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Botany Six - Flowers

A Beginner’s Guide to Botanical Terminology (Pt 6).

Botany and biological description are a bit more exacting than Latin nomenclature. Often there is only the correct, Latin-based name for a particular part of a plant, or animal. As a gardener you may not need to trouble with the finer details but if you get interested in how & why plants do what they do, or citizen science and recording the plants around you, you will need to know some of this to work with identification keys.   A passing familiarity with Botanical Latin can also be advantageous, try our Botanical Latin Lexicon.

This page is only a starting point in to how and why plants do what they do. The framework for Botanical Descriptions  has been used as starting point for items included and I hope that relating observable plant features and processes to the correct Biological terms makes this fascinating subject a little more accessible.

Flowers

→ Function
→ Perianth
→ Development
→ Androecium
→ Anatomy
→ Gynoecium

Flowers, probably the reason why so many gardeners expend time, effort, and money to sow, grow, tend, and propagate plants. While many have economic value in the modern world because of substances they contain their depiction and symbolism in Human culture goes at least as far back as the ancient Egyptians. Neither History nor Botany give us any real clues as to what evolutionary advantage plants get from our fascination with their Flowers. Perhaps it is because of their beauty we largely leave them alone to achieve their primary function for the plant, reproduction.

Flower Function

The primary function of a flower is the reproduction of the individual plant, and the continuation of its species. All flowering plants (Angiosperms) are said to be Heterosporous, i.e., each individual plant produces two different types of Spores. Microspores are produced inside anthers and Megaspores are produced inside Ovules. As both types of spores develop into Gametophytes inside Sporangia, they are said to be Endosporic.

In most plant species, individual flowers have both Male and Female parts, these are described Perfect or Bisexual flowers, and the species as Hermaphroditic. A minority of plant species have flowers of one Sex and are described as Imperfect or Unisexual. If the individual plants of a species each have Unisexual flowers of both sexes, then the species is Monoecious; if each individual plant has only unisexual flowers of the same sex, then the species is Dioecious.

Flower Development

The transition from a purely vegetative growth phase to that of flowering is one of the major changes in a plants life cycle and there are many interconnected triggering factors. In the first place the plant must have either, achieved sufficient stored energy reserves, of have the capacity to generate the required energy. For this component to be fulfilled it may be that the plant needs to achieve a sufficient overall biomass or quantity of leaves to proceed. In addition to this the plant needs to coordinate its flowering with the availability of is pollinators to achieve successful reproduction. For this component the plant is able to detect and respond to external variations in temperature, moisture and Photoperiod (Day / night length). This second component is sometimes referred to as Vernalisation. In broad terms plants growing in Temperate climate zones become Vernalised when they experience temperatures in the range of 1 to 7ºC for a predetermined length of time (Chill Hours). This ensures full dormancy is achieved and when temperatures rise flowering is triggered.

Climate change is already affecting Vernalisation of key food crops. As plants have an extremely limited ability to change their location, and with plant evolution and adaptation being known to be unlikely to be able to keep pace with the current rate of change the effect on successful reproduction for these plants will drive their extinction. The same will apply to any animals’ dependant on the plant or its fruit.

When the internal and external environmental factors are met changes in the hormones produced by the plants Meristems are triggered, the hormones influence gene expression in the cells of the Meristem and the floral growth phase begins.
Meristems are small areas of continually dividing undifferentiated cells in Stem (Bud) and Root tips, we have looked at how they generate the plants Indeterminate vegetative growth it the relevant previous sections. When floral growth is triggered the growth of the Meristem becomes Determinate (grows to a predetermined end state). In some cases, they become Inflorescence Meristems first before becoming Floral meristems. In either case it can be seen there is good correlation with the Indeterminate Apical Meristem as Leaf, Bud and Stem tissue becomes transformed into the reproductive organs.

There are three key groups of Genes whose expression is altered by the transition to Floral growth. The locations where these three groups are expressed, and how they inter-react determines the part of the flower that that tissue develops into.

Flower Anatomy

A Compete mature Flower comprises of four whorls of structures. (An Incomplete flower is missing one or more whorls). The inner two whorls are known as the Essential or Reproductive whorls and the outer two whorls are the Accessory or Helping whorls. These four whorls are attached to the vegetative tissue of the central Receptacle which is located near the end of the flowering stem and may sit below or enclose the reproductive whorls. Each whorl is dealt with separately in the following sections.

The Perianth

The Perianth is the collective term for the outer two Accessory whorls, the Calyx, and the Corolla. When the two whorls are both present and clearly separate the Perianth is said to be Heterochlamydeous. When the Petals and Sepals are indistinguishable, they are referred to as Tepals, and the Perianth is said to be Homochlamydeous and is referred to as the Perigone, this is often the case in Monocots.

Aestivation

Aestivation is the positional arrangement of parts of a flower within the bud before it has opened. The positioning persists to greater or lesser degrees as the flower opens and it can be an important taxonomic diagnostic for identification. Arrangements include:

  • Crumpled.
  • Decussate – in opposite pairs with each successive pair at right angles.
  • Imbricate – overlapping.
    • Contorted or twisted – every petal or sepal is outside its neighbour on one margin, and inside its neighbour on the other margin.
      • Cochleate – spirally twisted.
      • Contortiplicate – contorted and also plicate (pleated).
    • Quincuncial – with five parts, where two petals or sepals are outside all others, two are inside all others, and the fifth is outside on one margin and inside on the other.
  • Induplicate – folded inwards.
  • Open – petals or sepals do not overlap or even touch each other.
  • Reduplicate – folded outwards.
  • Valvate – margins of adjacent petals or sepals touch each other without overlapping.
  • Vexillary – a special type of aestivation occurring in plants like pea; in this type of aestivation a large petal called standard encloses two smaller petals.

Imbricate.

Contorted.

Quincuncial.

Valvate.

Vexilliary.

Involute.

Calyx

The Calyx is the collective term for the outermost whorl of the Flower and Perianth. The individual units it is comprised of are Sepals.

Sepals – are modified leaves, perform a protective function for the flower bud, and are usually, but not always green. They can photosynthesise but at a lower rate than leaves. The quantity of Sepals often mirrors the quantity of Petals, in Dicots the number is often four, five, or multiples of these, and in Monocots the number is often three or multiples of three. Sepals may be free (Aposepalous), or fused (Synsepalous), or fused towards the base into a Calyx Tube.

Tepals – are also modified leaves and believed to be the ancestral condition of flower Calyxes.
Like Sepals they are often green but may also be highly coloured and function as both protection and attraction. Tepals, they appear coloured like Petals are referred to as Petaloid Tepals. Like Sepals they may be free (Apotepalous) or fused (Syntepalous).

Aposepalous & Apopetalous.

Synsepalous & Apopetalous.

Aposepalous & Synpetalous.

Synsepalous & Synpetalous.

Corolla

The Corolla is the collective term for the innermost whorl of the Perianth. The individual units it is comprised of are Petals. When all the Petals in a Corolla are free from each other it is said to be Apopetalous, when petals are at least partially fused it said to be Synpetalous. Corollas may be irregular or possess either bilateral or radial symmetry.

Petals are modified leaves, are brightly coloured or unusually shaped, and perform the function of protecting the reproductive parts and attracting pollinators to the Flower. The quantity of Petals often mirrors the quantity of Sepals, in Dicots the number is often four, five, or multiples of these, and in Monocots the number is often three or multiples of three. Petals may be free (Polypetalous), or fused (Synpetalous), or fused towards the base into a Corolla Tube. Like Leaves petals often exhibit a wide upper part, The Blade, and a lower narrower part like a petiole, the Claw. Across the multitude of plant species Petals are incredibly diverse, in size, shape, and especially in colour, pattern and texture. Many have makings known as Nectar Guides that reflect light in the Ultraviolet part of the spectrum that is particularly visible to insects. In many plants of the Boraginacea family petals change colour once a flower is pollenated saving wasted effort for either party. Scent, shape and size can also play a part in attracting specific pollinators or repelling predators.

The Androecium

The word Androecium derives from the Greek for Mans House, it is the outer of the two Reproductive Whorls and is the collective name for the plant’s male reproductive organs, the Stamens. The androecium may contain a single or multiple whorls of Stamens.

Stamen

The typical Stamen is made from three parts, a stalk – the Filament, the Anther – lobed structures containing microsporangia, and sterile Connective tissue that connects the two. Depending on the species the quantity of Stamen in the Androecium can range from as few as one to over three thousand. There is also huge diversity in the arrangement of Stamen within the Androecium.

Stamens may be …

  • Arranged in one of two different patterns: Spiral; or whorled in one or more discrete whorls.
  • Adnate – fused or joined from more than one whorl. When Adnate to the corolla they are termed Epipetalous and when adnate to the Tepals are termed Epiphyllous.
  • Of differing lengths to each other-
    • Didymous: two equal pairs.
    • Didynamous: occurring in two pairs, a long pair and a shorter pair.
    • Tetradynamous: occurring as a set of six stamens with four long and two shorter ones

Didymous.

Didynamous.

Tetranamous.

  • With respect to the rest of the flower (Perianth), Stamens may be –
    • Exserted: extending beyond the corolla.
    • Included: not extending beyond the corolla.
  • With respect to the Petals they may be –
    • Alternipetalous (Antisepalous) having a single whorl of stamens, equal in number to the proper number of petals and alternating with them (inline / above the Sepals).
    • Alternisepalous (Antipetalous) having a single whorl of stamens, equal in number to the proper number of petals and inline / above them (alternating with the Sepals).
    • Diplostemonous: in two whorls, the outer alternating with the petals, while the inner is opposite the petals.
    • Obdiplostemonous: in two whorls, with twice the number of stamens as petals, the outer opposite the petals, inner opposite the sepals.

Antisepalous-alternipetalous.

Antipetalous-alternisepalous.

Diplostemonous.

Obdiplostemonous.

Filament

The Filament is a thin stalk that supports the Anther, they can be Connate (fused or joined to other individual organs of the same type) in the same whorl as follows:

  • Monadelphous – fused into a single, compound structure known as a Androphore.
  • Diadelphous – joined partially into two androecial structures.
  • Pentadelphous – joined partially into five androecial structures.
  • Declinate – curving downwards, then up at the tip.
Anther

The Anther is a lobed structure at the tip of the Stamen; it contains the Microsporangia which produces the plants Pollen. Anthers occur in a variety of shapes, the most frequent are described as Linear, Rounded, Sagittate (arrow shaped), Sinuous (wave like), or Reniform (kidney shaped). The division between the Anthers lobes (its Dehiscence), which is where they split open to release the pollen may be orientated in one of three ways; it may face away from the centre of the flower, be Extrorse: may face inwards, be Introrse, or may be Latrorse and face towards the side. There are two ways the Anther may be attached to the Filament via the Connective; by its base, Basifixed; or by the centre, Dorsifixed. Dorsifixed Anthers are usually able to move at the connection point, said to be Versatile.

Connective

The Connective is a small area of tissue that connects the Anther lobes to the Filament and to each other. Where the connective is very small, or imperceptible, and the anther lobes are close together, the connective is referred to as Discrete. Where the connective separates the anther lobes, it is called Divaricate. The connective may also be long, stalk-like, and crosswise on the filament, this is a Distractile connective. And the connective may also bear appendages and is called Appendiculate.

The Gynoecium

The word Gynoecium derives from the Greek for Woman’s House, it is the inner of the two Reproductive Whorls, the inner of all whorls in the Flower, and is the collective name for the plant’s female reproductive organs, the Carpel. The Gynoecium may contain a single or multiple Carpels.

Carpel

A Gynoecium may consist of one or more separate Carpels (sometimes referred to as Pistils). Carpels are thought to be modified leaves. The typical Carpel is composed of the following parts – 

  • Ovary – the enlarged basal part of the Pistil which contains a chamber, the Locule, within which a ridge of tissue, the Placenta, bears the Ovules.
  • Style – a pillar-like stalk that the pollen tubes grow through to reach the Ovary. Not all flowers have distinct Styles.
  • Stigma – structure at the apex of the Style on which Pollen is received and germinates.
Gynoecium Types
There are three possible types of Gynoecia – 
  • Monocarpus Gynoeciums – are comprised of a single Carpel.
  • Apocarpus Gynoeciums – are comprised of multiple, distinct, unfused Carpels.
  • Syncarpous Gynoeciums – are comprised of multiple, fused (connate) Carpels. The degree of fusion is variable; Carpels may be fused at the base only and have their own Styles and Stigmas; They may be fused entirely in to a Compound Ovary but still have their own Styles and Stigmas; They are occasionally fused by the Styles and Stigmas but retain separate Ovaries.
Gynoecium Position

The position of the Gynoecium with regard to other flower parts can be a very important taxonomic character for species identification. In some flowers, stamens, petals, and sepals are said to be “fused” into a “floral tube” or Hypanthium. (This is not strictly the case, but the term Hypanthium still applies.)

  • If the Hypanthium is absent, and Stamens, Petals, and Sepals are all attached to the receptacle below the Gynoecium the flower is Hypogynous. This is the typical arrangement in most flowers and these flowers are often referred to as having a Superior Ovary.
  • If a Hypanthium is present but is either free from the gynoecium (appearing to be a cup or tube surrounding the gynoecium), or connected partly to the gynoecium (with stamens, petals, and sepals attached to the Hypanthium part of the way up the ovary), the flower is Perigynous. These flowers are often referred to as having a Half-inferior Ovary (or, Partially inferior or Half-superior). This arrangement is characteristic of the Rose and Saxifrage families.
  • If a Hypanthium is present up to the base of the style(s), and Stamens, Petals, and Sepals are attached to the Hypanthium at the top of the ovary, (or occasionally, the hypanthium may extend beyond the top of the ovary) the flower is Epigynous. These flowers are often referred to as having an Inferior Ovary. This arrangement is characteristic of the Orchid and Aster families.
  • The Dark Grey coloured structure in all three diagrams below is know as the Receptacle.
Ovary & Placentation

When Carpels are fused, they may form an Ovary with just one Locule (chamber) and the ovary is termed Uni-locular. In other cases, thin Septa may remain between the Locules of the fused Carpels and the Ovary is termed Bi-locular, Tri-locular etc as dictated by the number of Locules.
In Botany Placentation is the term used to describe the arrangement of Ovules inside the ovary. If you save your own seed, or grow your own veg you will witness this, and if you trying to identify wild plants Placentation can be a diagnostic feature. There are several Placentation types.

  1. Basal Placentation – is found in single to muticarpelleary, Syncarpous Ovaries, the ovule is attached at the base of the Ovary.
  2. Apical Placentation – is found in single to muticarpelleary, Syncarpous Ovaries, the ovule is attached at the Apex of the Ovary.
  3. Parietal Placentation – is found in bicarpellary to multicarpellary syncarpous Ovaries. A Unilocular ovary becomes bi (tri, multi etc) locular due to formation of false septum. E.g., Cucumber. The Placentas may intrude into the Ovary or be Apical.
  4. Axile Placentation – is found in bicarpellary to multicarpellary syncarpous Ovaries. Carpels fuse to form septa forming a central axis and ovules are arranged on the axis. E.g., Hibiscus, lemon, tomato, Lilly. Placentas may intrude into the Ovary or be Apical.
  5. Free Central Placentation – is found in bicarpellary to multicarpellary syncarpous Ovaries. Due to septum degradation a unilocular condition is formed and ovules are arranged on the central axis. E.g., Dianthus, Primula.
  6. Marginal Placentation -is found in monocarpellary unilocular ovaries, placenta forms along ventral side and ovules are arranged in two vertical rows. E.g., Pisum satvium (pea).
Style

Styles are generally tube like and may be long, short, or occasionally absent (when the Stigma is said to be sessile). The tube may be open, filled with mucilage or with special cells that allow the passage of the pollen tube. Where the Gynoecium is composed of more than one carpel, each Carpel may have its own style, or share a fused one. Taxonomically the position that the style attaches to the Ovary can be diagnostic. The attachment position may be –

  • Terminal (apical), attachment at the apex of the ovary is the most common pattern.
  • Subapical, the style arises to the side slightly below the apex.
  • Lateral, the style arises from the side of the ovary and is found in Rosacea.
  • Gynobasic, the style arises from the base of the ovary, or between the ovary lobes and is characteristic of Boraginacea.
  • Subgynobasic, characteristic of Allium.
Stigma

The Stigma is the distal portion of the style, (unless it is Sessile), and is composed of Stigmatic Papillae whose cells are receptive to pollen. The Stigma may be restricted to the apex of the style but, especially in wind pollinated species, may cover a wide surface. The Stigma is adapted to capture Pollen with various hairs, flaps, other sculpturing or stickiness. The stickiness helps in the rehydration of Pollen grains and its germination. Pollen may be captured from the air, Anemophily, from visiting insects or other animals Biotic, or in rare cases from surrounding water, Hydrophily. Stigma also ensure proper adhesion of the correct species of pollen and can play an active part in pollen discrimination and some Self-Incompatibility reactions that reject pollen from the same or genetically similar plants. The Stigma is often lobed and exhibits considerable variation in shape.

Pollination

Pollination should be the next consideration, but as is not a part of plant descriptions or identification and as it is such a big topic in its own right, I will not be covering this here. Successful pollination is obviously critical to the survival of flowering plant species, but it is also critical to Human survival. Humans are omnivorous predators, we can eat plants, or animals that eat plants, or animals that eat animals that eat plants, etc. The bottom line is always Plants and therefore anything that limits their ability to achieve pollination affects the whole food chain. If you would like to look in to the mechanisms and detail of pollination you could start Here or Here.

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Further Resources

Most of the main search engines will give you a result if you are looking for the meaning of a particular botanical word. If you wish to explore the subject in more detail here are a few website links to give you a starting point.