Encyclopedia of Biology

statocyst A vesicle, organ of balance, or mechanore-
ceptor in invertebrates that serves to regulate equilibri-
um by using statoliths, a granule of sand or limestone
that responds to gravity by stimulating sensory cells
when an animal is in motion.

statoliths Equilibrium organs; starch grains in plant

cells that act as a gravity sensor. Found in root-tip cells

ortissues close to vascular bundles.

steady state If, during the course of a chemical reac-

tion, the concentration of an intermediate remains con-

stant, the intermediate is said to be in a steady state. In a

static system, a reaction intermediate reaches a steady

state if the processes leading to its formation and those

removing it are approximately in balance. The steady-

state hypothesis leads to a great simplification in reaching

an expression for the overall rate of a composite reaction

in terms of the rate constants for the individual elemen-

tary steps. Care must be taken to apply the steady-state

hypothesis only to appropriate reaction intermediates. An

intermediate such as an atom or a free RADICAL, present

at low concentrations, can usually be taken to obey the

hypothesis during the main course of the reaction. In a

flow system, a steady state can be established even for

intermediates present at relatively high concentrations.

stele The location of xylem and phloem in roots; cen-

tral vascular cylinder.

medulla

steles with leaf gaps meristeles

leaf gap

leaf trace

KEY

xylem

phloem

pericycle and

endodermis

solenostele dictyostele

haplostele medullated

protostele

plectostele actinostele mixed

protostele

A schematic of different types of stele, where xylem and phloem are located in roots.

314 statocyst

stamen

The male organ of a pollen-bearing flower.