Banana taxonomy and identification: the petiolar canal
The shape of the petiolar canal in banana is an important character for the taxonomic determination of the plant. It has long been used as one of the fifteen characters defined by Simmonds and Shepherd in 1955 to distinguish acuminata from balbisiana types. In particular, this trait makes it possible to assess the relative presence of the two species Musa acuminata and Musa balbisiana in cultivated bananas. The vast majority of cultivated bananas are in fact the result of hybridizations between these two species, or between subspecies of Musa acuminata, although the two phenomena are not exclusive. You can consult the article on banana naming for a complete review of their classification, and it's a good idea, in order to understand the rest of what's explained here.
Note: the shape of the petiolar canal explained here only applies to forms derived from the two species Musa acuminata and Musa balbisiana. In particular, it is of no use for other species, generally wild and/or ornamental (Musa itinerans, Musa velutina, Musa textilis, Fe'i, etc.).
We observe the shape of the petiolar canal section. Two elements are involved: the general shape of the canal, and the presence of a thinner margin at the edges, sometimes with more or less dry wings. There is a gradient between completely open shapes, with outward-facing margins, and, on the other hand, entirely closed shapes, even overlapping.
Open types with margins are linked to the Musa acuminata species, while closed types are linked to Musa balbisiana. In between, there are intermediate forms. Wild types perfectly illustrate the two extremes of the character. In the case of Musa acuminata, this applies to all subspecies, including malaccensis, siamea, burmannica, zebrina and banksii, which, although sometimes elevated to species status, is closely related to Musa acuminata, as all genomic analyses have shown for a long time.
As explained in the article cited above, the vast majority of cultivated forms derive from Musa acuminata and Musa balbisiana, and their genomic composition can be described in terms of the proportions of these two ancestors. Cavendish bananas are exclusively derived from Musa acuminata (AAA composition), as are Red bananas and East African Mutika bananas. In these types, the petiolar canal is open, with outward-facing margins. This is also the case for diploid cultivated types (AA composition) such as Sucrier or Mshare.
At the other end of the spectrum are ABB banana plants (67% balbisiana and 33% acuminata), such as Namwah and Orinoco, the best-known examples. Here, the canal is closed and sometimes overlapping.
In between, there are plants with less open channels, or with straight edges, sometimes a little curved. These are all AAB composition types. These include Prata (Hawai, Brazil...), Mysore (India), Silk (e.g. Lady Finger), Plantain (Central Africa, Central America) and many others.
Please note: In iNaturalist, all ABB and AAB types appear under the name Musa × paradisiaca, without distinction.
The shape of the petiolar canal is therefore an important characteristic of banana taxonomy. To observe it, it's always best to look at several leaves on a plant, and even at the leaves of the plant's suckers. There can always be variation in natural conditions, due to the effect of the environment. The assessment of the character must therefore be a sort of "average" or compromise between the various observations. And then, as always, there are (fortunately rare) exceptions. Finally, banana taxonomy is not based on this single trait, and definitive identification involves taking many other traits into account.
References
Simmonds, N. W, et Kenneth Shepherd. 1955. "The taxonomy and origins of the cultivated bananas". J. Linn. Soc. (Bot.) 55:302‑12.
International Plant Genetic Resources Institute (IPGRI); Centre de Cooperation Internationale en Recherche Agronomique pour le Developpement (CIRAD); International Network for the Improvement of Banana and Plantain (INIBAP). 1996. "Descriptors for banana (Musa spp.)". International Plant Genetic Resources Institute 55 p.
