stem

stem

stem, supporting structure of a plant, serving also to conduct and to store food materials. The stems of herbaceous and of woody plants differ: those of herbaceous plants are usually green and pliant and are covered by a thin epidermis instead of by the bark of woody plants. There is relatively more pith in herbaceous stems, and the cambium, which increases the diameter of woody stems, is usually almost inactive; it is therefore characteristic of herbaceous stems that, although they increase in height, their increase in diameter is small. Most herbaceous plants are annuals; some have specialized underground stems (see bulb, corm, rhizome, and tuber) that store food and enable the plant to survive unfavorable growing conditions. Aerial stems may be specialized as tendrils, thorns, or runners (stolons); another specialization is the fleshy, moisture-retaining stem of many arid-land plants (such as most cacti and other succulents). Aerial stems are usually erect; however, in the climbing plants they require support and in others (e.g., melons) they are prostrate. The vascular system in the stem consists chiefly of xylem (upward-conducting) and phloem (downward-conducting) tissue, usually in vascular bundles arranged concentrically on either side of the cambium—the xylem (wood) inside, the phloem outside. In monocotyledonous plants, which generally lack cambium, the bundles are scattered throughout the stem tissue. The sap ducts are formed of elongated cells joined end to end; in the xylem the cell ends dissolve away completely to form continuous tubes and in the phloem they develop perforations and are called sieve plates. Herbaceous stems are marked externally by leaf and bud nodes; woody stems also bear lenticels (pores for transpiration), scars where leaves, twigs, and fruits have dropped off, and bud scars. The annual extension growth of a woody stem develops from a terminal bud usually protected by bud scales or stipules; when the scales fall away, a characteristic bud scar remains. The sap of certain stems contain gums, latexes, and resins used commercially; many are the source of wood of great economic importance.

In living organisms, an undifferentiated cell that can produce other cells that eventually make up specialized tissues and organs. There are two major types of stem cells, embryonic and adult. Embryonic stem cells are located in the inner mass of a blastocyst (an embryo at a very early stage of development), and they eventually give rise to every cell type of the adult organism. Adult stem cells are found in some tissues in the adult body, such as the epidermis of the skin, the lining of the small intestine, and the bone marrow, where they serve in the regeneration of old or worn tissue. In cancer treatment, blood-forming adult stem cells are routinely harvested from bone marrow, stored, and then reinfused into patients to replace blood cells destroyed by chemotherapy or radiation therapy. This potential for replacing damaged tissues has aroused great interest in using embryonic stem cells to treat a number of other conditions, such as Parkinson disease, severe burns, and damage to the spinal cord. Mouse embryonic stem cells are widely used to create genetically modified mice that serve as models for investigating human disease. However, the use of human embryonic stem cells, which requires destroying the blastocysts from which they are obtained, has raised objections by those who feel blastocyst-stage embryos are human beings. The first human stem cell line was created in 1998, using cells harvested from embryos produced through in vitro fertilization. The use of human embryonic stem cells is allowed in some countries and prohibited or restricted in others.

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Plant axis that emerges from the roots, supports the branches, bears buds and shoots with leaves, and contains the vascular (conducting) tissues (xylem and phloem) that transport water, minerals, and food to other parts of the plant. The pith (a central core of spongy tissue) is surrounded by strands (in dicots; see cotyledon) or bundles (in monocots) of conducting xylem and phloem, then by the cortex and outermost epidermis, or bark. The cambium (an area of actively dividing cells) lies just below the bark. Lateral buds and leaves grow out of the stem at intervals called nodes; the intervals on the stem between the nodes are called internodes. In flowering plants, various stem modifications (rhizome, corm, tuber, bulb, stolon) let the plant survive dormantly for years, store food, or sprout asexually. All green stems perform photosynthesis, as do leaves; in plants such as the cacti (see cactus) and asparagus, the stem is the chief site of photosynthesis.

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Stem is a town in Granville County, North Carolina, United States. The population was 229 at the 2000 census.

Geography

Stem is located at (36.200527, -78.723605).

According to the United States Census Bureau, the town has a total area of 0.9 square miles (2.4 km²), all of it land.

Demographics

As of the census of 2000, there were 78 people, 53,493 households, and 6389 families residing in the town. The population density was 24457.5 people per square mile (9434.1/km²). There were 10224 housing units at an average density of 10877.9/sq mi (4167.9/km²). The racial makeup of the town was 99.39% White, 0.18% African American, 0.01% Native American, 0.18% Asian, 0.53% from other races. Hispanic or Latino of any race were 0.68% of the population.

There were 53,493 households out of which 31.2% had children under the age of 18 living with them, 52.7% were married couples living together, 14.0% had a female householder with no husband present, and 31.2% were non-families. 23.7% of all households were made up of individuals and 12.9% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 2.95.

In the town the population was spread out with 24.9% under the age of 18, 5.2% from 18 to 24, 35.8% from 25 to 44, 22.7% from 45 to 64, and 11.4% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 84.7 males. For every 100 females age 18 and over, there were 79.2 males.

The median income for a household in the town was $12,938 and the median income for a family was $21,000. Males had a median income of $8,333 versus $5,625 for females. The per capita income for the town was $3,362. About 94.5% of families and 91.7% of the population were below the poverty line, including 15.9% of those under the age of eighteen and 22.2% of those sixty five or over.

References

External links

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