Habituation
The phenomena that, after a number of sub-cultures, cells can grow, without the addition of specific factors, such as no longer needing exogenous growth regulators in the tissue culture medium. Such cells are autonomous.

Haemoglobin
Conjugated protein compound containing iron, located in erythrocytes of vertebrates; important in the transportation of oxygen to the cells of the body.

Hairy root culture
A fairly recent development in plant culture, consisting of highly branched roots of a plant. A plant tissue is treated with a culture of the bacterium Agrobacterium rhizogenes which transfers part of its own plasmid DNA to the cells of an infected plant. This alters the plant' metabolism, including alterations in hormone levels, which in turn cause the explant to grow highly branched roots from the sites of infection. The roots branch much more frequently than the usual root system of that plant, and are also covered with a mass of tiny root hairs. Their most significant feature is that they produce secondary metabolites at levels similar to those made in the original plant. Thus they can be used as replacement plants  for making such compounds as food flavours or fragrances.

Halothane
A volatile anaesthetic.

Hanging droplet technique
See microdroplet array.

HAPs       
Hazardous air pollutants.

Haploid
(Gr. haploos, single + oides, like) A cell or organism containing  only one representative from each of the pairs of homologous chromosomes found in the normal diploid cell. Having a single complete set of chromosomes, or referring to an individual or generation containing such a single set of chromosomes per cell. Usually a gamete. See monoploid.