The Difference Between a Bulb and a Corm

On Bulbs and Corms

06 May

Posted By kristi 0 Comment(s) 58 View(s) Horticultural Bits , Botanical Bits

First published 6th May 2025

On Bulbs

“Bulb” in the botany world has a very precise meaning — it is the underground stem (yes, stem) of a plant, made of fleshy leaves and used for food storage during dormancy.

A bulb is an organ. Other plant organs with similar roles as bulbs include corms, rhizomes and tubers, but these are structurally different to one another as well as to bulbs.

As corms look very similar to bulbs at first glance, these and their differences to bulbs will be covered below.

Here is a classic botanical bulb, a red onion (Allium cepa), showing the dried leaf remnants at top:

and the root remnants below:

Let’s do a longitudinal section (a cut through the vertical), and label what we see:

The basal plate is a highly reduced stem, from which grow the fleshy modified leaves (called scales*) to form what we’d call an onion. These are actually the leaf bases (bottoms) — the part which attaches directly to a stem or to the stem via a stalk called the petiole.

* Scales aren’t unique to bulbs. They are simply modified leaves with a protective role, and tend to be reduced in size and of a different texture to foliage leaves. Scale leaves can be drier, more papery, more fleshy, or even more leathery than the foliage leaves. You may know scale leaves as the tiny protective leaves covring the buds of fruit trees through winter.

These scales store the sugars this bulb would draw on after winter dormancy, to shoot new leaves from the top of the basal plate and new roots from underneath the basal plate. Eventually a seeding flower stalk would also grow from this plate. (Many bulbs cycle between a vegetative stage (growing a bulb) and a reproductive stage (growing a flower with seeds).

The very thin papery outer layer is called the tunic — not all bulbs have tunics, but those that do are called tunicate bulbs.

On Corms

“Corm” in the botany world also has a very precise meaning. A corm at first glance looks very much like a bulb, and is often mistaken for, and called a, bulb. This is easy to do, as a corm is also both an underground stem of a plant and a storage organ. Some corms also have tunics.

Many people may never have even heard the word “corm” to know to use it at all.

Below are saffron (Crocus sativus) corms, and the external similarities to a bulb are evident. It is very easy to identify the leaf end from the root end. The long, vertical fibres following the shape of the corm from root end to leaf end are the tunic:

However, intenally, while a bulb is a stem made of fleshy layers of modified leaves, a corm is a stem made of solid tissue.

Another cormous species is the water chestnut (Eleocharis dulcis). The plant here is 600 mm in length but can easily get to a metre:

Here are two corms, and the one on the left also has the remains of two daughter plants above it:

Corms also have scale leaves:

And here is a longitudinal section, showing the more starchy solid tissue interior:

Tags: bulb, corm, difference, identification,

Short Bio

Kristi Ellinopoullos
BSc(Hons), U.Syd. - double major in biochemistry and microbiology, with honours in microbiology
PhD, U.Syd - soil microbiology
Stumbled into IT and publishing of all things.
Discovered jujube trees and realised that perhaps I should have been an agronomist…
So I combined all the above passions and interests into plant-related websites and blogs, on which I write about plants, gardening, botany, soil chemistry, soil microbiology and biochemistry!

If you have any deep interest in microbiology and/or biochemistry, and how these apply to plants, I’m writing a deep-dive online book From Soil to Fruit here.