Encyclopedia of Biology

long-day plant Aplant affected by photoperiodism.
Aplant that needs more light than dark for flowering.
Long-day plants are spinach, lettuce, and wheat. Others
such as calceolaria, philodendron, and tuberous bego-
nias need 14 to 18 hours of light for flowering. Long-
day plants form flowers during day lengths of more than
12 hours and are both indoor and outdoor plant types.
See alsoPHOTOPERIODISM.

longevity The act of living for a long period of time.

loop of Henle One of the six structural and function-
al parts of the kidney’s nephron, along with Bowman’s
capsule, glomerulus, proximal convoluted tubule, distal
convoluted tubule, and collecting tubule. The long hair-
pin turn, or loop of Henle, extends through the medulla
from the end of the proximal convoluted tubule to the
startof the distal convoluted tubule. It has a descending
limb called the proxima straight tubule that reabsorbs
water, and an ascending limb that reabsorbs NaCl, end-
ing with the distal straight tubule. It is the major site of
water and salt reabsorption. Some parts of it are perme-
able to water and impermeable to materials such as salt
or ammonia in the urine, and vice versa. Also called the
ansa nephroni.

low-spin In any COORDINATIONentity with a particu-
lar dn(1 < n < 9) configuration and a particular geome-
try, if the n electrons are distributed so that they occupy
the lowest possible energy levels, the entity is a low-spin
complex. If some of the higher-energy d orbitals are
occupied before all the lower-energy ones are complete-
ly filled, then the entity is a high-spin complex.

luminescent The act of emitting light without caus-
ing heat (called cold light). Bioluminescence is the act
of producing light by biological organisms, e.g., firefly;
luminescent bacteria (Photobacterium phosphoreum);
and fox fire in the form of Clitocybeilludens
(Omphalotus olerius), Panellus stypticus,and Armillar-
ia mellea.The lanthanides are a special group of ele-
ments (elements cerium [Ce, atomic no. 58] through
lutetium [Lu, atomic no. 71] within the periodic table
that have trivalent cations that emit light. When these

elements are absorbed into materials, the materials can

become luminescent after being excited by an electrical

current (electroluminescence) or by absorbing light

(photoluminescence). As the ions relax to their ground

state, they release light.

lumper A taxonomist who prefers to classify organ-

isms into relatively small groups, emphasizing similari-

ties. Opposite of a splitter.

See alsoSPLITTER.

lung The basic respiratory organ of air-breathing

vertebrates. The basic function of the respiratory sys-

tem is for lungs to supply oxygen to tissues and remove

excess carbon dioxide from the blood. This is accom-

plished by inspiration, the movement of air into the

lungs, and expiration, movement out of the lungs.

Exchange of gases occurs at the internal surface of the

lungs by diffusing oxygen from the lungs into the

lung 209

Computer-enhanced image of a resin cast of the airways in the

lungs. The trachea (windpipe, top center) divides into two

bronchi, which divide further into small bronchioles. The bronchi-

oles terminate in alveoli (not seen), grapelike clusters of air sacs

surrounded by blood vessels. Here the blood takes up oxygen and

releases carbon dioxide to be exhaled.(Courtesy © Alfred

Pasieka/Photo Researchers, Inc.)