free-living flatworms (Turbellaria), and it is estimated
that more than 20,000 species exist. Millions of
humans are host to these parasites.
flavin APROSTHETIC GROUPfound in flavoproteins
and involved in biological oxidation and reduction.
Forms the basis of natural yellow pigments like
riboflavin.
flea A major group of bloodsucking insects that feed
on animals, belonging to the order Siphonaptera. There
are about 2,000 known species existing on all conti-
nents. Some species are vectors for diseases. They are
wingless, flattened-body types with legs with long
claws. They can jump from 14 to 16 inches.
While they tend to be associated with pets such as
cats and dogs (Ctenocephalides canis[dog flea] and
Ctenocephalides felis [cat flea]), they do include
humans as hosts.
Fleming, Sir Alexander (1881–1955) BritishBacte-
riologist Sir Alexander Fleming was born on a farm
at Lochfield near Darvel in Ayrshire, Scotland, on
August 6, 1881. He attended Louden Moor School,
Darvel School, and Kilmarnock Academy before mov-
ing to London, where he attended the Polytechnic Insti-
tute. He spent four years in a shipping office before
entering St. Mary’s Medical School, London University,
wherehe received an M.B., B.S., with gold medal in
1908, and became a lecturer at St. Mary’s until 1914,
when he served during World War I, returning to St.
Mary’s in 1918. He was elected professor of the school
in 1928 and emeritus professor of bacteriology, Univer-
sity of London, in 1948.
Fleming was interested in the natural bacterial
action of the blood and in antiseptics, and he worked
on antibacterial substances that would not be toxic to
animal tissues. In 1921 he discovered an important
bacteriolytic substance that he named lysozyme. In
1928 he made his most important discovery while
working on an influenza virus. He noticed that mold
had developed accidentally on a staphylococcus culture
plate and that the mold had created a bacteria-free cir-
cle around itself. Further experiments found that a
mold culture prevented growth of staphylococci, even
when diluted 800 times. He named the active substance
penicillin.
Sir Alexander wrote numerous papers on bacteriol-
ogy, immunology, and chemotherapy, including original
descriptions of lysozyme and penicillin. He was the
recipient of numerous awards and honors in scientific
societies worldwide. Fleming shared the Nobel Prize in
physiology or medicine in 1945 with Ernst Boris CHAIN
and Howard Walter FLOREY, who both (from 1939)
carried Fleming’s basic discovery forward in the isola-
tion, purification, testing, and quantity production of
penicillin. Fleming died on March 11, 1955, and is
buried in St. Paul’s Cathedral.flicker fusion rate(critical flicker frequency) The
rate beyond which the human eye can no longer recog-
nize discontinuous changes in brightness as a flicker,
i.e., the rate is the frequency at which the “flicker” of
an image cannot be distinguished as an individual event.
The flicker fusion rate (FFR) is 31.25 Hz, or 60 frames
per second (bright light) and 24 frames per second (dim
light) in humans. When a frame rate is above this num-
ber, the eye sees the signal as a consistent image (as on
television). A fly has an FFR of 300 frames per second.flora The term for all plants in a given location or,
collectively, on the planet.Florey, Sir Howard Walter(1898–1968) Australian
Pathologist Sir Howard Walter Florey was born on
September 24, 1898, in Adelaide, South Australia, to
Joseph and Bertha Mary Florey. His early education
was at St. Peter’s Collegiate School, Adelaide, and then
Adelaide University, where he graduated M.B., B.S., in- He was awarded a Rhodes Scholarship to Mag-
dalen College, Oxford, leading to the degrees of B.Sc.
and M.A. in 1924. He then attended Cambridge as a
John Lucas Walker student.
In 1925, he visited the United States on a Rocke-
feller traveling fellowship for a year, returning in 1926
to a fellowship at Gonville and Caius College, Cam-
bridge, receiving a Ph.D. in 1927. At this time he also
held the Freedom Research Fellowship at the London
Hospital. In 1927, he was appointed Huddersfield
Lecturer in Special Pathology at Cambridge. In 1931 he
Florey, Sir Howard Walter 131