Angiosperms: Classification and Features

In this article we will discuss about the Angiosperms:- 1. Systems of Classification of Angiosperms 2. Features Used to Describe an Angiospermic Plant.

Systems of Classification of Angiosperms:

Different taxonomists have proposed three different types of systems of classification, i.e., artificial, natural and phylogenetic.

In the artificial system of classification, only one or few characters were taken into consideration. The drawback of this system is that very closely related genera were placed in different groups. The best known artificial system is that of Linnaeus (1707-1778).

In the natural system of classification, all the available important characters are taken into consideration. Plants are classified according to their related characters.

Well known and widely accepted natural system of classification of angiosperms was given by two English botanists, George Bentham (1800-1884) and Joseph Dalton Hooker (1817-1911). Their system was published in the form of three volumes in Genera Plantarum in Latin language.

In the phylogenetic system of classification, the plants are classified according to their evolutionary and genetic relationship. Some of the well-known phylogenetic systems of classification are proposed by Engler (1886) a German botanist, Hutchinson (1920) an English botanist, and Tippo (1942) an American botanist.

Radford (1986) has mentioned the existence of four types of systems of plant classification, i.e., artificial classifications, mechanical classifications, natural classifications and phylogenetic classifications, and separated them as follows:

According to Lawrence, all classification systems can be tabulated into four different periods as under:

The natural system proposed by Bentham and Hooker (1862-1883) is most convenient and most suitable for the practical purposes, and the same has been followed in the present book.

An outline of the classification, proposed by Bentham and Hooker, with few points of each class and important orders, is given in the annexed chart. In a separate chart, an outline of classification by Armen Takhtajan (1969) is given.

Features Used to Describe an Angiospermic Plant:

Following is the scheme which should be used for the description and identification of a flowering plant:

Habitat:

Natural living place of the plant should be described. (This can be described only in the natural surroundings of the plant and not in the laboratory).

Habit:

1. Annual, biennial or perennial.

2. Herb (whether erect, prostrate, trailing, twining or climbing), undershrub, shrub (erect, straggling or climbing) or tree.

3. Any other peculiar habit such as epiphyte, saprophyte, parasite, etc.

Root:

1. Tap root or adventitious root.

2. Branched or unbranched.

3. Any other special form such as fusiform, napiform, conical, tuberous, nodulose, annulated, moniliform, etc.

Stem:

1. Herbaceous or woody.

2. Erect, prostrate, climbing or twining. In case of a climber, note the means of climbing, i.e. tendril, hook, spine or any other outgrowth.

3. Cylindrical or angular (note the number of angles in latter case).

4. Branched or unbranched (note the mode of branching).

5. Hollow or solid, jointed or unjointed.

6. Smooth, waxy, hairy or spiny.

7. Colour of the stem.

8. Any special modifications such as rhizome, tuber, bulb, corm, runner, sucker, stolon, offset, phylloclade, cladode, etc.

Leaf:

1. Evergreen or deciduous.

2. Cauline, ramal or radical.

3. Phyllotaxy:

Whether alternate, opposite (decussate or superposed) or whorled. In case of alternate phyllotaxy whether 1/2 (2-ranked or distichous), 1/3 (3-ranked or tristichous) or 2/5 (5- ranked or pentastichous).

4. Stipulate or exstipulate. If stipulate, then mention about the type of stipules (i.e., scaly, free-lateral, adnate, interpetiolar, ochreate, foliaceous or spinuous).

5. Petiolate or sessile.

6. Simple or compound. In case of compound leaf, mention whether pinnately compound or palmately compound. In case of pinnately compound leaf, mention whether unipinnate (paripinnately compound or imparipinnately compound), bipinnate, tripinnate or decompound. In case of palmately compound leaf, mention abount uni-, bi-, tri-, quadri- or multifoliate.

7. Then mention abount the shape, margin, apex, surface and venation of the leaf one after the other.

Various types of shape of leaves:

Acicular, Linear, Lanceolate, Elliptical, Oval, Ovate, Obovate, Oblong, Rotund, Cordate, Reniform, Oblique, Spathulate, Sagittate, Hastate, Cuneate, Falcate, Deltoid, Lyrate and Pedate.

Various types of leaf margins:

Entire, Repand, Sinuate, Serrate, Biserrate, Dentate, Runcinate, Crenate, Fimbricate, Ciliate and Spinuous.

Various types of leaf apices:

Obtuse, Acute, Acuminate, Cuspidate, Truncate, Retuse, Emarginate, Mucronate and Cirrhose.

Various types of leaf surfaces:

Glabrous, Rough, Glutinose, Glaucous, Spiny and Hairy.

Various kinds of venation in leaves:

Reticulate and parallel. After differentiating between the two, then mention whether unicostate or multicostate, and whether divergent or convergent in case of multicostate.

Inflorescence:

1. Mention whether racemose, cymose, mixed, compound or any other special form.

2. If racemose, then mention whether raceme, spike, spikelet, catkin, spadix, corymb, umbel or head.

3. If the inflorescence is of special type, mention whether cyathium, verticillaster or hypanthodium.

Flower:

1. Bracteate or ebracteate.

2. Bracteolate or without bracteoles.

3. Pedicellate or sessile.

4. Complete or incomplete.

5. Unisexual or hermaphrodite.

6. Di-, tri-, tetra-, or pentamerous.

7. Hypogynous, perigynous or epigynous.

8. Colour of the flower.

Calyx:

1. Number of sepals.

2. Polysepalous or gamosepalous or modified into pappus.

3. Aestivation, i.e., whether valvate, twisted, imbricate, quincuncial or vexillary.

4. Colour of sepals, i.e., whether green or petalloid.

5. Caducous or persistant.

6. Shape and size of the sepals.

Corolla:

1. Number of petals.

2. Polypetalous or gamopetalous.

3. Aestivation as in case of calyx.

4. Superior or inferior.

5. Shape and size of the petals. In the shape, mention whether cruciform, caryophyllaceous, rosaceous, campanulate, tubular, funnel-shaped, rotate, papilionaceous, bilabiate, ligulate or personate.

6. Appendages of corolla such as corona, nectary or spur.

Perianth:

Perianth is present in monocots, and can be described in the same way as calxy and corolla. In place of polysepalous and polypetalous only the term polyphyllous is used, and in place of gamosepalous and gamopetalous only gamophyllous is used.

Androecium:

1. Number of stamens. Write “indefinite” if more than ten.

2. Count the number of whorls and see whether some stamens are reduced into staminodes or not.

3. Free or united.

4. Nature of cohesion, i.e., monadelphous, diadelphous, polyadelphous, syngenesious or synandrous.

5. Nature of adhesion, i.e., whether epipetalous, gynandrous or any other special character.

6. Whether alternate with the petals or obdiplostamenous (opposite).

7. Inserted or exerted.

8. Nature of the filament, whether long, short or flattened. Note the didynamous or tetradynamous condition also.

9. Monothecous or dithecous.

10. Attachment of the anthers, i.e., whether basifixed, adnate, dorsifixed or versatile.

11. Anthers introrse or extrorse.

12. Appendages of stamens, i.e., presence of hair, scales, staminal corona, etc.

Gynoecium:

1. Number of carpels.

2. Syncarpous or apocarpous.

3. Superior, semi-inferior or inferior ovary.

4. Number of locules.

5. Number of ovules in each locule.

6. Type of placentation i.e., whether marginal, axile, parietal, free-central, basal or superficial.

7. Number of styles.

8. Number, shape and any modification of stigma.

9. Presence or absence of nectar-secreting disc below the ovary.

Fruit:

Kind of fruit.

Seed:

1. Number of seeds in fruit.

2. Endospermic or non-endospermic.

3. Number of cotyledons.

Floral Formula:

Different parts of the flower are represented by different symbols which form a formula called floral formula.

Various parts are represented as follows:

Number of the floral parts:

= By different numbers as 1, 2, 3, 4, 5 and so on, e.g., K₅ means 5 sepals and polysepalous

Cohesion of whorl:

= Enclosing the number in the bracket, e.g., K(5) means 5 sepals are fused or gamosepalous.

Adhesion of the floral parts:

= Line drawn on the top of the two concerned whorls, e.g., C₅ A(α) indicates that many fused stamens are epipetalous.

Superior ovary:

= A line below gynoecium as G.

Inferior ovary:

= A line on the top of gynoecium as G̅ .

Different whorls of floral parts:

= Different figures separately, e.g., C₅₊₅ means 10 petals are arranged in two whorls of 5 each.

Absence of a parts = O.

Indefinite number of a part = α

Floral Diagram:

After carefully drawing the other necessary diagrams (like a part of the plant, structure of the flower, L.S. of the flower, a stamen, gynoecium, and T.S. of the ovary) of the given plant, the most essential and most important diagram is the floral diagram.

It is actually an ideal ground plan of a flower which represents all floral parts in a perfect manner. It expresses the number, fusion, symmetry and other similar characteristics of the floral parts in a flower.

Different floral parts in a flower are expressed in a circular manner. In the different concentric circles of a floral diagram the sepals are drawn in the outermost circle. The sepals are followed by petals, stamens and carpels towards inner sides, respectively. Gamosepalous or polysepalous condition is made by joining the sepals or making them free. Same is the case with petals.

Position of sepals and petals are drawn in the respective circles corresponding to their actual position in section. A small circle above the floral diagram represents the mother axis. In zygomorphic flower the mother axis is shown as Φ while in actinomorphic flower it is drawn as . The bract, if present, is drawn below the floral diagram while the bracteoles are shown on the sides.

For drawing the floral diagram note whether a sepal or space between two sepals stands towards mother axis. Start from this particular sepal and mark the position of other sepals. Petals are drawn alternate to sepals. In actinomorphic flowers all sepals and petals are drawn of same size. But in case of zygomorphic flowers unequal sized sepals or petals are drawn. Spur in a sepal or petal is drawn in the form of a loop.

In case of epipetalous condition of stamens join the stamens to petals with a line. Stamens are shown by transverse section of anthers. In obdiplostamenous condition the stamens of outer whorl are drawn opposite to petals.

Introrse stamens face towards gynoecium while extrorse towards petals. Use a cross (x) or asterisk (*) in place of a staminode, if present. The gynoecium is drawn in the form of transverse section of the ovary.