rumex obtusifolius
rumex obtusifolius
rumex cristatus
Flower Development
A fruit of Rumex crispus (Curled Dock), a member of the Dock Family
(Polygonaceae) consists of three petals, each usually bearing a tubercle of
reddish corky tissue which aids dispersion of the fruit in water by acting as
flotation devices. Inside the triangular pyramid of the three petals is the fruit
proper, which is a hard three-angled nut.
Plants are so incredibly diverse in their forms and the morphology of their parts that making sense of them is tricky
for the student of botany. However, these things make much more sense when one realises the origins of plant
organs or appendages, such as leaves, sepals, petals, stamens and carpels. All these structures are essentially
modified leaves. Here we illustrate flower development using
Rumex crispus as an example, because this plant is
common and easy to find and illustrates some interesting peculiarities, and also because  Dudgeon conducted a
good study of flower development in this species in 1918 (
Botanical Gazette, Vol. 66, No. 5, pp. 393-420).
Left: fruit of Rumex obtusifolius (Broad-leaved Dock)
has narrower and more triangular petals with at least
one bearing a tubercle.
Rumex species are diverse and
differences in morphology assist the identification of
these different species in the field.
Left and right: the flowers of Rumex
obtusifolius
. The petals are green
but their protuberances have an
attractive reddish tinge. The flowers
are borne on the inflorescence which
is invested in sheaths of developing
bracts when young, high;y branched
and with its young parts covered in
protective mucilage. The flowers
develop in clusters at each node.
The oldest flowers areearest the
stem, the younger flowers outside
these on the upper surface of the
swollen nodes.
In flowering plants, each flower is a modified branch in which the internodes are very short and the 'leaf'-bearing
nodes close together, such that the distinction between nodes and internodes is usually lost. The leaves of these
nodes are highly modified to form the floral appendages. The branch typically emerges from the axil of a modified
leaf called a bract (or secobdary bract if we consider the bracts beneath the inflorescence as primary bracts). The
bottom-most node of the floral stem develops into the outer sepals. Within the sepals the petals develop at the
next node, followed by the stamens (male reproductive organs) and finally the carpel or carpels (female organs).

In
Rumex crispus these appendages develop from the outside towards the centre (i.e. the bottom-most node
develops its appendages first) but the development of the petals and carpels is delayed. There are three sepals,
which begin developing first as three thick prominences which grow rapidly over the yound flower.
Left: a diagram of a vertical section through a
developing flower of
Rumex crispus based on
Dudgeon (1918). The developing organs begin
as protuberances or
primordia. N = nucellus; S
= sepal; St = stamen; P = petal. The
nucellus
will form the
megasporangium or spore case in
which the
megaspore will later develop. The
megaspore gives rise to the female
gametophyte or haploid generation which
develops as the pollen sac containing the egg
cell or
ovule.

Before reading any further it may help to review
the structure of the carpel in our
Flowers exhibit.
Next the three petals begin to develop more or less at the same time as the 6 stamens. The stamens develop in 3
pairs. The stamens in each pair are joined at their bases which also join to the base of a sepal, such that a pair of
stamens join to each sepal. This suggests that each pair of stamens represents a single modified leaf. The carpels
begin to develop as a thick ring around the base of the nucellus, which represents the modified shoot tip and which
will form the embryo sac. Initially the nucellus is not enclosed by the carpels surrounding it. The carpels form
initially as a continuous ring with three growing tips (suggesting that the carpels are three modified leaves). The
mature
gynoecium (female structure) is usually described as three fused carpels. These three growing points will
later enclose the nucellus when the megaspore mother cell has formed and enlarged within it. This forms the ovary
wall surrounding the
ovarial cavity containing the developing ovule. The three points of the enclosing carpels
form the three
styles and stigmas.
Left: a vertical section through a
generalised carpel. The developing
carpel appendage(s) form the ovary
wall, style(s) and stigma(s). Within
them is the ovarial cavity containing
the ovule.

In
Rumex crispus, the enclosed ovule
can be interpreted as the growing
shoot tip, with the integuments
representing the most terminal leaves.
The three styles curve backwards and
terminate in branched stigmas which
are held between the anthers. The
embryo sac is the haploid
gametophyte formed by meiosis. In
meiosis, the nucleus doubles the
number of chromosomes and then
divides twice, resulting in four haploid
daughter cells (haploid means they
contain half the normal number of
chromosomes, that is a single set).
Meiosis is therefore described as a
reduction division.

In
Rumex crispus, the innermost
integument appears during prophase
of the first meiotic division and closes
over to form the micropyle. The outer
integument appears during the second
meiotic division. Both integuments
consist of a double layer of cells.
The diagrams below summarise the arrangement of parts in the developing flower of Rumex crispus.
The developing flower of Rumex
Above: horizontal sections through a developing flower of Rumex crispus (the section on the right is lower
down than the section on the left) based on Dudgeon, 1918. C = developing carpel (three carpel valves
around the central developing nucellus); N = nucellus; P = petal; S = sepal and St = stamen.
Development of the embryo sac and megasporogenesis

The cells of the outer layer of the outer integument and the ovary epidermis thicken and lose their contents to form a
continuous impervious layer around the ovule. The chalaza remains as the only region for nutrients to reach the
developing ovule. The terminal cell of the axial row of the nucellus develops into an archespore which divides by
mitosis to form the primary parietal cell and the
megaspore mother cell (MMC). The MMC divides by meiosis to form
a tetrad of four haploid cells, the innermost of which becomes the
megaspore and develops by mitosis into the
haploid embryo sac. The megaspore elongates, develops a vacuole at each end, and then divides by mitosis three
times to form the 8 nuclei of the embryo sac. The primary parietal cell divides twice by mitosis to form a cap of 4 cells.
In the haploid state, each cell has one complete set of 32 chromosomes, giving a diploid number in the parent plant of
64 chromosomes. To review the quite complex embryo sac development and fertilisation in flowering plants visit our
exhibit on
flowers.

Development of the stamens

The stamens begin as ovoid primordia each of which develops into an anther borne on a filament. Within each pollen
sac, the archespores each divide into a primary parietal cell, which goes on to develop the pollen sac wall, and a
primary microsporogenous cell which undergoes three or four mitotic divisions to produce sporogenous tissue. The
cells of the sporogenous tissue each undergo meiosis to produce a
tetrad of four cells surrounded by a common
cellulose cell wall. Each tetrad develops into four
microspores. The nucleus of each microspore divides into a tube
cell nucleus
and a generative nucleus. The generative nucleus forms a small cell whose nucleus divides into two
haploid male nuclei.

The innermost cell layer of the anther, enclosing the developing pollen (microspores) is the
tapetum which acts as a
nutrient layer and consists of binucleate or multinucleate cells in
Rumex crispus. The epidermal cells of the developing
anther enlarge, except where the two compartments of the anther (loculi) join, where they remain small and form a line
of weakness along which break down to join the two loculi into a single
pollen chamber and then to cause the anther
to split open during
dehiscence (splitting open upon drying), opening to release the pollen.

The mature flower

The three sepals and three petals of the Rumex flower are often referred to as tepals, meaning that it is not certain
which are petals and which are sepals. However, Dudgeon concluded, from his detailed study, that the tubercles are
borne on the petals, with the sepals remaining as tiny flaps at the bases of the petals. The petals enlarge considerably
as the flower matures, whilst the sepals remain small. On the basis of his observations, Dudgeon concluded that
fertilisation in
Rumex crispus is probably rare with the embryo usually developing by apogamy (without fertilisation). He
reported never observing normal-looking pollen and pollen grains on stigmas were never seen to germinate but
appeared instead to be degenerating. The stigmas withered early, suggesting they may not be able to support pollen
tube growth (unless pollen tube growth inhibits stigma withering) and no pollen tubes were observed. Finally,
fertilisation was not seen. However, haploid embryo sacs were seen, though it is possible that some remain diploid due
to failed meiosis in the MMCs and maybe these develop into embryos without fertilisation.

Genetic Control: The ABC of Flower Development

How exactly does a primordium know whether or not to become a leaf, sepal, petal, stamen or carpel? The process of
morphogenesis, the development of form and shape in living organisms has fascinated people since times immemorial.
The mystery of life is its ability to 'spontaneously' form from basic elements and then pass on its pattern to offspring.
Though some of the mystery still remains, science is rolling back the veil and considerable insight has already been
obtained. Flowers make good subjects for study in attempting to answer such questions, because of their symmetry
and the remarkable modifications that ancestral leaves have undergone in order to effect efficient reproduction.

Homeotic genes are genes regulating the structure and morphology of organisms and their parts. The ABC model
relies on different genes to be expressed (switched on) in different floral organs.

      Thus, expression of the A genes gives rise to sepals;
      Expression of the A and B genes gives rise to petals;
      Expression of the B and C genes gives rise to stamens;
      Expression of the C genes gives rise to carpels.

The exact genes belonging to each group (A, B or C) depends on the plant. In
Arabidopsis, for example, the A genes  
are two genes called apetal1 and apetal2, but in
Antirrhinum one identified A gene is called ovulata. In the absence of
one or more of the ABCs things go awry! This can happen if the gene is faulty, missing or switched off when it should
be on.

For example:  

In flowers lacking the B genes, the second whorl (W2, counting from the base upwards or outside-in) of floral parts
(normally petals) develop into sepals while W3 (usually stamens) develops into carpels!

In flowers lacking the C genes, the first whorl (W1, normally sepals) develops into sepals as expected, W2 develops
into petals as expected, but W3 also develops into petals (instead of stamens) and whorl 4 develops into new flowers
(instead of carpels) each possessing a whorl of sepals around two inner whorls of petals. Not surprisingly, one C gene
in
Arabidopsis is called agamous since flowers lacking it can not develop sexual organs and so develop no gametes!
(In
Antirrhinum the C gene is called plentiflora).

Things, of course, are not quite so simple and a more advanced model of the genetic control of flower development in
Arabidopsis is the ABCDE model. In this model, A and E genes are both needed for sepal development; A,B and E for
petal development; B, C and E for stamen development and C and E for carpel development.
Article updated:
11/10/14
13/01/16 ...