Difference Between Corm and Bulb

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Introduction

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A Gladiolus corm and an onion bulb


What are Corms and Bulbs?

The stem is modified into various forms in different plants to suit their adaptation needs. These modifications can be underground, aerial, or even sub-aerial. Underground modifications of stems consist of rhizomes, bulbs, corms, and tubers. They may resemble the plants' roots, but they differ in the presence of nodes and internodes, leaves, and buds. The main reason for the development of stem modifications is to allow the plant to survive in all kinds of environments and habitats, which are distinct from the regular ones.

Nature has modified stems and made them capable of performing the functions of food storage, plant support, protection, and vegetative propagation. These types of plants come under the category of Geophytes. Both corms and bulbs need a rest period that follows a time of active growth. Garlic is an example of a bulb, while corm examples are crocus, gladiolus, autumn crocus, etc.

Let us study two such underground stem modifications, the corm, and the bulb, and the difference between corm and bulb.

What is a Corm?

A corm is a swollen, underground plant stem modification that is present in some plants. Corms are built for nutrients and food that the plant can use in due course of time. These nutrients are generally stored in the corm during one growing season. They help produce various parts of the plants, like, roots, leaves, and flowers, for the next growing season of the plant.

Corms look similar to bulbs but are different from them as they do not possess the layered scales present in the bulbs. Corms reproduce via cormlets or individual corms. These cormlets can be separated from the parent to produce exact copies of the plant.

Corms are flattened or slightly round in appearance. Roots can grow from the corm base under the soil. In most cases, the parent corm dies back, and the corm lets it become the source of the plant the next year. The corm has membranous to scaly leaves and absorbs nutrients. It stores them for the next season. The corm can be divided and transplanted into other areas of the garden for vegetative propagation. Gladiolus, crocus, and crocosmia are classic corm examples.


What is Bulb?

A bulb is another stem modification. Most perennial monocotyledons have this form of a stem that provides a resting stage to their seeds. A bulb has a large, central bud that grows underground and is usually globe-shaped. The center of the bulb contains a bud that produces leaves and flowers. It also has membranous or fleshy leaves that arise from a short stem. A cluster of adventitious roots arises from the bulb base. The lateral buds give rise to shoots of the bulb. The plant’s food is stored in the leaves. New bulbs replace the old bulbs as they grow. 

Onion is the best example of a bulb, apart from garlic. Lily plants, tulips, and hyacinths are also bulb plants. Some bulbs are grown for their decorative flowers, like, lily, tulip, and iris.


Difference Between Corm and Bulb

Now that we know what is a corm and a bulb and how they store food let us try to distinguish between the two. The table given below provides the difference between corm and bulb.

Feature

Corm

Bulb

Shape

Spherical, elongated, or compressed

Spherical or Pyriform

Structure

Round and swollen stem base with scaly leaves

Short stems with fleshy leaf bases

Type

Underground stem modification

Condensed shoot that grows underground

Stem

Enlarged and swollen

Conical shape

Buds

External buds

Internal buds

Adventitious Roots

Develop all over the stem

Develop on the ventral side

Food storage

In stem

In leaf bases 

Tunic

Absent 

Present

Nodes

Distinct circular nodes present

No clear cut nodes are present

Cork

The corm is surrounded by cork.

No presence of cork in bulbs

Growth and Propagation

On the sides or above the stem

New bulbs replace the old one, which gets decayed.


Fun Facts

The largest corm is that of Titan Arum that weighs more than 150 kg.

FAQ (Frequently Asked Questions)

Q1. How do Corms Grow and Propagate?

Answer: A corm structure generally has one or more internodes. There is at least one growing point, with the leaves modified into tunics. The tunic of a corm forms from the initially dead petiole sheaths. They are the leaves produced in previous year. These leaves cover and protect the corm from insects, rhodents, and water loss. 

The internal structure of a corm consists of parenchyma cells. They are rich in starch. They have a circular node from which roots grow. Corms can be dug up and used. They propagate or can be redistributed by cutting the corms into sections and replanting them. Each section has at least one bud that can generate a new corm.

Corm plants have different long-term development stages. Some replace their older corms with a stack of younger corms regularly. This happens every season. Other species of corms simply grow larger in the next seasons. Buds or stolons can sprout independently on a corm and can split, forming a tussock.

Q2. How do bulbs p=Propagate?

Answer: Bulbs contain food and nutrients needed for their entire lifecycle in their underground root structure. The bulb has leaf bases or scales that do not support leaves. It contains food reserves to help the plant to survive adverse conditions. At the center of the bulb is a flowering shoot that becomes a vegetative growing point. Bulbous plant species have two stages of growth, vegetative growth and then reproductive growth.

Stage 1: During the vegetative stage, the bulb grows to the flowering size.

Stage 2:  In the reproductive stage, the plant flowers. 

Environmental conditions, like season shifts, are needed for the transition from one stage to the next. After the flowering period, the plant enters foliage or a resting period. In this period, the plant stores food by absorbing nutrients from the soil. This is required for growth in the next season.  Many bulbs self-propagate naturally. This is done through the formation of offsets, bulbils, or seeds. Others may need some human interaction through various gardening techniques.