The underground plant movement

Plants in general must bask in the sun day after day to soak up the energy they need for life activities.  In doing so, of course, they face certain hazards.  Being immobile, they are essentially "sitting ducks" for everything from plant-munching animals to fire, drought, and winter freezes.  As discussed earlier in "How the grass leaf got its stripes," plants in general employ one of two basic strategies for coping with such hazards.  Trees and shrubs tough it out with permanent woody shoot systems that resist damage with protective tissues or toxins or by dropping their leaves during the adverse seasons.  Their dormant buds are typically wrapped in tough scales.  They may simply rise above threats from grazing animals or ground fires, as do the Acacias of the tropical savannas.  Their competition with one another for light often results in forests. 

Grasses dominate in areas where moisture is too sparse for forests.  They have plenty of light, and escape most environmental threats by keeping their main stem system and buds below ground.  Like the periscopes of submarines, their leafy shoots and flowerstalks rise above ground to do their business, sending food reserves to storage organs below ground, or drawing upon those reserves to make seeds.  When things get too rough upstairs, these aerial shoots are abandoned, and replaced when better growing conditions return.

Grasses are not the only plants to "go underground."  A great many perennial herbs follow similar strategies, as do the unique class of plants called biennials.  They all achieve similar results, but what differs is the nature of the underground storage structures.  In different groups of plants, roots, stems, and even leaves, are recruited for this job.

The carrot, Daucus carota (Apiaceae) is mostly a thick taproot.
A short stem at the top produces the cluster of leaves.  Photo
by jonathunder, Wikimedia commons.

In most biennial herbs, a specialized taproot, or sometimes a hypocotyl  (to be explained shortly!) swells up with food reserves, resulting in a carrot, radish, beet, parsnip, turnip, or one of many other type of root vegetable.  These are radially symmetrical plants with an axial root system, similar to trees and shrubs.  The expansion of their storage organs in fact is due to the activity of a type of cambium similar to that which produces layers of wood in a tree.  They can be considered highly reduced trees in this respect. 

Biennials store up food reserves during their first season of growth, and use up those reserves making flowers and seeds during the second season.  Then they die.  Of course such swollen taproots are great sources of food for humans and other animals, and we cheat them out of reproducing by harvesting after the first season.

In the radish, Raphanus sativa (Brassicaceae), the
short section of the seedling stem below the cotyledons
(the hypocotyl) swells into a storage organ. From Transeau, et
al., 1940, Fig. 114.

All of the above-mentioned "root vegetables" appear to be actually roots, but some are actually a different, adjacent part of the plant.  When a seedling emerges from a seed, it consists of three sections: the root, the shoot, and something inbetween: the hypocotyl ("below the cotyledons").  The hypocotyl is the usually inconspicuous section of stem between the root proper and the seedling leaves (cotyledons) that mark the beginning of the leafy shoot.  In some biennials, such as radishes, it is this tiny section of stem that swells into the underground storage organ, not the root proper.  This is of utterly no importance to the master chef, but an essential piece of information for the botany-geek-wannabee.

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In the sweet potato (Ipomoea batatas),
adventitious roots from the vine swell to
become storage organs. Photo by H. Zell,
Wikimedia commons.

A sweet potato (Ipomoea batatas in the Morning Glory Family, Convulvulaceae) is also a root, but not a taproot as in a carrot. A sweet potato forms from an adventitious root that emerges from the vine creeping along the ground. Similar are the tuberous roots of Dahlias.  Regular ("Irish," "white," "Idaho") potatoes, on the other hand are specialized stems called tubers. The "eyes" on a potato are buds that can sprout into new leafy shoots.

The potato, Solanum tuberosum, is a
swollen underground stem with many
buds ("eyes") that can develop into new
leafy shoots. Photo by Donna, Wikimedia
Commons.

A ginger rhizome (Zingiber officianale), with a new rhizome
section developing (upper right).

The most widespread kind of underground stem is the rhizome.  This horizontal stem creeps along under the ground, or just above it, branching to form new rhizome sections, and sometimes expanding into an extensive colony.  A ginger or bearded iris is a good example of a rhizome that is also a food storage organ.

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A new shoot is developing on top of the corm of this
Amorphophallus titanum. It will form a new corm at its
base, while the old corm withers. Photo by stickpen,
Wikimedia commons.

A more specialized type of underground stem is the corm.  Often confused with a bulb (see below), a corm is a stem filled with solid storage tissue.  Found mostly in the Iris and Aroid Families, but also in water chestnuts (Sedge Family). Corms are short and fat, with a single dominant bud facing directly upward.  As that bud begins growth, the base of the new shoot swells to form a new corm on top of the old one, and sends out new adventitious roots.  The old corm gradually decays, and the newest corm is pulled down to replace it by the roots, which physically contract and shorten (contractile roots).
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An onion, Allium cepa, is a bulb
made up of the swollen, concentric leaf
sheaths of regular foliage leaves. Photo
by Amada44, Wikimedia Commons.

Finally, a true bulb is actually a big underground bud, made up of a cluster of modified leaves or leaf bases that swell up with food.  An lily, onion, or amaryllis bulb is a good example.   The bulb of the onion or amaryllis is made of of the concentric fleshy leaf sheaths of ordinary leaves. As each new leaf forms in the center of the bulb, it produces a long photosynthetic blade that emerges from the top, while the cylindrical sheath slowly expands within the bulb.  The collective fleshy sheaths ("onion rings") constitute the dormant bulb when the leaf blades wither at the end of the growing season. The oldest leaf sheaths on the outside of the bulb eventually dry up, becoming thin and paper-like.

In the true lily (Lilium), on the other hand, the bulb is a loose cluster of short, modified leaves, swollen with food and water. Like the vegetative leaves that develop on the elongate aerial shoot, these leaves have a relatively narrow base, rather than a cylindrical leaf sheath.

In the bulbs of Lilium (left) the food-storage organs are short, modified leaves, rather than the
sheaths of vegetative leaves.  From Brown, the Plant Kingdom, 1935.

License for photos from Wikimedia Commons: http://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License_1.2