Loading...
 

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.

Wild species of bananas share the same body plant as cultivated bananas, but differ in that they also produce seeds, in addition to suckers. The variability observed in morphological traits is used to characterize banana plants[2].

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 the banana plant's true stem. It 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.

The terminal growing point of the rhizome, the apical meristem, is a flattened dome from which the leaves and the inflorescence are formed.

Pseudostem

Main page on the banana pseudostem

The true stem is visible in the center of the pseudostem
The true 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 true stem[6] (which is often called the floral stem because it supports the inflorescence) 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.

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

Banana plants stripped of their leaves and leaf sheaths to reveal the true stem.
Banana plants stripped of their leaves and leaf sheaths to reveal the true stem.

The inflorescence is a complex structure that includes the flowers that will develop into fruits. It is supported by the aerial true stem, which is often called the floral stem[6]. The aerial true stem is produced by the terminal growing point on the rhizome. It grows through the pseudostem and emerges at the top of the plant soon after the last cigar leaf.

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.

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. The male flowers in the male bud produce pollen that may or may not be sterile. A third type of flowers called hermaphrodite, or neutral, may be present on the stalk between the female flowers and the male bud, traditionally called the rachis, but which some scientists argue is part of the peduncle[6]. They generally do not develop into fruit and their stamens do not produce pollen.

The inflorescence emerges at the top of the plant and soon starts pointing down, with the exception of Fei bananas.
The inflorescence emerges at the top of the plant and soon starts pointing down, with the exception of Fei bananas.
The bract lifts, or curls up at the tip, exposing the female flowers that will develop into fruit. The flowers are arranged in clusters, the future hands.
The bract lifts, or curls up at the tip, exposing the female flowers that will develop into fruit. The flowers are arranged in clusters, the future hands.
Below the female flowers, some cultivars possess hermaphrodite flowers, also called neutral flowers.
Below the female flowers, some cultivars possess hermaphrodite flowers, also called neutral flowers.
The last type of flowers to appear are the male flowers, which are also subtended by a bract.
The last type of flowers to appear are the male flowers, which are also subtended by a bract.
Photo diagram of the parts of inflorescence

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 starts at the first visible node and ends at the meristem in the male bud[6].  For what is traditionally called peduncle, they propose transitional peduncle because it supports organs that are in transition from leaves to bracts. They propose female peduncle for the section that supports the female flowers that become fruits, and male peduncle for the section that supports the nodes of male flowers, the traditional rachis.

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[7].

Rachis

The rachis traditionally refers to the part between the male bud and the last hand of fruit, or the first one depending on the author, but Australian scientists argue that it is the continuation of the peduncle, obviating the need for the ambiguous term rachis, which in botany has also been used for vegetative structures[6]. The part below the last hand (which they call the male peduncle[6]) can be bare or covered with persistent bracts. The scars indicate where the bracts were attached. They are called nodes.

Male bud

The male bud contains the male flowers enclosed in their bracts. It is sometimes called the bell. As the fruits mature, the rachis (or male peduncle[6]) and male bud continue to grow. 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. Blog post Would the true peduncle please stand up? published 3 March 2016 in Under the peel, the blog of the ProMusa community.

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