haploid -- Having a single set of chromosomes in the nucleus of each cell. Mosses, and many protists and fungi, are haploid, as are some insects, bryophytes, and the gametes of all organisms. Contrast with diploid.
haploid life cycle -- Occurs when the only multicellular stage in an organism's life cycle is haploid.
haploid-diploid life cycle -- Occurs when a multicellular diploid phase, or sporophyte, alternates with a multicellular haploid phase, or gametophyte. Only plants and certain algae possess this kind of life cycle, which is also called "alternation of generations".
haptonema -- Peg-like structure unique to the Prymnesiophyta; its function is not known.
hemoglobin -- protein complex found in the blood of most chordates and the roots of certain legumes. It binds oxgen molecules, and in chordates serves as the means by which the oxygen is supplied to the cells of the body.
herbivore -- Literally, an organism that eats plants or other autotrophic organisms. The term is used primarily to describe animals.
heterosporous -- Producing two different sizes or kinds of spores. These may come from the same or different sporangia, and may produce similar or different gametophytes. Contrast with homosporous, and compare with heterosporangiate.
histones -- proteins attached to the DNA of eukaryotes which allows it to be packaged into chromosomes.
histostructure -- n. The organization and arrangement of tissue (“histo” is from the Greek word for tissue). Since eggshell is a tissue, eggshell histostructure describes the two- and three-dimensional organization of mineral crystals and shell components.
homology -- Two structures are considered homologous when they are inherited from a common ancestor which possessed the structure. This may be difficult to determine when the structure has been modified through descent.
hotspot -- n. A localized, somewhat persistent (tens of millions of years), and relatively fixed zone of mantle melting, inferred from surface volcanic activity. Evidence of a hotspot's existence is left behind as tectonic plates move across it, for example, the Hawaiian Islands.