Hexose. Sugar (monosaccharide) with six carbon atoms.

Hibernation. Dormancy during winter. Occurs in many mammals, most reptiles and Amphibia, and many vertebrates, of temperate and arctic regions. Metabolism is greatly slowed and in mammals temperature drops to that of surroundings. Some, e.g. bats wake to feed from time to time.

Hierarchy. The systems of ranking in a graded order from species to kingdom.

Higher category. Any category higher than the rank of species.

Higher taxon. Any Taxon higher than the one in question.

Hilium.
Scar on seed coat marking point of former attachment of seed to funicle.

Histocompatibility. The acceptance by a recipient of tissue transplanted from a donor, a state that is determined by histocompatability Antigens.

Histogenesis. Differentiation of tissue.

Histogens. Distinct tissue zoned recognizable in apical meristems of many plants, especially in roots.

Histogram. A diagram showing a frequency distribution by means of rectangular areas.

Histology. Study of tissue.

Histolysis. Dissolution of tissue

Histone. Any of a group of water-soluble proteins found in association with the DNA of plant and animal chromosomes. They contain a large proportion of the basic (positively charged) amino acids lysine arginine, and histidine. They are believed to be involved in the condensation and coiling of chromosomes during cell division and have also been implicated in nonspecific suppression of gene activity. Histones do not occur in vertebrate sperm cells (see protamines) or ini bacteria and blue-green algae.