called Rhus toxicodendron. Western poison oak is
known as Toxicodendron diversilobum.
These plants can cause a skin reaction. No reaction
usually occurs the first time the skin is exposed to the
plant. Subsequent contact with the plant or plant resin,
however, can result in an allergic skin reaction that
usually appears seven to 14 days after contact. Subse-
quent contact results in a more rapid reaction, usually
within two to five days postcontact. The severity of the
reaction is related to the amount of plant material that
comes in contact with the skin, as well as to the degree
of allergic sensitivity of the individual. The allergen
(irritant from the plant) is often transferred from the
hands or clothing to other parts of the body.
The poison ivy plant and its relatives are common
throughout the United States. Poison ivy leaves are coat-
ed with a mixture of chemicals called urushiol. When
people get urushiol on their skin, it causes allergic con-
tact dermatitis. The body’s immune system treats urushi-
ol as foreign and attacks the complex of urushiol
derivatives with skin proteins. The irony is that urushiol,
in the absence of the immune attack, would be harmless.
Poison ivy can affect two out of three Americans,
and of these, 15 percent may have severe allergic reac-
tions that require medical treatment. Millions of Amer-
icans yearly seek remedies for the irritation caused by
poison ivy, oak, and sumac.
polar covalent bond A type of chemical bond,
based on electron affinity, where electrons shared by
atoms spend a greater percentage of time closer to an
oxygen nucleus rather than a hydrogen nucleus; bonds
arepolar, i.e., they have a partial electric charge across
the molecule due to their geometry and the electronega-
tivity difference between the two atoms (hydrogen is
positive, oxygen is negative); in organisms, they can
form weak hydrogen bonds. Water is an example, but
peptide bonds and amines also form polar covalent
bonds; these molecules can attract each other.
polar molecule A molecule that has both a positive
and a negative end, such as water.
pollen Microscopic grains produced by plants in
order to reproduce. Each plant has a pollinating period
that can vary depending on the plant, climate, and
region.pollen allergy Ahypersensitive reaction to pollen.
While grass pollens are generally the most common
cause of hay fever (seasonal allergic rhinitis), other
pollen types are also important. These include tree pol-
lens such as alder, hazel, birch, beech, cypress, pine,
chestnut, and poplar, and weed pollens such as plan-
tain, mugwort, and ragweed. The relative importance
of the kinds of pollen that can cause hay fever varies
between different climatic and vegetation zones. For
example, ragweed pollen, although very common in
North America, is present in Europe only in the French
Rhône valley and some areas of Eastern Europe, while
the pollen most associated with seasonal allergy in
Mediterranean regions is the olive tree. A person aller-
gic to one pollen is generally also allergic to members
of the same group or family (e.g., Betulaceae). Pollen-
induced reactions include extrinsic asthma, rhinitis,
and bronchitis.pollination The first step in plant reproduction.
Occurs when the male germcell of a plant, a pollen
grain, reaches the female reproductive part, or stigma,
of the same species of plant. This happens by wind
transportation or by animal carriers, although 90 per-
cent of flowering plants rely on animal delivery.polyandry Arare mating system where one female
mates with more than one male, although each male
mates with only one female. Two types of polyandry
exist: simultaneous polyandry, where each female
maintains a large territory that contains smaller nesting
territories of two or more males who care for the eggs
and tend to the young; and sequential polyandry, where
a female mates with a male, lays eggs, and terminates
the relationship and leaves that male. While the male is
left to incubate the eggs, she repeats the sequence with
another male. The latter is more common. In human
society, it is the practice of a woman to have more than
one husband at a time.polydentate SeeCHELATION;DONOR ATOM SYMBOL.polydentate 271