The Evolution of Bulbs

With winter being as mild as it has been, spring bulbs are starting to peek up out of the ground all over the place. Bulbs are interesting structures. They certainly make gardening a whole lot easier. Like all things botanical, however, it is the evolutionary and ecological significance that I find most interesting.

Functionally speaking, bulbs are storage organs. They contain preformed (albeit underdeveloped) shoots and plenty of energy to fuel rapid growth. Their ability to maintain dormancy is something most of us will be familiar with. They are incredibly hardy at this stage. As you might expect, bulbs are an adaptation for short growing seasons. Their ability to rapidly grow shoots gives them an advantage during short periods of time when conditions improve. Despite the energetic costs associated with supplying and maintaining such a relatively large storage organ, the ability to rapidly deploy leaves when conditions become favorable is nonetheless quite advantageous.

Contrast this with rhizomatous species, which are often associated with a life in the understory. Their ambling subterranean habit allows them to vegetatively "explore" for light and nutrients. What's more, the connected rhizomes allow the parent plant to provide nutrients to the developing clones until they grow large enough to support themselves. Under such conditions, bulbs would be at a disadvantage.

Bulbs have evolved independently throughout the angiosperm tree. Many instances of this switch from rhizomatous to bulbous growth habit occurred during the Miocene (23.03 to 5.332 million years ago) and has been associated with a global decrease in temperature and an increase in seasonality at higher latitudes. The decrease in growing season would have favored the evolution of bulbous plants such as those in the lily family. Today, we take advantage of this hardy habit. As such, bulbous species are some of the most common plants used in gardens.

Photo Credit: Pixel Addict (Wikimedia Commons)

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