Encyclopedia of Biology

of a fertilized egg or spore through each stage of
change, guided by genetics and environment until the
final adult stage is completed. A process where a geno-
type becomes expressed and transformed into a final
phenotype.

epiglottis A leaflike cartilaginous flap that closes and
covers the glottis (middle part of the larynx) to prevent
food and other objects from entering the trachea and
lungs while ingesting.

epilepsy A neurological or brain condition in
which a person has a tendency to have repeated
seizures. Clusters of nerve cells, or neurons, in the
brain sometimes signal abnormally. It affects more
than 2 million Americans, with over 180,000 new
cases each year.

epinephrine Another name for adrenaline. A hor-

mone and neurotransmitter secreted by the adrenal

gland (adrenal medulla) to react to stress, exercise, low

blood glucose. It is also a major component of the

fight-or-flight reaction, the reaction that happens in the

body when, faced with a sudden unexpected threat or

stress situation, both epinephrine and norepinephrine

are released.

The drug of choice for the treatment of anaphylax-

is. Individuals who are allergic to insect stings and cer-

tain foods should always carry a self-injecting syringe

of epinephrine.

Epinephrine increases the speed and force of heart-

beats and, therefore, the work that can be done by the

heart. It dilates the airways to improve breathing and

narrows blood vessels in the skin and intestine so that

an increased flow of blood reaches the muscles and

allows them to cope with the demands of exercise. Usu-

ally treatment with this hormone stops an anaphylactic

reaction. Epinephrine has been produced synthetically

as a drug since 1900.

epiparasite Any organism that extracts nutrients

from its host plant by means of intermediates.

epiphyte Any nonparasitic plant, fungus, or microor-

ganism that grows on the surface of another plant for

support but provides its own nourishment. Epiphytes

can form “mats” that contain a surface of canopy

plants with suspended soil and other material. A facul-

tative epiphyte is one that commonly grows epiphytical-

ly and terrestrially, but will usually exhibit a preference

for one or the other habit in a particular habitat.

episome A plasmid (circular piece of DNA ) that can

attach to and integrate its DNA in a cell and at other

times exist freely and still replicate itself alone, e.g., cer-

tain bacterial viruses.

epitasis Interaction between nonallelic genes, with

one gene altering the expression of the other gene.

epithelial tissues Closely packed layers of epithelial

cells, a membranelike tissue that covers the body and

epithelial tissues 117

Endoscope view of a healthy larynx (voice box) showing resting
vocal cords (v-shaped, center left and right). The vocal cords are
responsible for the production of sound. Normally the epiglottis, a
leaflike cartilaginous flap, closes and covers the glottis (middle
part of the larynx) to prevent food and other objects from entering
the trachea and lungs while ingesting.(Courtesy © CNRI/Photo
Researchers, Inc.)