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Our mystery seeds from last week came from the ironwood tree, Olneya tesota.

This unique tree gets its common name from the extreme hardness of its wood. Because there are a number of other trees with the same common name, it is sometimes called desert ironwood.

Desert ironwood is a small, shrubby tree found throughout the Sonoran Desert. The lower branches droop, giving a lovely form in its natural state.

The bark of the younger trunks and branches are pale gray to green at the tips.

It is a legume, having compound leaves of narrow, elliptical leaflets.

As with many desert plants, it is well armed, with many pairs of curved spines.

Desert ironwoods produce many lovely purplish-pink flowers in the spring. See Firefly Forest for a photograph of ironwood flowers.

The seed pods mature on the plant and then fall off.

The clue I mentioned that you might have noticed in the the mystery seed photograph last week was a drying leaf toward the bottom of the shot.

You can grow new trees from these seeds, but people often chose to purchase larger trees because desert ironwoods are very slow growing.

If you travel through the low desert you will often see dead ironwood trees. That is because the wood contains strong chemicals that prevent decay after the tree has died and the wood remains in place, sometimes for hundreds of years.

For more detail, see Natural History of the Desert Ironwood Tree (Olneya tesota) from the Arizona-Sonora Desert Museum.

Our mystery seeds from last week were from a blue palo verde, Parkinsonia florida (previously Cercidium floridum).

The blue palo verde is unique to the Southwest and is the Arizona State Tree.

Palo verdes are small trees named for their green trunk and branches.

Why green? The top layer of the trunk and branches contains chlorophyll, which allow the tree to continue to make food via photosynthesis even if it doesn't have leaves.

A close up of the trunk...

Blue palo verdes do have leaves at certain times of the year, but they tend to be small and fall off when the plant is under water stress.

Often the trees are covered with brilliant yellow blossoms in the spring...

...which produce these seed pods full of seeds.

In addition to the blue palo verde, several other species of palo verdes grow in Arizona, including the foothills palo verde, Parkinsonia microphylla and the Mexican palo verde, Parkinsonia aculeata.

The flowers of the Mexican palo verde are unusual because the top petal changes color with age, until it is a deep reddish orange.

Firefly Forest has more photographs and facts about Mexican palo verdes, blue palo verdes, and foothills palo verde.

Desert Harvesters has information about how to identify blue and foothills palo verde trees, plus tips for harvesting the pods

Our mystery "seeds" showcased last week were from an alligator juniper, Juniperus deppeana.

Technically these are not the seeds, but berry-like cones that contain the seeds. Notice the white, waxy coating which is a common characteristic.

Alligator junipers are named for their distinctive bark.

Once you've seen the bark that forms rectangular blocks, it is pretty easy to identify again. People say it looks like alligator skin.

Here's another example. There are different subspecies throughout the range that have variations in growing shape and cone size.

It has the typical scale-like leaves of a juniper (Photograph from Wikimedia).

Alligator junipers are generally small, but in this photograph by Tom Check (from Wikimedia) you can see that older alligator junipers can be magnificent trees.

Have you ever seen an alligator juniper? Where did you see it?