Friday, March 27, 2009

Bulb Basics

Madonna Lily (Lilium candidum) Cyclamen (Cyclamen persicum) Saffron Crocus (Crocus sativus)

Above, from left to right: Madonna lily (Lilium candidum), cyclamen (Cyclamen persicum), and saffron crocus (Crocus sativus) in Bonnefont Herb Garden.

Many bulbous plants are grown in the gardens of The Cloisters throughout the seasons. In addition to their place in the collection as medieval species, they prolong the garden’s ornamental value, often blooming when there are no other flowers to be seen. The snowdrops (Galanthus nivalis) enliven Bonnefont Herb Garden while there is still snow on the ground. Certain bulbous plants, like the fall-blooming saffron crocus (Crocus sativus), possess a rich history that our garden lecturers are eager to discuss. However, the saffron crocus is actually not a true bulb, but actually what is known as a “corm.”

The term “bulb” is applied informally to the many different underground storage organs in which plants reserve energy in the form of carbohydrates. The proper term for an herbaceous plant that possesses any of these organs is “geophyte.” All such storage organs are made from modified plant tissue. The type of tissue is one of the factors by which the plant is classified into one of four categories: true bulb, corm, tuber, or rhizome.

For true bulbs, the storage organs are modified leaf tissue called scales. Scales may be individually propagated. They will develop bulbils, which will eventually become bulbs. The most common examples of true bulbs include daffodils, tulips, and lilies.

Unlike true bulbs, the storage organs in corms are modified stem tissue. The life cycle of the corm is radically different from that of a true bulb. Whereas individual bulbs may continue to grow from year to year, an individual corm usually dies after one season of growth. The dead corm may be replaced by a new one, called a cormel. The saffron crocus shown above is an example of a corm.

The thickened storage organs for tubers—the third category of geophyte—are also made from modified stem tissue. The meristems (or growing points) of this organ are often referred to as the “eyes.” The potato is probably the most common tuber. Tuberous roots differ from tubers. They are actually swollen, fleshy root tissue. The growing points on tuberous roots are the buds that arise from the crown. Dahlias are a prime example of these organs.

Rhizomes, whose storage organs also consist of modified stem tissue, are most commonly propagated through division. Lily of the valley (Convallaria majalis) and members of the Iris genus are examples of rhizomes.

Another important characteristic that distinguishes geophytes is the tunic—a layer of dry, papery leaves that surrounds the organ, primarily providing protection. The true bulb is an example of a “tunicate” organ. Bulbs that lack the tunicate covering, such as tubers, are termed “imbricate” and are more prone to desiccation and physical injury than tunicate organs.

—Kevin Wiecks

References

Bryan, John, and Mark Griffiths, eds. The New Royal Horticultural Society Dictionary Manual of Bulbs. Portland, OR: Timber Press, 1995.

Cornell University Flower Bulb Research Program, http://www.flowerbulbs.cornell.edu/horticult.html, (March 27, 2009)

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Comments (1)

  1. Kevin Wiecks Says:

    Some people consider cyclamen a tuber. Some people consider it a corm. It is actually an enlarged hypocotyl. The hypocotyl is an important part of an emerging seedling. It normally develops into the stem, but in the case of some plants like cyclamen it enlargens and becomes a primary storage organ.

    Tuberous begonias also posses enlarged hypocotyls.

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