A Brief Introduction to a Very Thorny Subject

We use the word "thorn" very generically to refer to any sharp pointy part of a plant. However, to be technically correct, there are three different types of sharp pointy things:

• Prickles are modified epidermal tissue. The epidermis is the surface layer of the stem. Prickles are found on many plants in the Rosaceae (Rose) Family, as well as other plants such as Greenbrier vine.
• Spines are modified leaf tissue. The sharp pointy things on cacti are spines. The sharp tips of Agarita leaves and the sharp projections along the margin of Sotol leaves are also spines.
• Then there are the true thorns. Thorns are modified branch tissue. They are essentially branches with sharp pointed tips. Numerous trees and shrubs have true thorns.

This is going to be a series of posts. Probably the best place to begin is with the Bluewood Condalia (Condalia hookeri). This Condalia generally grows as an understory shrub in wooded areas of the Texas Hill Country and to the south in the open brush throughout the South Texas Brush Country.

The leaves of Bluewood Condalia grow on twigs with a very sharp terminal thorn. This is by far one of the best examples demonstrating that thorns are in fact modified branches:

Lote-bush Condalia (Ziziphus obtusifolia) looks very similar to Bluewood Condalia and the two are easily confused. Their ranges overlap in Texas. While they are not in the same Genus, they are both in the Rhamnaceae (Buckthorn) Family. Both of these shrubs have leaves on twigs with terminal thorns. The Lotebush, as it is often called, has twigs with leaves as well as bare twigs both with terminal thorns.

This set of photographs was taken along the Yanaguana Nature Trail at Mission San Juan in the San Antonio Missions National Historical Park on Monday, October 28, 2013.

I have to give credit to Dr. Floyd Waller, my botanist friend and mentor, for the original inspiration for this series of posts.