Chapter 5 — Morphology of Flowering Plants

The root is the descending, non-green, underground part of the plant axis that develops from the radicle of the embryo. Its main functions are absorption of water and minerals from the soil, anchorage of the plant, storage of reserve food and synthesis of plant growth regulators such as cytokinins.

There are two main root systems. In dicotyledonous plants such as mustard and gram, the radicle grows directly into a long, deep primary root or tap root, which bears lateral secondary and tertiary roots. The whole group is the tap root system. In monocotyledonous plants such as wheat, rice and grasses, the primary root is short-lived and is replaced by a cluster of thin roots arising from the base of the stem; this is the fibrous root system. In some plants roots arise from parts other than the radicle, for example from the stem of the banyan tree or the nodes of the grass Cynodon and monstera; such roots are adventitious roots.

A typical root, from tip to base, shows four regions.

• Root cap: a thimble-like cap of parenchyma cells that covers the delicate apex and protects it while the root pushes through the soil. It is continually worn away and replaced from the meristem.
• Region of meristematic activity (region of cell division): a few millimetres above the cap; its cells are small, thin-walled, with dense protoplasm and they divide repeatedly.
• Region of elongation: cells produced by the meristem enlarge and elongate rapidly, pushing the tip forward. This region is responsible for growth in length.
• Region of maturation (root hair zone): cells become differentiated. Some epidermal cells grow out as fine, thread-like root hairs, which are unicellular and greatly increase the surface area for absorption of water and minerals. Root hairs are short-lived and new ones form as the root grows.

In water hyacinth and other floating plants, the roots are green in colour and take part in photosynthesis. In parasites such as Cuscuta, roots are replaced by haustoria that draw food from the host. Root branches always arise from inside (endogenous origin, from the pericycle), whereas branches of the stem arise from the surface (exogenous origin); this is one of the important differences between root and stem. Roots also lack nodes, internodes, leaves and buds, and normally do not bear chlorophyll.

Besides absorbing water and anchoring the plant, roots in many species change their shape and structure to do additional work. Such a permanent change in form to perform a special function is called a modification. Root modifications are studied under three headings: storage, support and respiration, together with a few special types.

1. Storage roots. The tap root becomes swollen with stored food. In carrot the root is conical (broad at the top, tapering downwards); in radish it is fusiform (spindle-shaped, widest in the middle); in turnip it is napiform (almost spherical with a thin tail). Adventitious roots may also store food: the fasciculated roots of sweet potato and Asparagus occur in bunches, and in Dahlia the swollen tuberous roots hang in a cluster.

2. Roots for support. The prop roots of the banyan tree (Ficus benghalensis) hang down from horizontal branches, enter the soil and thicken into pillar-like supports for the heavy crown. Stilt roots of maize, sugarcane and screw pine (Pandanus) arise from the lower nodes of the stem, grow obliquely and enter the soil, giving extra support like the ropes of a tent. Climbing roots of Piper betel and money plant arise from nodes and cling to the support.

3. Roots for respiration. In mangrove plants such as Rhizophora, Avicennia and Sonneratia, which grow in waterlogged, salty, oxygen-poor mud, some roots grow vertically upwards into the air. These pneumatophores (respiratory roots) bear numerous minute pores called lenticels through which gaseous exchange takes place.

4. Other special roots. The root nodules of leguminous plants (gram, pea, groundnut) house the nitrogen-fixing bacterium Rhizobium. Epiphytic roots of orchids such as Vanda hang in the air and absorb moisture through a spongy tissue called velamen. Haustorial (sucking) roots of the parasite Cuscuta penetrate the host's tissues and draw nourishment. Assimilatory roots of Tinospora and Trapa are green and photosynthetic.

In every case, the underlying anatomy still shows the root's character: no nodes, endogenous branching and a root cap on the growing tips. Examination questions usually ask you to name the modification, give one plant example and state the function, so learn each of these as a triplet of name, example and purpose.

The stem is the ascending part of the plant axis that develops from the plumule of the embryo. It bears branches, leaves, flowers and fruits, and it is distinguished from the root by having nodes (points where leaves arise) and internodes (the portions between two nodes). Buds occur at the tip (terminal bud) and in the axils of leaves (axillary buds). A young stem is usually green and photosynthetic; later it becomes woody and dark brown. The main functions of the stem are spreading out branches bearing leaves, flowers and fruits, and conducting water, minerals and food between roots and leaves. Some stems store food, give support, protect the plant or reproduce it vegetatively.

Underground stem modifications store food and help the plant survive unfavourable seasons (perennation).

• Rhizome (ginger, turmeric, banana): a horizontal, branched, fleshy stem with distinct nodes, internodes, scale leaves and buds.
• Tuber (potato): the swollen tip of an underground branch; its 'eyes' are nodes bearing buds in the axils of scale leaves.
• Corm (Colocasia, saffron, Amorphophallus): a short, vertical, fleshy stem with nodes and scale leaves.
• Bulb (onion, garlic): a very short, disc-like stem bearing fleshy scale leaves which store food.

Sub-aerial modifications serve vegetative propagation. A runner (grasses, Oxalis) creeps on the ground and roots at nodes; a stolon (jasmine, mint) arches over and touches the ground; an offset (water hyacinth, Pistia) is a short, thick runner in aquatic plants; a sucker (Chrysanthemum, mint, banana) is an underground lateral branch that comes up as a new aerial shoot.

Aerial modifications:

• Stem tendrils (cucumber, pumpkin, watermelon, gourds, grapevine): slender, spirally coiled, from axillary buds; help the plant climb.
• Thorns (Citrus, Bougainvillea): axillary buds become hard, woody, pointed structures that protect against browsing animals.
• Phylloclade (Opuntia, Euphorbia): a flattened or fleshy green stem that photosynthesises, while the leaves are reduced to spines to cut water loss. A cladode is a one-internode phylloclade (Asparagus).
• Bulbils (Dioscorea, Agave): fleshy axillary buds that fall off and grow into new plants.

The stem of the plant Euphorbia deserves special attention because it is a common examination example of a phylloclade in a xerophyte.

The leaf is a lateral, generally flattened, green structure borne on the stem at a node, with a bud in its axil. It develops from the shoot apical meristem in acropetal order (youngest nearest the tip) and is the chief organ of photosynthesis. A typical leaf has three parts: the leaf base, the petiole and the lamina (blade).

The leaf base attaches the leaf to the stem and may bear two small lateral outgrowths called stipules. In monocots such as grasses the base expands into a sheath that partially or wholly covers the stem. In leguminous plants the base is swollen into a pulvinus, which controls leaf movement (as in the touch-me-not, Mimosa). The petiole holds the blade out to the light and lets it flutter in wind, cooling the leaf and bringing fresh air to the surface; a leaf without a petiole is sessile. The lamina is the green expanded part with veins and veinlets. A prominent middle vein, the midrib, runs through the centre. Veins give rigidity to the blade and act as channels for water, minerals and food. The shape, margin, apex, surface and extent of incision of the lamina vary widely and are used to identify plants.

Venation is the arrangement of veins and veinlets in the lamina. In reticulate venation the veinlets form a network, as in most dicots (peepal, hibiscus, mango). Reticulate venation may be pinnate (one midrib with lateral veins, as in mango) or palmate (several main veins from the base, as in castor). In parallel venation the veins run parallel to one another, as in most monocots (grasses, banana, canna). Parallel venation may be pinnate (banana) or palmate (grasses). There are exceptions: Smilax and Dioscorea are monocots with reticulate venation, while Calophyllum and Corymbium are dicots with parallel venation.

Leaves also carry stomata, tiny pores bounded by guard cells, mainly on the lower surface of dicot leaves and equally on both surfaces in monocots; through them gases are exchanged and water vapour escapes in transpiration. Learn the three parts of a leaf with their variations and the two venation patterns with examples, since a diagram of a typical dicot leaf with parts labelled is a favourite very short answer question.

Simple and compound leaves. A leaf is simple when its lamina is entire or, if incised, the incisions do not reach the midrib (mango, hibiscus, banana). A leaf is compound when the incisions reach the midrib or petiole and divide the blade into a number of leaflets. The test for a compound leaf is that a bud is present in the axil of the whole leaf but never in the axil of a leaflet. Compound leaves are of two kinds. In a pinnately compound leaf the leaflets are arranged on a common axis, the rachis, which represents the midrib (neem, tamarind, rose). It may be unipinnate, bipinnate (Acacia) or tripinnate (Moringa). In a palmately compound leaf the leaflets are attached at a common point at the tip of the petiole, like the fingers of a hand (silk cotton tree Bombax).

Phyllotaxy is the pattern of arrangement of leaves on the stem or branch. It ensures that leaves do not shade one another. In alternate phyllotaxy a single leaf arises at each node, the leaves being placed alternately (China rose, mustard, sunflower). In opposite phyllotaxy a pair of leaves arises at each node, opposite each other (Calotropis, guava). If successive pairs are at right angles it is opposite decussate; if in one plane, opposite superposed. In whorled phyllotaxy more than two leaves arise at a node and form a whorl (Alstonia, Nerium).

Modifications of leaves. Leaves may be changed for functions other than photosynthesis. Leaf tendrils of pea are slender, coiling structures that help climbing (in pea the terminal leaflets become tendrils; in Smilax the stipules; in Clematis the petiole). Spines of cacti and Opuntia are leaves reduced to defensive, water-conserving structures. The fleshy, food-storing scale leaves of onion and garlic are storage leaves. In insectivorous plants the leaf becomes a trap: a pitcher in Nepenthes and a bladder in Utricularia. In Australian Acacia the petiole flattens into a green, leaf-like phyllode that does the photosynthesis while the true blade drops off. Some leaves are reproductive, bearing adventitious buds on their margins that grow into plantlets, as in Bryophyllum.

Examination questions frequently ask for the difference between a pinnately and a palmately compound leaf, and for the type of phyllotaxy of a named plant, so pair each term with its example.

A flower is a modified shoot in which the shoot apical meristem changes to a floral meristem. Internodes do not elongate and the axis becomes condensed, so the leaves are replaced by floral appendages. A flower may be borne singly (solitary), but usually flowers are arranged in a definite pattern on a specialised branch called the inflorescence. The axis of the inflorescence is the peduncle and each flower stalk is a pedicel. Depending on whether the apex of the axis continues to grow or ends in a flower, inflorescences are of two main types.

Racemose (indefinite) inflorescence. The main axis continues to grow indefinitely and produces flowers laterally in acropetal succession: the oldest flowers are at the base and the youngest near the growing apex. Examples of racemose types are:

• Raceme: pedicellate flowers on an elongated axis (mustard, radish, Crotalaria).
• Spike: sessile flowers on an elongated axis (Achyranthes, Amaranthus).
• Catkin (amentum): a pendulous spike of unisexual flowers (mulberry, Acalypha).
• Spadix: a spike with a fleshy axis enclosed by a large bract, the spathe (banana, Colocasia, palms).
• Corymb: flowers at different levels brought to the same height by pedicels of different lengths (Caesalpinia, candytuft).
• Umbel: pedicels of equal length arising from one point, often with an involucre of bracts (Allium, coriander – compound umbel).
• Capitulum (head): a flattened receptacle bearing many sessile florets, the outer ray florets and inner disc florets, surrounded by an involucre (sunflower, Tridax).

Cymose (definite) inflorescence. The main axis terminates in a flower and further growth is by lateral branches that in turn end in flowers. Flowers therefore open in basipetal succession: the oldest flower is at the centre or top and the younger ones towards the periphery or base. Examples: solitary cyme (hibiscus, Datura), monochasial (uniparous) cyme with one lateral branch each time, which may be helicoid (all on one side, as in Heliotropium) or scorpioid (alternate sides, as in Ranunculus), dichasial (biparous) cyme with two lateral branches below the terminal flower (jasmine, Dianthus), and polychasial (multiparous) cyme with many lateral flowers (Calotropis, Nerium).

The chief difference to remember: racemose has unlimited growth, acropetal order and the youngest flower at the top; cymose has limited growth, basipetal or centrifugal order and the oldest flower at the top or centre.

A few inflorescences do not fit neatly into the racemose or cymose plan and are described as special types. Three of these are required for the Intermediate examination: the cyathium, the verticillaster and the hypanthodium. In each, a cluster of small flowers is packed into a structure that looks, to the casual eye, like a single flower or a fruit.

Cyathium. This is characteristic of the genus Euphorbia (family Euphorbiaceae), for example Euphorbia pulcherrima (poinsettia) and Euphorbia hirta. A cup-shaped involucre is formed by the fusion of five bracts, often with conspicuous nectar glands on its rim. Inside, a single, centrally placed female flower, reduced to one pistil on a long pedicel, is surrounded by several groups of male flowers, each reduced to a single stamen. The arrangement is cymose: the central female flower is the oldest and the male groups mature towards the periphery. The whole cyathium looks like one flower, but it is really an inflorescence of many unisexual, naked flowers.

Verticillaster. This is found in the mint family (Lamiaceae), for example Ocimum (tulsi), Leucas and Salvia. Members of this family have opposite leaves and square stems. At each node, in the axil of each of the two opposite leaves, a dichasial cyme develops whose later branching becomes monochasial and scorpioid. The two clusters of sessile flowers, one on each side of the node, are crowded so tightly that they seem to form a single whorl (verticil) of flowers around the stem. Thus a verticillaster is a condensed dichasial cyme ending in monochasial cymes, arranged in false whorls.

Hypanthodium. This is found in the genus Ficus (fig, peepal, banyan). The receptacle is fleshy and grows into a hollow, flask-shaped cup with a narrow apical opening, the ostiole, guarded by scales. Flowers are borne on the inner wall of this cavity: male flowers near the ostiole, female flowers (long-styled) towards the base, and sterile gall flowers (short-styled) in between. Pollination is by a tiny fig wasp that enters through the ostiole. The 'fruit' of the fig that we eat is thus a whole inflorescence that has ripened into a multiple fruit called a syconus.

When answering, describe the involucre, the arrangement of male and female flowers and the plant family, and add a labelled diagram.

The flower is the reproductive unit of angiosperms and is meant for sexual reproduction. It is a condensed, modified shoot. A typical flower is borne on a stalk, the pedicel, whose swollen tip is the thalamus (receptacle), on which four whorls of floral appendages are arranged in succession from outside inwards: calyx, corolla, androecium and gynoecium. Calyx and corolla are accessory organs (protective and attractive); androecium and gynoecium are the essential (reproductive) organs. In some flowers such as lily the calyx and corolla are not distinct and are together called perianth, made of tepals.

Sex of the flower. A flower with both androecium and gynoecium is bisexual (hermaphrodite); a flower with only one of them is unisexual: staminate (male) or pistillate (female). A plant bearing both male and female flowers is monoecious (maize, cucurbits, coconut); if male and female flowers are on separate plants the species is dioecious (papaya, date palm, mulberry).

Symmetry. A flower is actinomorphic (radially symmetrical) when it can be divided into two equal halves by any vertical plane passing through the centre, as in mustard, Datura and chilli. It is zygomorphic (bilaterally symmetrical) when only one vertical plane divides it into two similar halves, as in pea, gulmohur, bean and Cassia. If no plane gives equal halves it is asymmetric (irregular), as in canna.

Number of floral parts. When floral appendages are in multiples of three the flower is trimerous (most monocots), in fours tetramerous (mustard), in fives pentamerous (most dicots). A flower with a small leaf-like bract at the base of the pedicel is bracteate; without it, ebracteate. A flower is complete when all four whorls are present and incomplete if any is missing.

Position of the ovary on the thalamus. In a hypogynous flower the gynoecium sits at the top of the thalamus and the other whorls are below it, so the ovary is superior (mustard, China rose, brinjal). In a perigynous flower the thalamus forms a cup and the gynoecium is at the centre with the other whorls on the rim at the same level, the ovary being half-inferior (plum, rose, peach). In an epigynous flower the thalamus completely encloses the ovary and fuses with it, so the other whorls arise above the ovary, which is inferior (guava, cucumber, sunflower ray florets).

Calyx. The calyx is the outermost whorl and its members are sepals, usually green and leaf-like, which protect the flower in the bud stage. When sepals are free it is polysepalous (mustard); when united, gamosepalous (Datura, hibiscus). Sepals may be coloured and attractive like petals, when they are called petaloid (Mussaenda, Caesalpinia). A calyx that falls off as the flower opens is caducous (poppy); one that falls after fertilisation is deciduous; one that persists into the fruit is persistent (brinjal, tomato, chilli). Below the calyx in some flowers (hibiscus, cotton) there is an extra whorl of bracteoles, the epicalyx.

Corolla. The corolla is the second whorl, made of petals, which are generally brightly coloured to attract insects for pollination. Petals may be free (polypetalous: mustard, rose) or united (gamopetalous: Datura, Petunia, Ipomoea). Corollas take many shapes: cruciform (four petals in a cross, mustard), papilionaceous (butterfly-like with standard, wings and keel, pea), tubular (disc florets of sunflower), infundibuliform (funnel-shaped, Datura), campanulate (bell-shaped, Physalis), rotate (wheel-shaped, brinjal), bilabiate (two-lipped, Ocimum) and ligulate (strap-shaped, ray florets of sunflower).

Aestivation. The mode of arrangement of sepals or petals in the floral bud with respect to the other members of the same whorl is called aestivation. Four types are recognised.

• Valvate: the margins of the sepals or petals just touch one another without overlapping, as in the sepals of Calotropis and the petals of Annona.
• Twisted (contorted): one margin of each appendage overlaps the next in a regular, spiral manner, as in China rose, lady's finger and cotton.
• Imbricate: the margins overlap but not in any particular direction; one petal is completely outside, one completely inside and the others partly overlap, as in Cassia and gulmohur.
• Vexillary (papilionaceous): of five petals, the largest, the standard (vexillum), overlaps the two lateral wings (alae), which in turn overlap the two smallest anterior petals fused into a boat-shaped keel (carina), as in pea and bean (Fabaceae).

In the examination, aestivation is asked both as a definition and as a set of four small diagrams (cross-sections of the bud). Practise drawing these: valvate as five segments touching edge to edge, twisted as a pinwheel, imbricate with one inside and one outside, and vexillary with the big standard covering the rest.

The androecium is the third whorl and the male reproductive part of the flower. It is composed of stamens, each of which is a microsporophyll. A stamen has a slender stalk, the filament, and a terminal anther. The anther is usually bilobed, each lobe having two chambers, the pollen sacs (so the anther is dithecous and tetrasporangiate); the two lobes are joined by a sterile band of tissue, the connective. Pollen grains are produced within the pollen sacs. A sterile stamen that does not produce pollen is a staminode.

Cohesion is the union of members of the same whorl. Stamens may cohere by their filaments or their anthers.

• Monadelphous: filaments of all the stamens united into one bundle, anthers free (China rose, cotton, lady's finger).
• Diadelphous: filaments united into two bundles, typically nine plus one (pea, bean, Tephrosia).
• Polyadelphous: filaments in more than two bundles (Citrus, Bombax).
• Syngenesious: anthers united into a tube around the style, filaments free (sunflower and other Asteraceae).
• Synandrous: both filaments and anthers united throughout their length (cucurbits).

Adhesion is the union of members of different whorls. Stamens are epipetalous when attached to the petals (brinjal, Datura, Solanum), epiphyllous (epitepalous) when attached to the perianth (lily, onion), and gynandrous when united with the gynoecium, stamen and carpel forming a column called the gynostegium (Calotropis).

Number and length. The stamens in a flower may be free (polyandrous) and their number may be equal to, double or many times the number of petals. When two stamens are long and two short the condition is didynamous (Ocimum, Lamiaceae); when four are long and two short it is tetradynamous (mustard, Brassicaceae). The anther may be attached to the filament at its base (basifixed, mustard), by its back (dorsifixed, Sesbania), at one point so that it swings (versatile, grasses) or along its whole length (adnate, Magnolia).

Variation in the length and arrangement of stamens is an adaptation for successful pollination, and these terms reappear in the family descriptions of Chapter 8, so learn the examples with the terms.

The gynoecium is the innermost whorl and the female reproductive part of the flower. It is made of one or more carpels (megasporophylls). Each carpel or pistil has three parts: the basal, swollen ovary, which contains one or more ovules attached to a cushion-like tissue, the placenta; an elongated tube, the style; and the terminal, receptive stigma on which pollen grains land. After fertilisation the ovules develop into seeds and the ovary into the fruit.

When a flower has a single carpel it is monocarpellary (pea, bean); with two, bicarpellary (Datura); with three, tricarpellary (lily); with five, pentacarpellary (hibiscus); with many, polycarpellary. Several carpels may be free from one another, the apocarpous condition (lotus, rose, Michelia), or fused together, the syncarpous condition (mustard, tomato, hibiscus). The ovary may have one chamber (unilocular) or several (bilocular, trilocular, multilocular), and the number of locules need not equal the number of carpels: mustard has two carpels but a bilocular ovary because of a false septum (replum); Datura has two carpels but becomes four-chambered.

Placentation is the arrangement of ovules within the ovary. The types are:

• Marginal: the placenta forms a ridge along the ventral suture of a single carpel and the ovules are borne on this ridge in one or two rows (pea, bean and all Fabaceae).
• Axile: the placenta is axial (central) in a multilocular syncarpous ovary and ovules are attached to it (China rose, tomato, lemon, lily).
• Parietal: ovules develop on the inner wall of the ovary or on peripheral placentae; the ovary is unilocular but in mustard it becomes two-chambered by the formation of a false septum, the replum (mustard, Argemone, cucumber).
• Free central: ovules are borne on a central axis that is not connected to the ovary wall, septa being absent (Dianthus, Primula, Stellaria).
• Basal: a single ovule is attached to the base of a unilocular ovary (sunflower, marigold, all Asteraceae).

The style may be terminal, or it may arise from the base of the ovary among the lobes (gynobasic style, Lamiaceae). The stigma may be simple, bifid, capitate or feathery (grasses). A single nectar-secreting disc may be present below the ovary. The examination usually asks for the five types of placentation with one example each and a set of transverse-section diagrams, so draw them until they are automatic: marginal with ovules along one side, axile with ovules at the centre of a wheel, parietal with ovules on the rim, free central with a column, basal with one ovule at the bottom.

The fruit is a characteristic feature of the flowering plants. It is a mature or ripened ovary, developed after fertilisation. If a fruit forms without fertilisation of the ovary it is called a parthenocarpic fruit and is seedless (banana, some grapes and oranges). Generally the fruit consists of a wall, the pericarp, and the seeds. The pericarp may be dry or fleshy; when fleshy it differentiates into an outer epicarp, a middle mesocarp and an inner endocarp.

A true fruit develops only from the ovary (mango, tomato, pea). A false fruit (pseudocarp) develops from the ovary together with other floral parts such as the thalamus, calyx or receptacle: in apple and pear the thalamus is the edible fleshy part; in cashew the swollen pedicel and thalamus form the 'apple'; in strawberry the fleshy thalamus bears small achenes.

On the basis of the number of ovaries and flowers involved, fruits are simple (from a single ovary of one flower, monocarpellary or syncarpous), aggregate (from the free carpels of a single apocarpous flower) and multiple or composite (from all the flowers of a whole inflorescence).

Simple fleshy fruits. The drupe develops from a monocarpellary, superior ovary and is one-seeded; the pericarp has a thin epicarp (skin), a fleshy mesocarp (pulp) and a hard, stony endocarp (stone) that encloses the seed, as in mango, coconut and peach. In coconut the mesocarp is fibrous, not fleshy. The berry arises from a syncarpous ovary with the whole pericarp fleshy and many seeds embedded in the pulp (tomato, grape, brinjal, banana). The pepo is a berry-like fruit from an inferior ovary with a hard rind (cucumber, watermelon). The hesperidium has a leathery epicarp with oil glands, a spongy mesocarp and juice-filled hairs from the endocarp (orange, lemon). The pome is a false fruit in which the fleshy thalamus surrounds the ovary (apple, pear).

Simple dry fruits. Dry dehiscent fruits split open at maturity: the legume (pod) splits along both sutures (pea, bean, groundnut does not dehisce), the follicle along one suture (Calotropis), the siliqua splits from below upward leaving the replum with the seeds (mustard), and the capsule opens by several valves or pores (cotton, lady's finger, Datura, poppy). Dry indehiscent fruits do not split: the achene (one seed, pericarp free from the seed coat, Clematis), the caryopsis (one seed with pericarp fused to the seed coat, rice, wheat, maize), the cypsela (from an inferior bicarpellary ovary, sunflower, Tridax), the nut (hard, woody pericarp, cashew, litchi) and the samara (winged, Hiptage). Schizocarpic fruits break into one-seeded pieces (mericarps), as in castor (regma) and Acacia (lomentum).

Aggregate fruits. When a single flower has an apocarpous gynoecium (many free carpels), each carpel develops into a small fruitlet and the group of fruitlets on one thalamus is an aggregate fruit or etaerio. It is named after the fruitlet type: an etaerio of follicles in Michelia and Calotropis, an etaerio of achenes in Clematis and strawberry, an etaerio of drupes in raspberry, and an etaerio of berries in Annona (custard apple), where the berries fuse into one fleshy mass.

Multiple (composite) fruits. These develop from a whole inflorescence, the flowers and their axis ripening together. The sorosis develops from a spike or spadix: in pineapple the fleshy fruit is formed by the fusion of the perianths, ovaries and axis of the whole inflorescence; in jackfruit and mulberry the perianths become fleshy. The syconus develops from a hypanthodium: the fleshy receptacle of the fig encloses many small achenes.

The seed. After fertilisation the ovules develop into seeds. A seed is made up of a seed coat and an embryo; the embryo consists of a radicle, an embryonal axis and one or two cotyledons.

Structure of a dicotyledonous seed (gram, pea, bean). The outermost covering is the seed coat, with two layers, the outer testa and the inner tegmen. The hilum is a scar on the seed coat through which the developing seed was attached to the fruit; above it is a small pore, the micropyle, through which water enters at germination. Within the seed coat is the embryo with two large, fleshy cotyledons that store food. The embryonal axis has the radicle (future root) below the cotyledons and the plumule (future shoot) above them; the portion between the cotyledons and the radicle is the hypocotyl and above the cotyledons the epicotyl. In such seeds the endosperm is consumed during development and the mature seed is non-endospermic (exalbuminous). In castor, the endosperm persists and the seed is endospermic.

Structure of a monocotyledonous seed (maize grain). Monocot seeds are generally endospermic; the seed coat is membranous and fused with the fruit wall. The bulky endosperm stores food and is separated from the embryo by a proteinaceous layer, the aleurone layer. The embryo is small and lies in a groove at one end of the endosperm. It consists of one large shield-shaped cotyledon, the scutellum, and a short axis with plumule and radicle. The plumule is enclosed in a sheath, the coleoptile, and the radicle in the coleorhiza; both protect the growing tips as they push through soil. Orchid seeds are non-endospermic monocot seeds.

Botanists describe a plant in a fixed order using a set of exact terms, so that any reader anywhere can picture the plant and place it in its family. This semi-technical description follows the sequence of the plant body: habit, vegetative characters and then floral characters, ending with the floral formula and floral diagram. Learning this sequence is the key to writing the family descriptions of Fabaceae, Solanaceae and Liliaceae in the next chapter.

Vegetative characters. The description begins with the habit (herb, shrub, tree, climber; annual, biennial, perennial) and the habitat. Then the root (tap or fibrous, with any modification), the stem (herbaceous or woody, erect or climbing, branched or unbranched, solid or hollow, hairy or glabrous, with modifications) and the leaves (arrangement or phyllotaxy; simple or compound; type of venation; shape of the lamina; presence of stipules; leaf base, petiole and margin).

Floral characters. The inflorescence type is stated first. The flower is then described as bracteate or ebracteate, pedicellate or sessile, complete or incomplete, bisexual or unisexual, actinomorphic or zygomorphic, tri-, tetra- or pentamerous, hypogynous, perigynous or epigynous. Each whorl follows in order:

• Calyx: number of sepals, free or fused, aestivation, any special features (persistent, petaloid).
• Corolla: number of petals, free or fused, shape, aestivation, colour.
• Androecium: number of stamens, cohesion and adhesion, length, anther type and dehiscence.
• Gynoecium: number of carpels, apocarpous or syncarpous, ovary position, number of locules, placentation, style and stigma.

Then the fruit type, the seed (endospermic or not, number of cotyledons) and finally the floral formula and floral diagram.

As an example, a brief description of the mustard plant (Brassica campestris): an annual herb with a tap root; stem erect, herbaceous, branched, green; leaves alternate, simple, exstipulate, reticulate venation, the lower ones lyrate; inflorescence a raceme; flowers ebracteate, pedicellate, complete, bisexual, actinomorphic, tetramerous, hypogynous, yellow; calyx of four sepals in two whorls, polysepalous, imbricate; corolla of four petals, polypetalous, cruciform, valvate; androecium of six stamens, tetradynamous, free, basifixed anthers; gynoecium bicarpellary, syncarpous, ovary superior, unilocular becoming bilocular by a false septum, parietal placentation; fruit a siliqua; seeds non-endospermic.

Write descriptions in this order every time; the marking scheme for the family question rewards each correctly placed character.

The floral formula is a shorthand way of representing the structure of a flower using symbols, letters and numbers, and the floral diagram is a ground-plan drawing of the flower as seen from above, showing the number, arrangement and relations of the floral parts and their position with respect to the mother axis. Together they summarise a whole page of description in one line and one drawing.

Symbols used in the floral formula.

• Br – bracteate; Ebr – ebracteate; Brl – bracteolate.
• ⊕ – actinomorphic; % (or an arrow) – zygomorphic.
• ⚥ – bisexual; ♂ – male; ♀ – female.
• K – calyx; C – corolla; P – perianth; A – androecium; G – gynoecium.
• A number after a letter gives the number of members, ∞ means indefinite (more than ten); the number in brackets, e.g. K(5), shows fusion of members of a whorl.
• A line joining two whorls, e.g. C(5) A5 with a bracket over them, shows adhesion (epipetalous stamens); a line above G shows an inferior ovary, a line below G shows a superior ovary.
• G(2) with the number of locules and the placentation may be written as G(2), bilocular, axile.

Examples. Mustard: Ebr ⊕ ⚥ K2+2 C4 A2+4 G(2). China rose (hibiscus): Br ⊕ ⚥ K(5) C5 A(∞) G(5). Pea: % ⚥ K(5) C1+2+(2) A(9)+1 G1. Datura: ⊕ ⚥ K(5) C(5) A5 G(2). Lily (Allium): Br ⊕ ⚥ P(3+3) A3+3 G(3). (In each of these the ovary is superior, shown in writing by a line drawn under G.)

The floral diagram. The mother axis (the stem on which the flower is borne) is shown as a dot at the top of the diagram, representing the posterior side; the bract, if present, is drawn at the bottom (anterior side). Working inwards, concentric circles of sepals, petals, stamens and the transverse section of the ovary are drawn in their actual number and orientation. Fusion within a whorl is shown by joining the members; epipetalous stamens are drawn attached to the petals; aestivation is shown by the overlapping of sepals and petals; the ovary section shows the number of locules and the placentation. In a zygomorphic flower, the unequal parts (for example the standard, wings and keel of pea) are drawn in their real positions relative to the mother axis, the standard being posterior.

In the examination, floral formula and floral diagram carry marks in the family question, so practise writing the formula for mustard, pea, China rose, brinjal and lily, and drawing the diagram for at least mustard and pea.