Encyclopedia of Biology

roundworm SeeNEMATODE.

royal jelly A milky white creamy material, thick and
nutritious, secreted by the hypopharyngeal glands of
nurse bees and supplied to female larvae in royal cells.
Needed for the transformation of larvae into queens.
Queen bees live exclusively on royal jelly.

Rplasmid Aplasmid is a mobile, extrachromoso-
mal, self-replicating DNA structure that is found in the
cells of bacteria. The R plasmid carries genes that
function to resist antibiotics. This is carried out by a
resistance transfer factor (RTF), the component of the
R plasmid that encodes the ability to conjugate and to
transfer DNA.

r-selection Selection that shows a pattern of species
reproduction that is fast, with many offspring that
mature quickly; such species have short life spans and
little parental involvement in rearing.

rubisco See RIBULOSE-1,5-BISPHOSPHATE CARBOXY-
LASE/OXYGENASE.

rubredoxin An IRON–SULFUR PROTEINwithout acid-
labile sulfur, in which an iron center is coordinated by
four sulfur-containing LIGANDs, usually cysteine. The
function, where known, is as an electron carrier.
See alsoACID-LABILE SULFIDE;COORDINATION.

rubrerythrin Aprotein assumed to contain both a
RUBREDOXINlike iron center and a HEMERYTHRINlike
dinuclear iron center.
See alsoNUCLEARITY.

ruminant A polygastric (having more than one
digestive cavity) herbivorous animal has a stomach
divided into four chambers. Usually a hoofed, even-
toed, usually horned mammal, such as sheep, goats,
deer, elks, camels, antelope, giraffes, and cows, that

chew a cud consisting of regurgitated, partially

digested food. The four stomachs consist of the

rumen, the first large compartment from which food

is regurgitated after cellulose has been broken down

bythe action of the symbiotic bacterial, protozoal,

and fungal populations in the rumen. The second

stomach or reticulum (or honeycomb) consists of

folds of the mucous membrane that form hexagonal

cells and joins the omasum or third stomach by the

reticulo-omasal orifice. The digested material in the

rumen and the reticulum is exchanged about every 50

to 60 seconds by a rhythmic cycle of contraction.

Some of the material from the reticulum also goes

into the omasum through the reticulo-omasal orifice.

The omasum, or third stomach, located between the

reticulum and the abomasum, has muscular leaves

that may serve an absorptive function. The aboma-

sum, or fourth stomach, of a ruminant is the true

digestive stomach. A ruminant can use cellulose as an

energy source because of fermentation by bacteria in

the rumen.

runner A modified horizontal, aboveground stem,

e.g., strawberry.

rusts(Uredinales; Urediniomycetes) Rusts are obli-

gate parasites, fungi, of vascular plants and cause eco-

nomic problems with food crops. Rusts can complete

their life cycle on one host (autoecious) or alternate

between two different hosts (heteroecious). Some eco-

nomically important rusts affect

barleystem rust—Puccinia graminis f. sp. secalis,

P. graminisf. sp. tritici;leaf rust—P. hordei;stripe

rust—P. striiformis

oats stem rust—P. graminis f. sp. AVENAE; crown

rust—P. coronata

rye stem rust—P. graminisf. sp. AVENAE; crown rust—

P. coronata

triticalestem rust—P. graminisf. sp. tritici;leaf rust—

P. recondite

wheat stem rust—P. graminisf. sp. tritici;leaf rust—

P. recondite;stripe rust—P. striiformis

There are about 7,000 species of rusts.

300 roundworm