Lewis acid Amolecular entity that is an electron-
pair acceptor and therefore is able to react with a LEWIS
BASEtoform a LEWIS ADDUCTbysharing the electron
pair furnished by the Lewis base.
Lewis adduct The adduct formed between a LEWIS
ACIDand a LEWIS BASE. An adduct is formed by the
union of two molecules held together by a coordinate
covalent bond.
See alsoCOORDINATION.
Lewis base A molecular entity able to provide a pair
of electrons and thus capable of COORDINATIONto a
LEWIS ACID, thereby producing a LEWIS ADDUCT.
lice Small (<^1 / 6 in. long) wingless insects (Arthropoda:
Insecta) that have three pairs of legs, and a body com-
posed of a head, thorax, and abdomen. They have flat-
tened, elongate bodies and somewhat oval heads and a
three-stage life cycle (egg, nymph, and adult). The
nymphs molt three times before reaching maturity.
There are two suborders of lice, Mallophagorida
(chewing lice, broad heads with biting mouthparts) and
Anoplurorida (sucking lice, narrow heads with sucking
mouthparts), and they are found worldwide.
Human lice come in three forms: head lice (Pedicu-
lus humanus capitis), body lice (Pediculus humanus
humanus), and pubic lice (Pediculus pubis). They feed
exclusively on humans, and their survival depends on
temperature and humidity conditions associated with
human bodies. They desiccate rapidly if they are away
from humans for more than 24–36 hours.
lichen Lichens are two organisms in one: Having no
roots, stems, or leaves, they actually are symbiotic
associations of a fungus and a photosynthesizing part-
ner that results in a stable thallus of specific structure.
The fungus is almost always an ascomycete, very
rarely a basidiomycete (about 20 basidiolichens are
known). Lichens are classified as fungi, and their sci-
entific names formally refer to the fungal partner
(mycobiont). The photosynthesizing partner (photo-
biont) can be a green alga, such as Trebouxia or
Trentepohlia, or a cyanobacterium (“blue-green
alga”), such as NostocorScytonema.The symbiotic
unit has a definite, reproducible form that is different
from the nonlichenized fungus or alga. A few lichens
have two photobionts, and the fungus looks different
where each occurs within the thallus; i.e., it may be
flat where the green alga grows but have dark bumps
where the cyanobacterium occurs, as in Peltigera leu-
cophlebia.The lichen symbiosis thus involves closely
merged members from usually two but sometimes
three biotic kingdoms.
Worldwide in distribution, lichens can be found on
bare soil or sand, grow over mosses and low plants, or
become attached to rocks, trees, or almost any other
substrate. They are good measures of air quality, since
they have great sensitivity to sulfur dioxide (SO 2 ), an
indicator of local pollution and acid rain. In addition,
their tolerance and accumulation of metals, including
heavy metals and radioactive isotopes, are useful indi-
cators of industrial pollution.
Lichens can be classified into several types based
on appearance and structure: Fruticose (shrublike)
lichens, such as British soldiers (Cladonia cristatella)
and common reindeer lichens (Cladina rangiferina),
include species that look like tiny trees or columns but
also include old man’s beard (genus Usnea), which can
dangle from tree branches. Foliose lichens, such as the
puffed shield lichen (Hypogymnia physodes) are flat
and leaflike. Crustose species, such as the map lichen
(Rhizocarpon geographicum), form a “crust” on rocks,
wood, or soils, and are very abundant. Squamulose
lichens, somewhat similar to crusts, are composed of
tiny overlapping or shingle-like flakes; examples are the
primary (basal) thalli of Cladonia,and the oyster lichen
(Hypocenomyce scalaris), which grows on acidic wood.
Umbilicate lichens have leaflike thalli that attach to
rocks at a central point; smooth rock tripe (Umbilicaria
mammulata) is an example. Jelly lichens, such as the
blue jellyskin (Leptogium cyanescens), have a flabby
thallus and a cyanobacterial photobiont, and usually
occur in damp habitats.
Many lichens have a layered internal morphology,
with the bulk of the thallus formed of fungal filaments
and the photobiont often restricted to a distinct layer.
Fungal hyphae are densely arranged in the upper cortex
to exclude other organisms and regulate light intensity
to the green algal cells. These are often located just
below the cortex. Underlying the algae is the medulla,
another layer of much less compact fungal filaments.204 Lewis acid