Floral
Structure And Inflorescence Type
Basic
Floral Structures
Most
identification and classification of flowering plants is based on floral
structure.
This laboratory will introduce you to the basics of floral structure
found in different members of the Anthophyta, and its terminology.
Flowering plants were once described as "Angiosperms"
which is derived from the Greek words "angeion" which means a
container, and "sperma" which means seed.
Probably the most distinctive structure of the flower is the carpel
(the container) which encloses and protects the ovules
which later develop into the seeds
i.e. after fertilization.
The
flower forms on a determinate shoot (one with limited growth).
This shoot bears both the sporophylls and sterile appendages.
Flowers may be clustered in various ways into aggregations called inflorescences.
The stalk of an inflorescence is called a peduncle,
whereas that of an individual flower in an inflorescence is called a pedicel.
The part of the flower stalk to which the floral appendages are
attached is termed the receptacle.
Flowers of most species contain two sets of sterile appendages, these are the sepals and petals. The sepals and the petals are attached to the receptacle below the fertile parts of the flower (the stamens and carpels). See Fig 1. The sepals occur below the petals and the stamens occur below the carpels. Collectively the sepals are known as the calyx, and the petals from the corolla. Together, the calyx and the corolla constitute the perianth. All of these structures are believed to have evolved from modified leaves. Usually, the sepals are green and the petals are brightly coloured, although there many flowers in which both parts are similar in colour.
The stamens are the microsporophylls. Collectively the stamens are called the androecium, which means "house of man". Most angiosperms have stamens which consists of two main parts; i) a slender stalk or filament, and ii) the four microsporangia or pollen sacs. The part of the stamen which is comprised of the pollen sacs is called the anther.
The
carpels are the megasporophylls.
Collectively the carpels are called the gynoecium
which, means "house of woman".
Carpels resemble leaves that are folded lengthwise, enclosing one or
more ovules.
A single flower may contain one or more carpels.
If several carpels are present, they may be either separate from each
other, or they may be fused together to various degrees.
Sometimes an individual carpel or a group of fused carpels is called a
"pistil".
Each carpel (or fused group of carpels) is differentiated into a lower,
middle and upper parts.
The lower part is called the ovary,
its function is to and enclose and protect the ovules.
The upper part or stigma is
exposed to pollinators, it receives the pollen and assists in its germination.
Carpels of many flowers possess a mid section which consists of a more
or less elongated tube-like stalk, the style,
which connects the stigma with the ovary.
The style elevates the stigma to a location where it is receptive to
pollen, it also provides a route and nutrition to the pollen tube of a
germinated pollen grain.
Many
variations exist to this basic flower design.
Species which have flowers containing both stamens and carpels are said
to be perfect.
The flower is said to be imperfect
if either stamens or carpels are absent.
Unisexual flowers are said to be either staminate
or carpellate.
If both staminate and carpellate flowers occur on the same plant (e.g.
corn, oats), the species is monoecious.
If they are found on separate plants (e.g. willows), the species is
said to be dioecious.
Any one of the floral parts (sepals, petals, stamens, carpels) may be
lacking from a flower.
Flowers with all four kinds of floral parts are called complete
flowers.
If any part of the flower is lacking, the flower is said to be incomplete.
Thus an imperfect flower is also incomplete, but not all incomplete
flowers are imperfect.
The floral parts may be either spirally arranged or whorled in arrangement with respect to their attachment to the receptacle. In addition, the place of attachment of the floral parts to the receptacle varies with respect to the position of the ovary (or ovaries). If the sepals, petals, and stamens are attached to the receptacle below the ovary, the ovary is said to be superior and the flower is said to be hypogynous. In some flowers with superior ovaries, the petals and the stamens are attached to the margin of a cup-shaped extension of the receptacle (the hypanthium). Such flowers are said to be perigynous (Fig.# 2). In other flowers the sepals, petals, and stamens apparently grow from the top of the ovary, which is therefore inferior and such flowers are said to be epigynous.
Finally,
the flower structure can have variations in symmetry.
In some flowers, the corolla is made up of floral parts of similar
shape and which radiate from the centre of the flower and are equidistant from
each other; that is, they are radially symmetrical.
Such flowers are said to be regular
or actinomorphic.
In other flowers, one or more members of at least one whorl of floral
parts (often the petals) are of a different form than other members of the
same whorl; that is, they are bilaterally symmetrical.
These flowers are said to be irregular
or zygomorphic.
Examine
the flowers on display and try to locate the flower structures described
above.
Also try and determine whether the flower is hypogynous, epigynous, or
perigynous; complete or incomplete; perfect or imperfect, and actinomorphic or
zygomorphic.
Placentation
The portion of the ovary where ovules originate and remain attached until maturity is called the placenta. In some flowers, the placentation is parietal; that is, the ovules are borne on the ovary wall or on extensions of it. In other flowers, the ovules are borne on a central column of tissue in a partitioned ovary with as many locules (chambers) as there are carpels. This is axile placentation. In others, the ovules are borne on a central column of tissue not connected by partitions to the ovary wall (free central placentation). Finally, in some flowers there is a single ovule at the very base of a unilocular ovary. This is basal placentation. See Fig.# 3.
Numerical
Plan of the Flower
Most
flowers are constructed upon a definite numerical plan.
In the monocotyledons (Liliopsida) the flowers usually have a numerical
plan of three (or multiples of three): e.g. the Tulip (Tulipa
spp.); 3 sepals, 3 petals, 6 stamens and 3 carpels.
These are call 3-merous
flowers.
Dicotyledon (Magnoliopsida) flowers are usually constructed on a
numerical plan of four or five and are referred to as 4-merous
or 5-merous
flowers.
The numerical plan of the flower is most evident in the sepals and
petals.
To some extent it is carried through to the stamens, but this feature
of the floral design does not often apply to carpels.
Examine
the slide of the cross-section of monocot-dicot flower buds.
Note the numerical plan for each.
Floral
Diagrams
A method of quickly summarizing the more important structural characteristics of a flower in a pictorial form is the use of the floral diagram. A floral diagram represents a cross-section of a flower as it would appear if all parts were at the same level. It might also be thought of as a sort of aerial view of a flower in diagrammatic form. For uniformity and convenience, the various parts are represented in diagrams by standardized symbols. The symbols and some examples are illustrated below.
As
an exercise, take one of the flowers from those on the side bench, determine
the floral structure and draw a floral diagram.