Indeterminate Inflorescences
In indeterminate inflorescences, the
terminal bud continues to grow. Lateral flower buds produce flowers in apical
succession. That is, the lower or outer flowers are older than the upper or
inner flowers. Therefore, the flowers open from the outside in; that is,
"centripetal." When the floral axis is flattened, the opening is
directed from the outside in.
Indeterminate inflorescences are
classified into three main groups: the first group has an elongated main axis,
the second group has a dwarf main axis, and the third group has a flattened,
concave, or convex main axis.
Simple raceme: The main axis of the raceme inflorescence is elongated. It bears a number of lateral, stalked flowers. The lower flowers have longer stalks than the upper ones, as in lupin, garden cress (Eruca sativa), and snapdragon (Figure 1). When the main axis branches into lateral branches, and the lateral branches bear flowers, the inflorescence is known as a compound raceme (panicle), as in grapes (Figure 2). If the inflorescence contains the main axis and lateral branches, the main axis and lateral branches are called a peduncle. The stalks that bear the flowers of the inflorescence are called pedicels. In the case of ray flowers, the stalk is also known as a peduncle. When the peduncle flattens and takes the shape of a concave platform, as in artichokes (Figure 3), or takes a pear-shaped form, hollow, as in the fig. The unbranched, leafless flower stalk that arises from the underground stems is surrounded by cotyledonary leaves and terminates in a single flower, as in the lotus. Or it bears many flowers, as in the onion, and is known as a root stalk or scape (Figure 4).
The spike: The main axis of the spike elongates. The larger, lower flowers open earlier than the upper ones, as in the racemose inflorescence. However, the flowers here are sessile, i.e., without stalks, as in the cockscomb [Celosia argentea var. cristata plant] (Figure 5).
Comb-like: In a comb-like inflorescence, the
main axis is relatively short. Lower flowers have longer stalks than upper
flowers, as in Matthiola and lantana (Figure 9). Therefore, the flowers appear
to be at the same level.
Umbel: The umbel has a short primary axis.
At its apex, it bears a cluster of flowers. Each flower arises from the axil of
a bract. The bracts form a periphery known as the involucre. The umbel may be
branched. Each branch bears several flowers, in which case it is known as a
compound umbel, as in anise and khella [Ammi visnaga] (Figure 10). It
is rarely unbranched, and the primary axis bears the flowers directly.
Head or Capitulum:
Its primary axis is limited and flat.
Large florets are located on the outside, and small ones on the inside. The
outer florets open earlier than the inner ones, as in sunflowers. Although the
inflorescence appears as a single flower, it is composed of a large number of
sessile flowers (florets). There are two types of florets: ray florets and disc
(tube) florets. The head may consist of only one type of floret. The
inflorescence is surrounded by one or more green bracts known as the involucre
(Figure 11). This inflorescence is common in the Asteraceae family and is also
found in Acacia (Figure 11b) and in sensitive plants such as Mimosa.
Cymose Inflorescences
In cymose inflorescences, the main
axis terminates in a flower. The lateral axis, which arises below it, also
terminates in a terminal flower. Therefore, its growth is halted. The flowers
may be stalked or sessile. Regarding the development of flowers in a cymose
inflorescence... The flowers develop in a basile sequence, with the largest
apical flower followed by the smallest lateral flower. They open centrifugally,
meaning they open from the center outwards. An indeterminate inflorescence can
be uniparous, biparous, or multiparous.
- Uniparous Inflorescence
In this type of inflorescence, the
main axis terminates in a terminal flower. It gives rise to one lateral branch,
which also terminates in a terminal flower. Each of the main branch, the
lateral branch, and subsequent branches gives rise to only one branch. Thus,
two types of Unipparous Inflorescence inflorescences are observed: helicoid and
scorpoid (Figure 12). When the lateral branches arise on the same side and the
bracts on the opposite side, the axis appears curved. Therefore, it is called a
helicoid uniparous inflorescence, as in begonias, sedges, and many plants
belonging to the Solanaceae family. Alternatively, when the branches emerge
from both sides consecutively, the axis appears straight or zigzagging, as in
the flax inflorescence.
In this type of inflorescence, the
main axis bears two lateral branches. Each of these branches terminates in a
flower, meaning the inflorescence bears three flowers. In this case, the
inflorescence is called a simple biparous inflorescence. In a compound biparous
inflorescence, the two lateral flowers are replaced by two biparous
inflorescences. This pattern of branching may be repeated several times, as in
Bougainvillea (Figure 13).
- Multiparous inflorescence
In this inflorescence, more than two
branches emerge surrounding the central flower, each ending in a flower, as in Eucalyptus,
Callisia, and Geranium (Figure 14). This inflorescence
differs from the umbel in that the oldest flowers are located in the center,
unlike in the umbel, where the largest flowers are located outwards.
- Mixed Inflorescences
The main axis branches indefinitely,
while the lateral branches branch indefinitely, or vice versa, as in the Olive
Family (Figure 15). The main branch bears an indeterminate inflorescence, and
the lateral branches bear determinate inflorescences. Similarly, in the
Boraginaceae family, the main inflorescence is racemose, and its branches are
uniparous. In the grapevine, the main branch bears a racemose inflorescence
(Figure 2), while the lateral branches bear indeterminate branched
inflorescences.
The main axis branches into lateral
branches that bear flowers. There are various forms of compound inflorescences:
compound racemose, as in the Poinciana (Delonix regia); compound spike, as in
wheat; and compound spadix, as in dates (Figure 16a). Compound comb-like, as in
the Candytuft [Iberis] flower. Compound umbel, as in coriander. Compound head,
as in the Camel Thorn flower (Echinops) (16b).
Other types of inflorescences:
- Cyathium:
A highly reduced, limited
inflorescence consisting of a single female flower at the tip. It is surrounded
by five groups of male flowers, each group being a scorpioidal inflorescence.
Each group is located in front of a bract. The five bracts surround the
inflorescence and fuse together to form a cup. These bracts are alternately
accompanied by crescent-shaped glands, either three or four, as in Euphorbia
peplus. In the case of Poinsettia, there is only one large gland. Each
male flower consists of a single stamen (Figure 17).
- Verticillate Inflorescence:
The verticillate inflorescence is a
determinate, compound inflorescence. The flower stalks are so short that they
appear sessile. It consists of two opposite inflorescences, each emerging from
a leaf axil on a stem. Each inflorescence then branches into two branches,
completely enclosing the stem, as in basil (Figure 18) and some species of the
mint family (Lamiaceae).
Syconium Inflorescence:
In the syconium inflorescence, the
flower axis, or spadix, is fleshy and hollow. The flowers are located inside.
The inside of the inflorescence is connected to the outside by an opening at
the top, which is often covered by a small leaf. Insects that perform
pollination enter through this opening. The flowers borne in the syconium
inflorescence are usually unisexual (Figure 19).
References
Benlloch R, Berbel A, Serrano-Mislata A, Madueno F. 2007.
"Floral Initiation and Inflorescence
Architecture: A Comparative View". Annals of Botany100
(3): 659–676.
Kirchoff BK, Claßen-Bockhoff R. 2013. "Inflorescences:
concepts, function, development and evolution".
Annals of Botany. 112 (8): 1471–6.
Tucker SC, GrimesJ. 1999. "The inflorescence:
Introduction". The Botanical Review. 65 (4):
303–316.
WyattR. 1982. "Inflorescence Architecture: How Flower
Number, Arrangement, and Phenology Affect Pollination
and Fruit-Set". American Journal of Botany. 69
(4): 585–594.
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