joint The contact area between two bones. There are
many different types of joint, including hinge, ball and
socket, universal, sliding, and slightly movable.

Joly, John An Irish geologist (1857–1933) who, in
1899, tried to determine the Earth’s age by calculat-
ing how long it would take for the rivers to dump
salt into the ocean to reach the present salinity lev-
els. It was based on the assumption that oceans were
freshwater when first formed. He estimated 90 mil-
lion years for the age of the Earth. He was more
accurate with dating a geological time period. Work-
ing with Sir Ernest Rutherford in Cambridge in
1913, and using the radioactive decay in minerals, he
estimated that the beginning of the Devonian peri-
od—the time between the Silurian and Carbonifer-
ous—was not less than 400 million years ago, an age
that is pretty well accepted today. Joly collaborated
with Henry Horatio Dixon and was the first to
explain how sap rises in plants largely due to evapo-
ration from leaves.

joule (J) A unit of work and energy; 1 J = 0.239 cal;
1 cal = 4.184 J. It is defined as being equal to the work
done when the point of application of a force of 1 new-
ton (N) moves in the direction of the force, a distance
of 1 meter (m).

junk DNA(noncoding DNA) Genomic DNA that

serves no apparent purpose; stretches of DNA that do

not code for genes.

Jurassic period The middle period of the Mesozoic

era, 213 to 145 million years ago. Age of the

dinosaurs. Named after the Jura Mountains between

France and Switzerland, where rocks of this age were

first studied.

See alsoGEOLOGICAL TIME.

juvenile A stage in development prior to adult stage.

juvenile hormone A chemical hormone in insects

secreted by a pair of endocrine glands, corpora allata,

close to the brain. It inhibits metamorphosis and main-

tains larva or nymph characteristics during develop-

ment and is responsible for determining the molt type.

One of three major insect development hormones. Used

as a pesticide, it retards the development of insects.

juxtaglomerular apparatus Agroup of specialized

cells or tissue in a kidney nephron that is located near the

point where the afferent arteriole meets the distal tubule.

It is composed of the macula densa, a specialized group

183

J