The auxiliary cell is a spore

Spore

In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

-like structure that form within the fungal

Fungus

A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

 family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Gigasporaceae

Gigasporaceae

The Gigasporaceae are a family of fungi in the order Diversisporales. Species in this family are widespread in distribution, and form arbuscular mycorrhiza in roots....

 (order Diversisporales

Diversisporales

The Diversisporales are an order of generally hypogeous arbuscular mycorrhizal fungi within the division Glomeromycota. Many have vesicles for energy storage, or auxiliary cells. Species produce a wide range of spore types, hence the name....

). Auxiliary cells have thin cell wall

Cell wall

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

s, echinulate

Echinulate

Echinulate is an adjective describing the spiny, or "spiky" nature of some spores found in plants and fungi; an example is the auxiliary cell. The words derives from the same root as the prefix echino, meaning that the shape is similar to the porcupine ....

 (spiny), papillate, knobby or sometimes smooth surfaces, and are formed from hyphae after spore germination before the formation of mycorrhizae, and then on the extraradical hyphae in the soil. They may not be 'cells' in the biological sense of the word, as they are structures found with coenocytic hyphae belonging to members of the phylum (division) Glomeromycota

Glomeromycota

Glomeromycota is one of seven currently recognized phyla within the kingdom Fungi, with approximately 230 described species. Members of the Glomeromycota form arbuscular mycorrhizas with the roots or thalli of land plants. Geosiphon pyriformis forms an endocytobiotic association with Nostoc...

. Mostly they are known from members of the Gigasporaceae

Gigasporaceae

The Gigasporaceae are a family of fungi in the order Diversisporales. Species in this family are widespread in distribution, and form arbuscular mycorrhiza in roots....

. Currently this family contains Gigaspora, Scutellospora and Racocetra, but there are other generic names that have not been widely accepted (Dentiscutata, Cetraspora, Fuscutata and Quatunica) — all of these form auxiliary cells. Members of the genus Pacispora (another genus in the Diversisporales

Diversisporales

The Diversisporales are an order of generally hypogeous arbuscular mycorrhizal fungi within the division Glomeromycota. Many have vesicles for energy storage, or auxiliary cells. Species produce a wide range of spore types, hence the name....

) are also said to produce a kind of auxiliary cell but this requires further confirmation.