Underground stem

Underground stems are modified plant structures that derive from stem tissue but exist under the soil surface. Plants have two axes of growth, which can be best seen from seed germination and growth. Seedlings develop two structures or axes of growth, one that develops upward out of the soil, called stems, and structures that develop downward which are called roots. The roots are modified to have root hairs and branch indiscriminately with cells that take in water and nutrients, while the stems are modified to move water and nutrients to and from the leaves and flowers. Stems have nodes with buds where leaves and flowers arise at specific locations, while roots do not.Plants use under ground stems to multiply their numbers by asexual reproduction and to survive from one year to the next, usually over a period of dormancy. Plants produce these modified stems so they can survive a cold or dry period which normally is a period of inactive growth, and when the cold or dry period is over the plants begin new growth from the underground stems. Being underground protects the stems from the elements during the dormancy period, such as freezing and thawing in winter or extreme heat and drought in summer or fire. They can also protect plants from heavy grazing pressure from animals, the plant might be eaten to the ground but new growth can occur from below ground that can not be reached by the herbivores. A number of "weed"y species use underground stems to spread and colonize large areas since the stems do not have to be supported or strong, less energy and resources are needed to produce these stems, often these weeds have more mass under ground than above ground.

Types of underground stems

Different forms of underground stems include:

*Bulb - Short, upright organ used by plants for food storage or reproduction, with specialized leaves modified into thick flesh scales. Tulips and Lilies.
*Corm - Short, upright, hard or fleshy stems covered with thin, dry papery leaves.
*Rhizome - With reduced scale like leaves. The top can generate leafy stems while the bottom can produce roots. Iris.
*Stolon - Horizontal stems that run at or just below the soil surface with nodes that root and long internodes, the ends produce new plants. When above ground they are called "runners".
*Tuber - An enlarged fleshy end of a stem, generally from rhizomes but often also referring to thickened roots too.

A number of underground stems are consumed by people including; onion, potato, ginger and taro.