Texas Bull-Nettle

I was out botanizing a vacant lot just a few blocks down from my house recently when I came across a plant that is more than capable of defending itself from potential herbivores or the unwary naturalist.  This is none other than Texas Bull-Nettle or Cnidoscolus texanus.

cnidoscolus_flw2 The white flowers are mostly harmless but the remainder of the plant is loaded with trouble.  With other common names like Bull Nettle, Treadsoftly, and Mala Mujer (I get the direct translation of the Spanish but there must be some cultural context tacked onto this that I am ignorant of–Can anyone place this in the correct context?),  there is no doubt that this plant means business.

These are herbaceous plants, about 80-100cm tall.  An individual plant has separate male and female flowers together on the same inflorescence.  I didn’t get any closer to this plant than I had to so can’t be for sure which sex these flowers are.  The next photo reveals why I kept my distance from this plant.


Notice both the main stem and the stem branches.  All are covered with hispid or bristly hairs. But these are not normal hairs; they are extremely painful, stinging hairs. The leaves are covered with the same stinging hairs as well.  Here is how this plant defense mechanism works: If the foliage or stems are touched, the glass-like hairs break off in the skin (yours or a hapless four-legged fellow creature) and act like hypodermic needles. The “needles” release a toxin which causes an intense burning sensation. This effect is a type of allergic response known as contact urticaria and the reaction can last for several days.

The genus name Cnidoscolus says it all. The Greek cnide means nettle and scolopes means prickle or sting. From the flowering plant family Euphorbiaceae.  A good plant species to know and respect!


A small thought for Earth Day

During my lectures on cell division I tell my plant biology students that if there is only one reason they could ever care about the environment or ecology then they should be concerned about the loss of plant biodiversity, especially in the tropical rainforests. Reason being is that in the medical treatment of cancer we have two main successes, both using chemical compounds produced by plants. Vincristine is used in the treatment of childhood leukemia with good success and Vinblastine has fairly good results with Hodgkin’s Disease.  One came from the Pacific Northwest of the U.S. and the other from a plant in the tropics.  Plants synthesize these compounds for use in defense and they work the same way in the cells of our bodies as they do in the plant cell.  We are not even close to being able to dream up chemical compounds like these in the laboratory.  Maybe in a hundred or two hundred years, just as likely never.  There are other compounds in plants waiting to be discovered in the tropical rainforests that undoubtedly have the same or greater medical potential as Vincristine and Vinblastine.  The problem is that many parts of tropical rainforest are being cleared, sending hundreds of plant species to extinction, never to be examined by humans.  These plant species have never even been given a scientific name.  We are seeing a great potential benefit to humanity being erased from the planet forever.

Just a small thought on Earth Day….

How do yucca moths find yucca flowers in the dark?

If you have read the previous post about how yucca flowers are pollinated by the yucca moth you might wonder how the moth is able to navigate to flowers in the dark.  Well, it’s fairly well known that many insects have visual capabilities outside of the visible portion of the electromagnetic spectrum.  What many people may not be aware of is that many parts of flowers have evolved to take advantage of an insect’s visual range, especially in the UV or ultraviolet part of the spectrum.

It just so happens that Yuccas have “UV-coded” their stamens (male reproductive part of the flower) and especially the anthers, which produce the pollen.  Take a look at the UV photograph below of the stamens from a Yucca flower.  The bright white parts of the stamens are the anthers.

UV photo of Yucca stamens

UV photo of Yucca stamens

I would be willing to bet that the yucca moth has  visual capabilities in the UV range and is assisted in its pollination duties by the UV-visual anthers.  Would other insects be drawn in as well?  Maybe.  But there are a couple of things to consider.  First, many insects are not active at night.  Second, yucca flowers do not have nectaries which provide nectar as a reward for transferring pollen to other flowers.  Most insect-pollinated flowers have nectaries to provide a food reward for the insect.   For yucca flowers  the reward or offering to the moth is the flower’s ovules or immature seeds.

Just another interesting aspect of the natural history of Yucca torreyi. Thanks to Monique Reed of Texas A&M University for suggesting this topic and providing the photo.  Photo credit goes to Robert Corbett.

Torrey’s Yucca: great natural history


Torrey’s yucca, Yucca torreyi, is one of two yuccas that we have in our area.   I was out botanizing yesterday south of Abilene (Texas,USA) in Abilene State Park and found a good number of these monocots in full flower.  Torrey’s Yucca is a member of the plant family Agavaceae.  The plant pictured may be a little non-typical in that the panicle inflorescence, which is the entire mass of flowers, is not sitting up higher in comparison to the long leaves.  Typically, the upper half of the panicle of flowers will stand above the top leaves.  However, the length of the sword-like leaves on this particular plant are about 90cm, which is typical of this species.  The other species of yucca found here on the Rolling Plains, Yucca constricta, has leaves no longer than about 65cm. One final supporting morphological feature found on the leaves of this specimen are the narrow threads on the margins.

Below is a close up image of the inflorescence.  Note the large number of pendant or drooping flowers.


Below, a close up of the inside of one flower:


To the best of my knowledge the yuccas (species in the genus Yucca) are endemic only to North America with 35 species total in the genus.  They are found in warm areas and were used extensively by Native Americans as a source of food, fiber, soap, and medicine.  Torrey’s Yucca is named for John Torrey (1796-1873), a distinguished American botanist, physician, and collector of many western North American plants.

Pollination in yuccas is interesting.  All species are dependent on the yucca moth for pollination. Apparently, if the moths are not present Yuccas can reproduce vegetatively (which is a nice evolutionary adaptation to have).  Quoting from Powell (1988):

The yucca moth (Tegeticula = Pronuba) flies at dusk to a flower where she climbs stamens to collect pollen and pack the pollen in a large ball-like mass under her neck.  She then visits another flower where she inserts her ovipositer directly through the ovary wall and deposits 20-30 eggs, one at a time, each directly in to an ovule.  She then climbs  to the stigma of the same flower and spreads the pollen, thus ensuring pollination, subsequent fertilization, and developing seeds that provide nourishment for the moth larvae. Each larva ultimately destroys the seed in which it grows, but there are many undamaged seeds left in the yucca capsule.

From Google a picture of a yucca moth (http://en.wikipedia.org/wiki/Prodoxidae):


A species of yucca moth in the family Prodoxidae

The relationship between the yucca and yucca moth is termed a mutualism,  in a general ecological context.  More specifically, this is a marvelous example of an obligate mutualism for the moth species of the genera Tegeticula, which both pollinates and deposits eggs in Yucca flowers.  For the moth, the relationship to yucca is termed an obligate mutualism because the moth larvae feed only on yucca seeds.  The yucca moth has special mouth parts which allow it to package and transfer the flower pollen. For the yucca plant the relationship is mostly obligate because only yucca moths transfer their pollen between yucca flowers for fertilization.  I say mostly obligate since yuccas can reproduce vegetatively, so they can perpetuate themselves without sexual reproduction.  There are advantages and disadvantages to asexual reproduction.  However, the bottom line for yuccas is that they have coevolved with the yucca moth to form a very unique mutualistic relationship.  Therefore, it’s probably fairly safe to say that sexual reproduction via the yucca moth is most likely the primary means of reproduction for the yucca.  Such a highly evolved relationship with an insect is not happenstance.

The evolutionary history of the other genera of the moth family Prodoxidae is quite fascinating.  A second genus in this moth family, Parategeticula, also has an obligate relationship with yucca flowers as well. Individuals of a third genus of yucca moths, Prodoxus, do not have a mutualistic relationship with yucca plants, neither depositing their eggs in yucca seeds nor moving around pollen from the flowers. However, Prodoxus eggs are deposited in the fruits (mature ovary from a flower) and leaves of yuccas where they eat and grow, which is more of a herbivorous relationship.

Hopefully, I have all of this evolutionary history straight.  I have a huge interest in plant-animal coevolution and, as an evolutionary botanist, delving into a story such as this is like dining at a five-star restaurant (minus the huge bill at the end!).  A reference for those wanting more is the article by  Pellmyr et al. (1996), referenced below.  This is recommended reading for those who find this plant-moth saga as fascinating as I do.

One never knows where an afternoon of botanizing will lead….


Powell, A.M., 1988. Trees and shrubs of Trans-Pecos Texas including Big Bend and Guadalupe Mountains National Parks. Big Bend Natural History Association.

Pellmyr, O., J. N. Thompson, J. Brown, and R. G. Harrison. 1996. Evolution of pollination and mutualism in the yucca moth lineage. American Naturalist. 148: 827–847.

Copyright © Rick L. Hammer 2009

Wavy-leaf Gaura


Wavy-leaf Gaura, Gaura sinuata, with a number of flowers clustered together in the above image.  There are 4 clawed petals (slender stalk at the bottom of the petal), almost standing erect and spaced equally apart (see the flower in the upper right corner), and 8 stamens (male flower parts) hanging down in front.

This is one of several species of Gaura found here in the Rolling Plains and is widespread in Texas. The Flora of the Great Plains describes this particular species of Gaura as “often too common in sandy prairies & open woodlands, stream valleys, roadsides, waste places.” Nevertheless, an attractive part of our local flora.

Greenthread on the Rolling Plains


This might be the second most dominant composit, or member of the sunflower family, that is flowering out here in the western part of north-central Texas.  We are experiencing a very dry spring so I feel lucky to have the few early spring wildflowers that are presently in bloom.  Greenthread, or Thelesperma filifolium, is an annual or short-lived perennial commonly found in prairies, roadsides and disturbed areas.

For those who are not familiar with the floral structure of flowers in the sunflower family the first thing to know is that the above image is actually an entire inflorescence, a collection of about 20-25 flowers.  The yellow “petals” you see along the outside of the flower head are called ray florets and the different-looking florets in the middle of the head are called disk florets.  I really like the reddish-brown color of the disk florets in Greenthread.


The above image shows the plant habit with the linear leaves.  Also notice that the grasses surrounding are colorless, a good confirmation of our state or dryness.  Greenthread occurs in the same habitat as Engelmann’s Daisy (previous post) and the two make a good spring duo.

Engelmann’s Daisy in full bloom


We have a number of wildflowers in bloom on the Rolling Plains but at least in and around Abilene it seems that Engelmann’s Daisy, Engelmannia peristenia, is the most common at this time, followed by Greenthread, Thelesperma filifolium (more on this species later). Engelmann’s Daisy is abundant along Interstate 20 north of Abilene and also in town along the US83/84 freeway. The name for this species that many may be familiar with is Engelmannia pinnatifida (published 1840).  But some enterprising botanists dug up an even older name from the scientific literature that was published in 1832, which is E. peristenia.  In taxonomy the oldest, validly published name takes precedence, even though we don’t sometimes discover these older names until 100s of years later.

I have enjoyed seeing this plant lining the roadsides over the last week.  While out walking among the populations today I could see many plants that have yet to bloom.  So, we should be in for a good stretch of blooms from this species for the next several weeks. Engelmann’s Daisy is supposedly found throughout all of Texas except for the forested sandy areas in the eastern part of the state. According to the Flora of North Central Texas, farmers and ranchers refer to this as an “ice cream” plant because it is preferred by livestock. This is why it is so abundant along highway right-of-ways and not found in areas where grazing animals can get at it.

Engelmann’s Daisy is a member of the sunflower family or Asteraceae.  Here’s a photo which shows the stem and leaves:


In the taxonomy world we call this genus (Engelmannia) a monotypic genus, since it has only one species which is this one. Engelmann’s Daisy is named for Dr. George Engelmann (1809-1884), a German-born botanist and physician of St. Louis.  This attractive plant is native to North America and I think it would look nice  in a cultivated bed.

While out scouting for blooms today I also found Greenthread in flower (Thelesperma filifolium) and Texas Sage (Salvia texana).  Stay tuned.  I have pictures…..