In this article we will discuss about:- 1. Characters of Asteraceae 2. Distribution of Asteraceae 3. Economic Importance 4. Affinities 5. Important Types.
Characters of Asteraceae:
Mostly herbs or shrubs or rarely trees; leaves alternate rarely opposite, exstipulate rarely stipulate; inflorescence capitulum or head surrounded by involucre of bracts; ray and disc florets, flower tubular or ligulate, flowers bi- or unisexual or outer male or female, pentamerous, actinomorphic or zygomorphic, caryxpappus, corolla gamopetalous, petal lobes 5, stamens 5, epipetalous, usually dithecous, filament free and anthers united i.e. syngenesious, introrse, ovary unilocular, inferior, with basal placentation, style slender stigma bifid; fruit cypsela.
A. Vegetative characters:
Herbs (Ageratum, Lactuca, Dahlia, Sonchus), shrubs (Inula, Senecio) rarely trees (Vernonia arborea and Leucomeris). Many of the plants are xerophytes (Proustia), hydrophytes (Cotula) some are semiaquatic (Caesulia axilaris).
Tap root, sometimes modified into tubers (Dahlia).
Erect, or prostrate, herbaceous or woody (Artemisia), hairy, sometimes with latex. Stem tubers are also present (Helianthus); tubers are edible (H. tuberosus); cylindrical; glabrous, solid or fistular, stem may be leaf-like (Baccharis).
Alternate rarely opposite (Zinnia, Dahlia) or whorled; leaves may be radical, petiolate or sessile, exstipulate, mostly simple sometimes scale-like (Senecio), unicostate or multicostate reticulate venation.
B. Floral characters:
A head or capitulum, consisting of a few or large number of flowers or florets closely arranged on an axis surrounded by involucral bracts. The whole head or capitulum is apparently similar to a single flower because the involucral bracts perform the function of protection.
In Helianthus the outer or peripheral, ligulate and zygomorphic florets are called ray-florets; whereas inner or central, tubular and actinomorphic ones are called disc-florets.
In capitulum or head the form of flowers and distribution of sex also varies.
On the basis of form of flowers the heads are of three types:
1. Heterogamous or radiate heads. The outer or ray-florets are ligulate and zygomorphic and inner or disc-florets tubular and actinomorphic e.g. Helianthus.
2. Homogamous-rayed or ligulate heads. All the flowers in the head are ligulate, zygomorphic and alike; e.g. Sonchus.
3. Homogamous-non-rayed or discoid heads. All the flowers are tubular, actinomorphic and alike, e.g., Ageratum.
Distribution of sex:
The flowers of a head may be all hermaphrodite (Ageratum), or ray-florets are female or neuter and inner ones hermaphrodite, or male; rarely the complete head bears unisexual flowers.
Bracteate, sessile, (Sonchus, Ageratum), complete or incomplete, hermaphrodite or unisexual, pentamerous, tubular (actinomorphic) or ligulate (zygomorphic), epigynous and inconspicious.
Zygomorphic, ligulate, pistillate, or neuter or sometimes also bisexual, epigynous.
Modified into pappus or absent or scale-like.
Petals 5, gamopetalous, highly coloured, ligulate, strap-shaped, valvate.
Either absent or if present then bicarpellary, syncarpous, inferior, unilocular with basal placentation, one anatropous ovule; style one; stigma bifid.
Absent; if present cypsela.
Bracteate, sessile, complete, hermaphrodite, actinomorphic, pentamerous, epigynous and tubular.
Modified into pappus or scale, persistent.
Petals 5, gamopetalous, tubular, coloured.
Stamens 5, epipetalous, syngenesious, dithecous, introrse, dehiscing longitudinally.
Bicarpellary, syncarpous, inferior, unilocular with single anatropous ovule, basal placentation; style simple, long, stigma bifid.
Distribution of Asteraceae:
The family is commonly known as Sunflower family. It is the largest family of dicotyledons, comprising 950 genera and 20,000 species, out of which 697 species occur in India. They are world wide in distribution and abundant in tropics and in cold arctic or alpines regions.
Economic Importance of Asteraceae:
Leaves of Lactuca sativa are used as salad. The roots of Helianthus tuberosus are edible.
The seeds of Helianthus and Artemisia yield oil.
Solidago used in dropsy. Artemisia yields santonin which is used as vermifuge. The roots of Taraxacum used in bowel disorders. The juice of Emillia sonchifolia leaves has cooling effect and is used in eye inflammation and also for night blindness. Eclipta alba used as tonic in spleen enlargement. Centipeda orbicularis is used in cold and toothache.
It is obtained from Solidago laevenworthii and Taraxacum.
The capitula of Chrysanthemum roseum and C. cinerriefolium are dried, powdered and used as insecticide.
Zinnia, Dahila, Cosmos, Chrysanthemum, Calendula, Helichrysium, Aster Helianthus etc. are well known garden plants.
Xanthium, Blumea, Sonchus, Vernonia are the common weeds.
1. Some plants are woody and perennial.
2. Leaves alternate and simple.
3. Capitulum of only actinomorphic, hermaphrodite flower in some genera viz. Vernonia, Ageratum, Mikania.
4. Ovules anatropous.
5. Pollination by insects.
The family Asteraceae (Compositae) is regarded as the most advanced and highly evolved and is considered to occupy the highest position in the plant kingdom.
1. It includes maximum number of genera (950) and species (20000).
2. The members of this family are worldwide in distribution.
3. Plants mostly herbaceous annuals, biennials or perennials.
4. Leaves exstipulate, opposite or whorled.
5. Floral buds are well protected by involucral bracts.
6. Flowers sessile, small, inconspicuous.
7. Flowers arranged to form capitulum inflorescence.
8. Flowers epigynous and in many species zygomorphic.
9. Calyx reduced to pappus or scales.
10. Corolla gamopetalous and tubular.
11. Reduction in the number of stamens.
12. Stamens epipetalous and syngenesious.
13. Gynoecium bicarpellary, syncarpous, inferior, unilocular.
14. Single ovule.
15. Basal placentation.
16. Fruit simple, in some mechanism of wind dispersal (parachute).
17. Some plants are wind pollinated.
18. Seed non-endospermic.
19. Due to small flowers much of the material is saved.
Besides these morphological features and specialization there are also anatomical characters viz.:
1. Homogeneous medullary rays.
2. Small vessels and fibres.
3. Fibres with simple pits.
4. Presence of intraxylary phloem, medullary and cortical bundles and anamolous secondary growth.
These suggest that the Asteraceae is the most highly evolved and most successful family in the plant kingdom.
Affinities of Asteraceae:
Taxonomists have assigned different systematic position to the family. Bentham and Hooker placed the family under Gamopetalae just after Rubiales. Hutchinson did not treat the family as the last evolved and put the Asterales under the 6th series far before the last series Laminales; but he placed it near Rubiaceae.
Engler and Rendle placed it as the highest evolved dicot family. If polyphyletic origin of the family is taken into account it may be assumed that the origin of Rubiales and Asterales is from Apiaceae (Umbelliferae), which shows a tendency of aggregation of flowers and reduction of calyx lobes, carpels and ovules.
Takhtajan traces the origin of Campanulales, Calycerales and Asterales from the order Gentianales. He also relates Asteraceae with Calyceraceae and other families of the other Campanulales.
Division of the family and chief genera:
The family Asteraceae is divided into 2 sub-families and 12 tribes:
1. Sub-family. Asteroideae:
Flowers disc not ligulate, latex rare. This sub-family is subdivided into 11 tribes viz., Heliantheae, Astereae, Anthemideae, Arctotideae, Inuleae, Seneciodeae, Calenduleae, Eupatorieae, Vernonieae, Cynareae, Mutisieae with Helianthus, Aster, Artemisia Arctotis, Inula, Senecio, Calendula, Ageratum, Vernonia, Cynara, Mutisia as the genera of the tribes respectively.
2. Sub-family. Lactucoideae (Liguliflorae):
All flowers ligulate latex present; with single tribe. Tribe 12. Lactuceae – Lactuca, Sonchus.
Important Types of Asteraceae:
1. Helianthus annuus (Fig. 68.1):
Tap, root branched.
Erect, herbaceous above and woody below, cylindrical, solid, branched, glabrous ordinary.
Ramal and cauline, alternate, simple, petiolate, exstipulate, ovate, serrate, acute, surface hispid, unicostate reticulate venation.
Capitulum surrounded by involucre.
Capitula large with two kinds of flowers:
(a) The peripheral flowers or ray florets, which are large, attractive and ligulate.
(b) Disc florets, in the centre and tubular.
Bracteate, sessile, incomplete, zygomorphic, ligulate, pistillate or neuter, epigynous.
Petals 5, gamopetalous, a short basal tube and a large flat strap shaped limb, with 5 teeth (sometimes lesser) indicating the number of petals.
Absent, or if present then bicarpellary, syncarpous, inferior, unilocular, basal placentation, simple style, bifid stigma.
Bracteate, sessile, complete, hermaphrodite, tubular, actinomorphic, pentamerous, epigynous.
Pappus or reduced, modified into 2-3 scales, persistent.
5, gamopetalous, tubular, 5 toothed, teeth represents the number of petals, valvate.
Stamens five, epipetalous, filaments free, short, alternating with the petals, anthers syngenesious, basifixed, dithecous, introrse.
Bicarpellary, syncarpous, unilocular, inferior, basal placentation, single basal ovule, style single long; stigma bifid.
2. Sonchus asper:
An annual weed.
Tap, branched, and annual.
Herbaceous, erect, fistular, milky, branched, green, rough and angular.
Cauline, alternate, simple, lyrate-irregular shape, dentate spinous margin, unicostate reticulate.
Capitulum crowded in sub-corymbose umbellate panicles; capitulum of homogamous ray florets only.
Bracteate, sessile, complete, hermaphrodite, homogenous, all ligulate, zygomorphic, pentamerous, epigynous, small.
Five toothed, gamopetalous, ligulate, strap-shaped, superior, valvate, yellow.
Stamens 5, epipetalous, syngenesious, dithecous, sagittate, basifixed introrse, superior.
Bicarpellary, syncarpous, inferior, unilocular, basal placentation, ovule single, style single, stigma bifid.
3. Launea asplenifolia (Fig. 68.2):
A perennial herb, grows in rosettes.
Herbaceous, reduced but trailing, cylindrical, solid, glabrous.
Radical, alternate, sessile, simple, exstipulate, margin spinulose toothed, unicostate reticulate venation.
Head of homogamous ligulate flowers.
Bracteate, sessile, complete, hermaphrodite, zygomorphic, pentamerous, epigynous, all ligulate, yellow.
Represented by pappus i.e., tuft of hairs, superior.
Petals 5, gamopetalous, ligulate, valvate, superior, yellow.
Stamens 5, epipetalous, syngenesious, basifixed, dithecous, introrse, superior.
Bicarpellary, syncarpous, inferior, unilocular, basal placentation, single basal ovule; style simple, stigma bifid.
4. Ageratum conyzoides:
An annual weed.
Erect, herbaceous, cylindrical, solid, branched, hairy green.
Alternate above and opposite below, simple, petiolate, exstipulate, ovate, dentate margin, acute apex, unicostate reticulate venation.
Capitulum; of homogamous tubular flowers.
Bracteate, sessile, complete, hermaphrodite, actinomorphic, homogamous, tubular, pentamerous, epigynous, small, violet.
Petals 5, gamopetalous, tubular, valvate, violet, superior.
Stamens 5, epipetalous, syngenesious, basifixed, dithecous, introrse, superior.
Bicarpellary, syncarpous, inferior, unilocular, basal placentation, single ovule, style simple, stigma bifid and hairy.