Chapter: Introduction to Botany: The Origin of Flowering


A flower (Fig. 8.7) is a compact generative shoot that is comprised of three zones: sterile (perianth), male (androecium), and female (gynoecium) .

The Flower


flower (Fig. 8.7) is a compact generative shoot that is comprised of three zones: sterile (perianth), male (androecium), and female (gynoecium) (Fig. 8.6). 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 (Fig. 8.10). 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:


FLOWER PARTS occur in whorls in the following order—sepals, petals, sta-mens, pistils


PEDICEL flower stem


RECEPTACLE base of flower where other parts attach


HYPANTHIUM cup-shaped receptacle (Fig.8.8)




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, formula: P


ANDROECIUM collective term for stamens: formula: A




ANTHER structure containing pollen grains


FILAMENT structure connecting anther to receptacle


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


1.              A single CARPEL = simple PISTIL


2.              Two or more fused CARPELS = compound PISTIL


3.              Two or more unfused CARPELS = two or more simple PISTILS


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

PISTIL Collective term for carpel(s). The terms CARPEL and PISTIL are equiv-alent 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 de-velops 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 symme-try, formula: B


ZYGOMORPHIC FLOWER A flower having only one plane of symmetry, formula:"

PERFECT FLOWER A flower having both sexes


MALE / FEMALE FLOWER A flower having one sex, formula (Fig. 8.9)


MONOECIOUS PLANTS A plant with unisexual flowers with both sexeson the same plant


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


SUPERIOR OVARY most of the flower is attached below the ovary, formula:G...


INFERIOR OVARY most of the flower is attached on the top of ovary, formula:G...


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 struc-ture 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 spe-cific letter, numbers of parts with digits, and some other features (whorls, fusion, position) with other signs:


The following signs are used to enrich formulas:


PLUS “+” is used to show different whorls;minus“–” shows variation; “V” = “or


BRACKETS “[]” and “()” show fusion


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

MULTIPLICATION “ x” shows splitting


INFINITY ¥” 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 (Fig. 8.11); formula of the flower shown there is


ABC model


All parts of flower have a specific genetic developmental origin explained in the ABC model (Fig.8.12). There are three classes of genes with expression whichoverlaps 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 - >  calyx


                  A + B  - >  corolla


                  C + B  - >  androecium


                  C alone  - >  gynoecium




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 fructifi-cations (flower units, FU) were very primitive and did not yet form a compacted flower, instead, there were multiple free carpels, and paired stamens (Fig. 8.13).


Another primitive flower is Amborella,a small forest shrub of New Caledonia, which is an island in the Pacific Ocean. These plants have 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 (Fig. 8.14). Taxonomically, both Archaefructusand Amborella belong to magnoliids.

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