We are well in to autumn now and everywhere you look (in the gardening universe, anyway) you can find information on which bulbs to plant now for a colorful spring. But have you ever wondered, what exactly is a bulb?
If not, don’t worry. I won’t think any less of you.
But, if you have wondered what makes a bulb a bulb, well, friend, I’m here to answer that.
The text-book definition – literally, from the book Botany for Gardeners, the text book for one of my horticulture classes – of a bulb is “a short, flattened stem bearing fleshy, food-storage leaves”. To visualize this, think of an onion. (For a nice onion diagram, go here.) The flatish, roundish disc at one end of the onion is the stem. Growing out from the underside of the stem would be the roots. On the upper side of the stem – or the part of an onion you would eat – are the fleshy leaves. We humans use these leaves as food, but, were you to leave the onion in the ground, the onion itself would use those leaves as food storage to sustain itself until it could grow a new shoot the next year and start photosynthesizing again.
Garlic and tulips are also bulbs. At first glance they don’t seem to resemble an onion much, but the main difference lies in their axillary buds. Hidden deep between the bulbs leaves are axillary buds (axillary buds are just buds that are found in the axil – or angle – between the stem and a leaf). These axillary buds grow, enlarge, and develop into additional bulbs; or, in the case of garlic, the individual cloves.
What about crocus? or dahlias? or any other number of flowering plants that have their start as a knobby lump of tissue? Are they all bulbs?
I’m so glad you asked.
The answer is no, they are not.
In addition to bulbs, there are corms, tubers, and rhizomes. Similar to bulbs, they are all underground food storage organisms. But they all differ in specific structure and function.
Let’s start with corms. Even if you’ve never heard the word “corm” before, you would still probably recognize their progeny. Crocus and gladiolus both arise from corms. Ready for the textbook definition again? Here it is: corms are “short, swollen, underground stem(s) in which food is stored”. The papery coverings around a corm are the remnants of the previous year’s leaf bases. Like a bulb, roots will emerge from the underside of the corm stem. If you remove those papery old leaf bases you’ll see parallel lines around the circumference of the corm; these are nodes where leaves grow from. Because growing leaves and flowers is so energetically expensive for the corm, a new corm will develop above the old one, which will eventually wither away. Additional small corms (or cormels) might also form around the stem’s base. When these cormels grow to their full size, you could remove them from the main corm and grow additional plants from each individual.
Next up, are the rhizomes: “underground, horizontal stems”. The tall, bearded-irises all grow from rhizomes. Other, perhaps less desirable rhizomatous plants would include Canada thistle and field bindweed. Like their above-ground stem counterparts, rhizomes have nodes bearing leaves and axillary buds. From some of the buds roots will develop; from others, flower stalks.
Lastly, let’s talk about tubers. Tubers are further broken down into two categories: stem tubers and root tubers.
A stem tuber is “an enlarged tip of a rhizome containing stored food”. If that’s hard to visualize, just think of your regular ol’ baking potato. Like rhizomes, stem tubers have nodes that have axillary buds – the “eyes”. From those “eyes”, shoots and roots will develop, with more rhizomes forming below them. Those rhizomes will bear more tubers, which you will dig up and enjoy mashed at your Thanksgiving dinner.
You’ll probably also find root tubers on your Thanksgiving table. Sweet potato is one example of a plant that grows from root tubers. Dahlias, the darling of many floral designers, is another such example. A root tuber is “an enlarged, food-storage root bearing adventitious shoots” (click here for some root tuber photos). So, unlike all of the previous underground storage organs, root tubers are (as the name implies) root, not stem, adaptations. However, they do bear adventitious shoots which will form adventitious roots that will eventually expand to become tubers.
So, to quickly recap there are:
- bulbs – a flattened stem with fleshy food storage leaves
- corms – a swollen underground stem where food is stored
- rhizomes – a sometimes fleshy, underground, horizontal stem
- stem tubers – an enlarged tip of a (non-fleshy) rhizome containing stored food
- root tubers – an enlarged root where food is stored that also happens to have adventitious shoots
If that’s at all confusing or a bit vague all I can say is: Welcome to the world of botany.