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Morphology of the banana plant


Banana mat
Drawing of a banana mat showing the 'true' stem (shown in blue) inside the pseudostem. The clump formed by the fruit-bearing parent plant, its suckers and the rhizome is called a mat.
Drawing of a banana mat showing the 'true' stem (shown in blue) inside the pseudostem. The clump formed by the fruit-bearing parent plant, its suckers and the rhizome is called a mat.

The banana plant is a tree-like perennial herb. It is an herb because the aerial parts of the parent plant die down to the ground after the growing season. It is a perennial because one of the offshoots growing at the base of the plant, the sucker, then takes over. The parent plant and its suckers form what is commonly called a mat, or stool. The botanical term is genet[1].

What looks like a trunk is in fact a pseudostem made from tightly packed leaf sheaths. Most of the 'true' stem is inside the pseudostem. In a fruiting plant, it starts on the rhizome and ends with the meristem in the male bud (if present).

The variability observed in morphological traits is used to characterize banana plants[2]. Wild species of bananas share the same body plant as cultivated bananas, except that they reproduce through both seeds and suckers.

Root system

The root system is the means by which the plant takes up water and nutrients from the soil.

The roots are produced by the underground structure called a rhizome[3]. The primary roots originate from the surface of the central cylinder (see below), whereas secondary and tertiary roots originate from the primary roots.

Rhizome

Banana rhizome schema

The rhizome is commonly referred to as a corm, and occasionally as a bulb, but the botanically correct term is rhizome[4]. Rhizomes are characterized by horizontal underground growth; production of roots from multiple nodes; and production of clonal plants[5]. Corms, on the other hand, are vertical enlarged compact stems with a tunic of thin leaves and roots arising from a single node; features that do not describe well the banana plant's underground structure.

In the vegetative phase, the terminal growing point of the rhizome, the apical meristem, has the form of a flattened dome. At the transition from the vegetative to the floral stage, the meristem area becomes convex and rises above the surrounding leaf bases. Flower bracts appear in place of leaves. Following the formation of the flower, the aerial stem[6] begins to develop and carries the flower and leaf upwards, eventually emerging at the top of the pseudostem.

Pseudostem

Main page on the banana pseudostem

The stem is visible in the center of the pseudostem
The stem is visible in the center of the pseudostem

The pseudostem is the part of the plant that looks like a trunk. This 'false stem' is formed by the tightly packed overlapping leaf sheaths. The pseudostem continues to grow in height as the leaves emerge one after the other and reaches its maximum height when the aerial stem[6] emerges at the top of the plant.

Even though the pseudostem is very fleshy and consists mostly of water, it is quite sturdy and can support a bunch that weighs 50 kg or more.

Stem

Banana plants stripped of their leaves to reveal the stem.
Banana plants stripped of their leaves to reveal the stem.

The stem provides support to the leaves, and flowers and fruits. The leaves or flowers are attached to a node, and the sections between nodes are internodes. The stem develops from the apical meristem on the rhizome. Inside the pseudostem it is called the aerial stem[6]. When it emerges at the top of the plant, it is called the peduncle.

The leaves are attached to the aerial stem (erroneously called floral stem[7]), whereas the flowers and fruits are attached to the peduncle.

Leaf

Main page on the banana leaf

Photo diagram of leaf morphology

The leaf is the plant's main photosynthetic organ. Each leaf emerges from the center of the pseudostem as a rolled cylinder (see cigar leaf below). The distal end of the elongating leaf sheath contracts into a petiole, that is more or less open depending on the cultivar. The petiole becomes the midrib, which divides the blade into two lamina halves. The upper surface of the leaf is called adaxial while the lower one is called abaxial.

The first rudimentary leaves produced by a growing sucker are called scale leaves. Mature leaves that consist of sheath, petiole, midrib and blade are called foliage leaves.

Lamina veins run parallel to each other in a long S shape from midrib to margin. Veins do not branch, which results in leaves tearing easily.

Cigar leaf

cigar_leaf.jpg

The cigar leaf is a recently emerged leaf still rolled as a cylinder.

The lapse of time in which a leaf unfolds varies. Under favourable climatic conditions, it takes about seven days, but it can take up to 15 to 20 days under poor conditions.

The new leaf is tightly coiled, whitish, and particularly fragile.

The extension at the tip of the leaf is called the precursory appendage. After emergence, it withers and falls off.

Sucker

Main page on the banana sucker

From left to right: water sucker and sword sucker
From left to right: water sucker and sword sucker

A sucker is a lateral shoot that develops from the rhizome and usually emerges close to the parent plant. Other names for sucker are keiki (in Hawaii) and pup.

A sucker that has just emerged through the soil surface is called a peeper. A full grown sucker bearing foliage leaves is called a maiden sucker.

Morphologically, there are two types of sucker: sword suckers (right on the photo), characterized by narrow leaves and a large rhizome, and water suckers (left on the photo), which have broad leaves and a small rhizome. Water suckers have a weak connection to the parent plant and as such will not develop into a strong plant.

The number of suckers produced varies with the type of cultivar. The sucker selected to replace the parent plant after fruiting is called the follower or ratoon.

Inflorescence

The inflorescence is a complex structure that includes the flowers that will develop into fruits. The botanical term for the banana inflorescence is a thyrse[8] (an inflorescence in which the main axis continues to grow and the lateral branches have determinate growth[9]).

Female flowers
Female flowers

The female (pistillate) flowers appear first. In cultivated bananas, the ovary develops into a seedless fruit by parthenocarpy (without being pollinated). As it lifts, the bract (a modified leaf associated with a reproductive structure, such as a flower) exposes a cluster of female flowers that are normally arranged in two rows. These flowers will develop into a hand of fruit. The number of hands in the bunch depends on the number of female clusters in the inflorescence, and varies depending on the genotype and environmental conditions.

Male flowers
Male flowers

As the female flowers develop into fruit, the distal portion of the inflorescence elongates and produces clusters of male (staminate) flowers, each subtended by a bract. In cultivated bananas, the male flowers in the male bud produce pollen that is more or less fertile.

Neutral flowers
Neutral flowers
A third type of flowers called hermaphrodite, or neutral, may be present on the stalk between the female flowers and the male bud. They generally do not develop into fruit and their stamens do not produce pollen.
 

Peduncle

Traditional names given to parts of the peduncle.
Traditional names given to parts of the peduncle.

In botany, the peduncle is the stalk that supports the inflorescence. Yet, in the Descriptors for bananas, the peduncle refers only to the stalk between the leaf crown and the first hand of fruit, whereas the stalk that actually supports the female and male flowers is called rachis[2].  Australian scientists argue that in keeping with the botanical definition of the term, the peduncle extends to the meristem in the male bud and is composed of three sections[7]

Transitional peduncle

The transitional peduncle supports organs that are in transition from leaves to bracts: sterile nodes with a bract that abscises at bunch emergence. It corresponds that what is traditionally called the the peduncle.

Female peduncle

The  female peduncle  supports the female flowers that become fruits.

Bunch

The bunch is the descriptive term that includes all the fruits. The fruits are arranged into hands, the former clusters of flowers that were each subtended by a bract. By analogy, the fruits in a hand are often called fingers. The largest bunch, according to Guinness World Records, weighed in at 130 kg[10].

Male peduncle

The male peduncle  supports the male flowers in the male bud. It corresponds that what is traditionally called the rachis, an ambiguous term that in botany has been used in relation to both vegetative and reproductive parts, whereas the term peduncle is only used for stems that support flowers[7].

The part above the male bud can be bare or covered with persistent bracts. The scars (nodes) indicate where the bracts were attached. The male peduncle continues to grow as the fruits are maturing.

Male bud

The male bud contains the male flowers enclosed in their bracts. It is sometimes called the bell. In some cultivars, the male bud ceases to grow after the fruits have set and can be more or less exhausted by the time the bunch reaches maturity. The presence or absence of the male bud is one of the traits used to distinguish cultivars.

References

1. Clonal colony or genet, retrieved 20 July 2017
2. 1996. Descriptors for banana (Musa spp.). IPGRI, Rome (ITA). 55p.
4. Robinson, J.C. and Galán Saúco, V. 2010. Bananas and plantains. Crop production science in horticulture. CABI, Wallingford (GBR). 297p.
5. What is a rhizome, retrieved 16 March 2016
6. Skutch, A.F. 1932. Anatomy of the Axis of the Banana. The Botanical Gazette, 93(3):233-258.
7. Blog post Would the true peduncle please stand up? published 3 March 2016 in Under the peel, the blog of the ProMusa community.
8. Kirchoff. B.K. 1992. Ovary structure and anatomy in the Heliconiaceae and Musaceae (Zingiberales). Canadian Journal of Botany, 70(12): 2490-2508.

Also on this website

Would the true peduncle please stand up? published 3 March 2016 in Under the peel, the blog of the ProMusa community

Musa ontology developed for the Generation Challenge Progam Crop Ontology