Glossary terms starting with "H"
Headpole
Headpole is a term used to describe the broader end the valve in genera (Didymosphenia, Gomphoneis, Gomphonema) that are club-shaped.
Helictoglossa - terminal nodule, infundibulum, polar nodule
The internal, distal termination of the raphe is termed the helictoglossa. The structure is developed in many raphid diatoms, but not all. The helictoglossa is a lip-like structure. In many species, helictoglossae are distinguishable in the light microscope in valve view by optical dissection (focusing through many narrow, optical planes). The structure may also be visible in girdle view, as a thickened lip of silica.
Heteropolar
Taxa that are asymmetrical to the transapical plane have apices that are dissimilar in shape, and are termed heteropolar.
Heterovalvar
Frustules in which one valve differs in morphology from the other. Many of the diatoms are heterovalvar based on valve ornamentation or the presence or absence of a raphe system. The term is often used to refer to the monoraphid diatoms, in which one valve possesses a normal raphe valve and the other possesses a rapheless valve (that has been secondarily filled with silica).
Holotype
A holotype of a name of a species or infraspecific taxon is the one specimen or illustration used by the author, or designated by the author as the nomenclatural type. As long as a holotype is extant, it fixes the application of the name concerned
Hood - cavum
The hood is an internal silica extension that extends over the rimmed depression, found in species within the genus Planothidium.
Hyaline
'Hyaline' is a general term that is applied to any unornamented area (area lacking pores or other structures) of a diatom valve.
Hymenes
Very delicate silica membrane that occlude the pores
Hypotheca
The smaller valve of the two valves that make up the frustrule, the larger valve is the epitheca