8.2: The Flower and the Fruit

The Flower

A flowercompact generative shoot with sterile, male and female zones, specifically in that order, other flower terms see in the separate glossary in the text (Figure \(\PageIndex{3}\)) is a compact generative shoot that is comprised of three zones: sterile (perianth), male (androecium), and female (gynoecium) (Figure \(\PageIndex{2}\)). Perianth is typically split into green part (calyx, consists of sepals) and color part (corolla, consists of petals). Sometimes perianth consists of similar parts which are neither sepals nor petals: tepals. This might be seen in the tulip (Tulipa) flower where tepals change their color from green (like in calyx) to red, white or yellow (like in corolla).

The general characters that a flower has are sex, merosity, symmetry, and the position of the gynoecium. Merosity is simply the number of parts in each whorl of a plant structure, whether it is the number of sepals, petals in a corolla, or the number of stamens. The position of the gynoecium refers to whether the ovary is superior or inferior (Figure \(\PageIndex{6}\)). Inferior ovary (cucumber, Cucumis, apple Malus or banana Musa) will develop into a fruit where stalk and remnants of perianth are on the opposite ends, whereas superior ovary will make fruit where stalk is placed together with perianth (like in tomatoes, Solanum). More terms are described in the following separate small glossary:

Figure \(\PageIndex{1}\) The origin of fruit. Note correspondences between different parts (shown with color).

FLOWER PARTS occur in whorls in the following order—sepals, petals, stamens, pistils.

(The only exceptions are flowers of Eupomatia with stamens then perianth, Lacandonia with pistils then stamens, and some monocots like Triglochin, where stamens in several whorls connect with tepals.)

PEDICEL flower stem

RECEPTACLE base of flower where other parts attach

HYPANTHIUM cup-shaped receptacle (Figure \(\PageIndex{2}\))

PERIANTH = CALYX + COROLLA

SEPALS small and green, collectively called the CALYX, formula: K

PETALS often large and showy, collectively called the COROLLA, formula: C

TEPALS used when sepals and petals are not distinguishable, they form SIMPLE PERIANTH, formula: P

Figure \(\PageIndex{2}\) Zones in hellebore (Helleborus) flower: sterile perianth, male androecium and in the center, female gynoecium (inside, three ovules are well visible).

ANDROECIUM collective term for stamens: formula: A

STAMEN = FILAMENT + ANTHER

ANTHER structure containing pollen grains

FILAMENT structure connecting anther to receptacle

GYNOECIUM collective term for pistils/carpels, formula: G. Gynoecium can be composed of:

1. A single CARPEL = simple PISTIL, this is MONOMERY

2. Two or more fused CARPELS = compound PISTIL, this is SYNCARPY

3. Two or more unfused CARPELS = two or more simple PISTILS, this is APOCARPY

(Note that variant #4, several compound pistils, does not exist in nature.)

To determine the number of CARPELS in a compound PISTIL, count LOCULES, points of placentation, number of STYLES, STIGMA and OVARY lobes.

Figure \(\PageIndex{3}\) Most important parts of the flower.

PISTIL Collective term for carpel(s). The terms CARPEL and PISTIL are equivalent when there is no fusion, if fusion occurs then you have 2 or more CARPELS united into one PISTIL.

CARPEL structure enclosing ovules, may correspond with locules or placentas

OVARY basal position of pistil where OVULES are located. The ovary develops into the fruit; OVULES develop into seeds after fertilization.

LOCULE chamber containg OVULES

PLACENTA place of attachment of OVULE(S) within ovary

STIGMA receptive surface for pollen

STYLE structure connecting ovary and stigma

FLOWER Floral unit with sterile, male and female zones

ACTINOMORPHIC FLOWER A flower having multiple planes of symmetry, formula: \(\ast\)

ZYGOMORPHIC FLOWER A flower having only one plane of symmetry, formula: \(\uparrow\)

PERFECT FLOWER A flower having both sexes

MALE / FEMALE FLOWER A flower having one sex, formula: ♂ / ♀ (Figure \(\PageIndex{5}\))

Figure \(\PageIndex{4}\) Flower with hypanthium (cup-shaped receptacle).

MONOECIOUS PLANTS A plant with unisexual flowers with both sexes on the same plant

DIOECIOUS PLANTS A plant with unisexual flowers with one sex on each plant, in effect, male and female plants

SUPERIOR OVARY most of the flower is attached below the ovary, formula: \(G_{\underline{\dots}}\)

INFERIOR OVARY most of the flower is attached on the top of ovary, formula: \(G_{\overline{\dots}}\)

(Inferior ovary only corresponds with monomeric or syncarpous flowers.)

WHORL flower parts attached to one node

Flower formula and diagram

Since there are so many terms about flowers, and at the same time, flower structure and diversity always were of immense importance in botany, two specific ways were developed to make flower description more compact. First is a flower formula. This is an approach where every part of flower is designated with a specific letter, numbers of parts with digits, and some other features (whorls, fusion, position) with other signs:

\(\ast K_{4}C_{4}A_{2+4}G_{\underline{(2)}}\): flower actinomorphic, with four sepals, four petals and six stamens in two whorls, ovary superior, with two fused carpels

Figure \(\PageIndex{5}\) Diagram of male (left) and diagram and scheme of female (central and right) flowers of sedges (Carex). Note the perigynium (external cover of pistil).

Figure \(\PageIndex{6}\) Position of ovary: superior (left, hypogynous flower) and inferior (right, epigynous flower).

\(\uparrow K_{(5)}[C_{(1,2,2)}A_{2,2}]G_{\underline{(2\times2)}}\): flower zygomorphic, with five fused sepals, five unequal fused petals, two-paired stamens attached to petals, superior ovary with two subdivided carpels

\(\ast K_{(5)}C_{(5)}[A_{5}G_{\underline{(3)}}]\): actinomorphic flower with five fused sepals and five fused petals, five stamens attached to pistil, ovary inferior, with three fused carpels

The following signs are used to enrich formulas:

PLUS “+” is used to show different whorls; minus “\(-\)” shows variation; “\(\vee\)” = “or”

BRACKETS “[]” and “()” show fusion

COMMA “,” shows inequality of flower parts in one whorl

MULTIPLICATION “\(\times\)” shows splitting

Figure \(\PageIndex{7}\) How to draw a diagram (graphical explanation): compare numbers on plant and on diagram.

INFINITY “\(\infty\)” shows indefinite number of more than 12 parts

Flower diagram is a graphical way of flower description. This diagram is a kind of cross-section of the flower. Frequently, the structure of pistil is not shown on the diagram. Also, diagrams sometimes contain signs for the description of main stem (axis) and flower-related leaf (bract). The best way to show how to draw diagram is also graphical (Figure \(\PageIndex{7}\)); formula of the flower shown there is \(\ast K_{5}C_{5}A_{5}G_{\underline{(5)}}\).

ABC model

All parts of flower have a specific genetic developmental origin explained in the ABC model (Figure \(\PageIndex{8}\)). There are three classes of genes with expression which overlaps as concentric rings, and these genes determine which cells develop into particular organ of the flower. If there are A and C genes expressed, cells will make sepals and pistils. In areas where A and B are active, petals will form; areas where B and C are active are the sites where stamens will appear. A will make a sepal, C will “create” a carpel:

• A alone \(\rightarrow\) calyx

• A + B \(\rightarrow\) corolla

• C + B \(\rightarrow\) androecium

• C alone \(\rightarrow\) gynoecium

Figure \(\PageIndex{8}\) ABC model of flower development.

Origin of flower

An example of a primitive magnoliid flower would be Archaefructus which is a fossil water plant from the lower Cretaceous time period in China. Its fructifications (flower units, FU) were very primitive and did not yet form a compacted flower, instead, there were multiple free carpels, and paired stamens (Figure \(\PageIndex{9}\)).

Another ancestral flowering plant is Amborella,a small forest shrub of New Caledonia (Figure \(\PageIndex{10}\)), which is an island in the Pacific Ocean.

Amborella has irregular flowers, a stylar canal, unusual 5-celled embryo sacs that have one central cell, and only four other cells (egg cell and its “sisters”). A stylar canal is a canal that leads to the ovary that the pollen tubes pass through so these plants are not completely “angiospermic”, this represents one of the stages of the origin of pistil (Figure \(\PageIndex{11}\)).

Figure \(\PageIndex{9}\) Comparison of Archaefructus flower (left) and typical flower (note colors). (Modified from various sources.)

The Inflorescence

Inflorescence is an isolated generative shoot (shoot bearing FU). Together, inflorescences make generative shoot system. Its diverse structure is of not lesser importance than the structure of vegetative shoot system.

The vast diversity of inflorescences can be split into four groups, or “models” (Figure \(\PageIndex{12}\)). Sole flower is sometimes considered as a “Model 0”.

Two models are most widespread. Model I inflorescences are based on the racemebasic monopodially branched inflorescence (Model I) (monopodially branched generative shoot). They are simple or double and mostly monopodial (Figure \(\PageIndex{14}\)).

Model II inflorescences (Figure \(\PageIndex{13}\)) bear or consist of closed (sympodially branched) units. The most complete but more rare variant is thyrsus, whereas reduced variants (monochasia and dichasia) are more frequent.

Figure \(\PageIndex{10}\) Amborella trichopoda, sister group to all other flowering plants. White ruler equal to 1 mm.

Figure \(\PageIndex{11}\) Amborella pistil, longitudinal section: stylar canal is green, embryo sac red.

Figure \(\PageIndex{12}\) Four kinds of inflorescences (left to right): Model I (raceme-based), Model II (thyrsoid) , Model III (panicle) and Model IV (intercalate).

Figure \(\PageIndex{13}\) Model II inflorescences (from top to bottom): thyrsus, dichasium and monochasium (cincinnus).

Figure \(\PageIndex{14}\) Different Model I inflorescences and their evolutionary connections. Digits represent the simple encoding system: first position is main axis, second position are secondary axes (flower pedicels), 1 means developed, 0 reduced. Double inflorescences have four digit positions, for the first and second orders of branching. Some names: 11 raceme, 11/11 double raceme, 10 spike and spadix, 01 umbel, 01/01 compound umbel, 00 head.

Pollination

Pollination could be of two types: self- and cross-pollination. Cross-pollination can happen in both abiotic and biotic ways. Abiotic would be represented by gravity, wind, or water; biotic would be performed by agents like insects, birds, bats, or in some cases tree mammals like possums. Wind-pollination is seen as being wasteful and unintelligent due to the fact that the plant needs to produce so much more pollen without any precise targeting.

Adaptation to the particular pollination agent results in different pollination syndromes. For example, cup-shaped flowers are usually pollinated with massive animals like beetles and even bats. Funnel-shaped flowers as well as labiate flowers (with lips), are adapted to flies and bees. Flowers with long spurs attract butterflies and birds (like hummingbirds or sugarbirds).

Self-pollination often exists like a “plan B”, in case cross-pollination is, for some reason, impossible. Sometimes, self-pollinated flowers even do not open; these flowers are called cleistogamous.

If pollination needs to be avoided, apomixis will prevent it. Apomixis requires reproductive organs, but there is no fertilization. One type of apomixis is apospory when an embryo develops from the maternal diploid tissue when an embryo develops from the maternal diploid tissue, but does not go through the meiosis stage. In this process, asexual reproduction will have become vegetative. Another type of apomixis would be apogamy (parthenogenesis) when embryo develops from an unfertilized gamete after diploidization has occurred. Here, vegetative reproduction evolved from sexual reproduction.

The Fruit

A fruit is defined as ripened ovary, flower, or whole inflorescence. The origins of the fruit coat and the pericarp (Figure \(\PageIndex{15}\)) which is comprised of the exocarp, mesocarp, and endocarp, are mostly from the wall of the pistil.

Fruits can be simple, multiple, or compound. *Simple fruitssimple fruitfruit originated mostly from one pistil come from a single pistil (like cherry, Prunus). *Multiple fruitsmultiple fruitfruit originated from many pistils are formed from many pistils of the same flower (strawberry, Fragaria). A compound fruitfruit originated from the whole inflorescence: infrutescence (infructescense) would be a pineapple (Ananas) or fig (Ficus) which comes from multiple flowers (inflorescence).

Fruits can be dry or fleshy. An example of dry fruit is a nut like peanut (Arachis) or walnut (Juglans). Examples of fleshy fruits include apples (Malus) or oranges (Citrus).

Fruits also delegate dispersal function to their different parts. *Dehiscent fruitsdehiscentfruits which open (like canola, Brassica) open and delegate dispersal to individual seeds.

Figure \(\PageIndex{15}\) Scheme of drupe (e.g., peach) with three levels of pericarp. Note that pit is essentially endocarp + seed.

Indehiscent fruits (like papaya, Carica) will not open and will be dispersal units (diaspores) themselves.

Schizocarp fruits (like in spurge, Euphorbia or maple, Acer) are in between: they do not open but break into several parts, and each of them contains one seed inside. For example, maple fruit consists of two “wings”, each of them contains the part of fruit and one seed.

In addition, simple fruits could be monomerous (1-seeded) like nut or achene (sunflower, Helianthus), or bear multiple seeds (like follicle in tulip, Tulipa).

All these different variants have their own names partly described in the following table:

Type	Consistency	Opening	Example(s)
Simple	Fleshy	Indehiscent	Drupe, Berry, Hesperidium, Pome
Simple	Dry	Dehiscent	Capsule, Legume (pod), Silique (Figure 8.3.1)
Simple	Dry	Schizocarpic	Regma, Samara, Shizocarp
Simple	Dry	Indehiscent	Caryopsis (grain), Nut (incl. acorn), Achene
Multiple	Fleshy	Indehiscent	Multiple drupe
Multiple	Dry	Dehiscent	Follicle
Multiple	Dry	Indehiscent	Multiple nut
Compound	Fleshy	Indehiscent	Compound berry
Compound	Dry	Indehiscent	Compound nut