Aschelminth
The Aschelminthes (also known as Aeschelminthes, Nemathelminthes, or
Pseudocoelomata), closely associated with the Platyhelminthes, are an obsolete phylum of
pseudocoelomate and other similar animals that are no longer considered closely related
and have been promoted to phyla in their own right.
Assimilation
The energy-requiring process by which plant cells convert nitrate ions (NO 3 – ) taken up by
the roots of plants into ammonium ions (NH 4 +), which can then be used in the synthesis of
amino acids and other nitrogenous compounds.
Associative learning
The acquired ability to associate one stimulus with another; also called classical
conditioning.
Assortative mating
A type of nonrandom mating in which mating partners resemble each other in certain
phenotypic characters.
Astragalus
The bone of the ankle which articulates with the bones of the leg. Also known as talus.
Astrocytes
A star-shaped cell, especially a neuroglial cell of nervous tissue.
Asymmetrical
Irregular in shape or outline.
Asymmetric carbon
A carbon atom covalently bonded to four different atoms or groups of atoms.
Atactostele
A type of monocotyledonous siphonostele in which the vascular bundles are dispersed
irregularly throughout the center of the stem.
Atmospheric pressure
The weight of the Earth's atmosphere over a unit area of the Earth's surface.
Atom
The smallest unit of matter that retains the properties of an element.
Atomic mass
The mass of an atom of a chemical element expressed in atomic mass units.
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