Astragalus hypoxylus is a rare species of flowering plant in the legume family

Fabaceae

The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...

 known by the common name Huachuca milkvetch, or Huachuca Mountain milkvetch. It is endemic to Arizona

Arizona

Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...

 in the United States, where it is known only from the Patagonia

Patagonia Mountains

The Patagonia Mountains is a 15 mile long mountain range south of the Santa Rita Mountains. Sonoita Creek flows in the valley north of the Patagonias. Both ranges are east of the Santa Cruz River Valley...

 and Huachuca Mountains

Huachuca Mountains

The Huachuca Mountain range is part of the Sierra Vista Ranger District of the Coronado National Forest. The Huachuca Mountains are located in Cochise County, Arizona approximately south-southeast of Tucson and southwest of the city of Sierra Vista, Arizona...

. There are three populations.

This small perennial herb forms a patch on the ground no more than about 10 centimeters wide and 3 to 4 centimeters tall. It grows from a fibrous taproot

Taproot

A taproot is an enlarged, somewhat straight to tapering plant root that grows vertically downward. It forms a center from which other roots sprout laterally.Plants with taproots are difficult to transplant...

. The alternately arranged leaves are each made up of several pairs of gray-green leaflets which are a few millimeters long. The inflorescence

Inflorescence

An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Strictly, it is the part of the shoot of seed plants where flowers are formed and which is accordingly modified...

 is a flower cluster about a centimeter wide containing tiny white flowers with purple tips. The flowers bloom in April and May and are pollinated

Pollination

Pollination is the process by which pollen is transferred in plants, thereby enabling fertilisation and sexual reproduction. Pollen grains transport the male gametes to where the female gamete are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself...

 by bee

Bee

Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s of the genera Anthophora

Anthophora

The bee genus Anthophora is one of the largest in the family Apidae, with over 450 species worldwide in 14 different subgenera. They are most abundant and diverse in the Holarctic and African biogeographic regions. All species are solitary, though many nest in large aggregations...

, Bombus, and Osmia. The fruit is a yellow and purple legume pod about a centimeter long. This Astragalus looks so different from other species that it was once classified in a monotypic genus all its own.

This plant grows in woodlands dominated by Emory oak (Quercus emoryi), Mexican blue oak

Quercus oblongifolia

Quercus oblongifolia, commonly known as the Mexican blue oak or Sonoran blue oak, is an evergreen small tree or large shrub in the white oak group...

 (Q. oblongifolia), alligator juniper

Juniperus deppeana

Juniperus deppeana is a small to medium-sized tree reaching 10-15 m tall. It is native to central and northern Mexico and the southwestern United States...

 (Juniperus deppeana), and Mexican pinyon

Johann's Pinyon

Pinus johannis, the Johann's Pinyon, is a pine in the pinyon pine group, native to North America. The range extends from southeast Arizona and southwest New Mexico, United States, south in Mexico along the Sierra Madre Occidental and the Sierra Madre Oriental to southern Zacatecas and San Luis Potosí...

 (Pinus discolor). The substrate is a gravelly soil rich in limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 and metamorphic rock

Metamorphic rock

Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...

.