florets are ray florets and these can be pistillate or styliferous and sterile or neuter (no style present). Radiant capitula are discoid capitula with peripheral corollas having lobes variously expanded. In ligulate or liguliflorous capitula all the florets are bisexual and have ligulate corollas; this capitulum type is only found in members of tribe Cichorieae. Disciform capitula have florets with actinomorphic corollas with peripheral florets having tubular corollas. These peripheral corollas are pistillate. Homogamous capitula have florets exhibiting similar sexual forms whereas heterogamous capitula have florets with two or more sexual forms.
Most sunflower florets have five anthers corresponding to the number of lobes in the corolla. The anthers are positioned along the sinuses of the corolla lobes (alternate to the lobes). The anther filaments are free from the corolla just above the tube and the two thecae (pollen sacs) of each stamen are connate with the thecae of adjacent stamens producing a tube that surrounds the style. Pollen is shed to the interior of this tube (introrse dehiscence). The connective, the tissue connecting the two anther thecae of each anther, may continue beyond the anther thecae and produce an appendage. Some sunflowers do not have appendages (e.g., many Asteroideae: Eupatorieae). The anther collar is located at the end of the filament just below the connective and is an area of sclerified cells involved in the mechanical aspects of pollen presentation in sunflowers. The anther collar is progressively shorter on the adaxial side facing the style. If the thecae extend below the point of insertion between the filament and connective, the anther is calcarate; if not, the anther is ecalcarate. Anthers can have tails, sometimes very elaborate with branched projections. Calcarate and caudate (tailed) anthers are common among the basal lineages of the family whereas ecalcarate anthers are more common in the Asteroideae.
The styles of sunflowers have two stigmatic branches or arms. They may be smooth (glabrous) or pubescent with trichomes confined mostly to the distal end of the style and the abaxial surfaces of the style branches. In fertile florets, the adaxial surface of the style branch has stigmatic (fertile) papillae. In some groups the stigmatic papillae are continuous throughout the style branches, whereas in others the stigmatic papillae are confined to the margins of the style branches. As the floret matures, the style grows through the tube formed by the fused anthers pushing the pollen up. This secondary pollen presentation mechanism ensures pollen is available to insects visiting the capitulum on a daily basis as new florets open. The pollination biology of sunflowers has not been extensively studied but an excellent summary of the available literature is provided by Lane (1996).
Sunflowers produce dry, indehiscent fruits termed cypselae (achenes in many publications). A few species have fleshy fruits reminiscent of a drupe (Chrysanthemoides, Tilesia). Most species have a pappus on the distal end of the cypsela. This structure is derived from the calyx of the floret (Carlquist, 1976). The pappus aids in dispersal or defense against herbivory (Stuessy and Garver, 1996).
For additional general information concerning the morphology of sunflowers the reader is advised to consult the introductory chapter for the family in Flora North America (Barkley et al., 2006).
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Rights holder/Author | Jose L. Panero, Bonnie S. Crozier, Tree of Life web project |
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Flickr: Asteraceae - worldwide ( I )
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