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.
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.
|In the sweet potato (Ipomoea batatas),
adventitious roots from the vine swell to
become storage organs. Photo by H. Zell,
|The potato, Solanum tuberosum, is a
swollen underground stem with many
buds ("eyes") that can develop into new
leafy shoots. Photo by Donna, Wikimedia
|A ginger rhizome (Zingiber officianale), with a new rhizome
section developing (upper right).
|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,
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).
|An onion, Allium cepa, is a bulb
made up of the swollen, concentric leaf
sheaths of regular foliage leaves. Photo
by Amada44, Wikimedia Commons.
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