Apologies for the very late entry today — we were waiting for either of the final two entries for UBC Research Week. It looks like that won’t happen, so we’ll just move on. Connor Fitzpatrick wrote today’s entry:
Beaucarnea recurvata is a slow-growing monocot evergreen tree in the Ruscaceae. Native to Mexico, it’s distinctive features (PDF) include a swollen base capable of achieving considerable girth (2m) and clusters of long, slender leaves produced on branch tips. The graceful descent of the leaf clusters elicits the image of a pony’s tail aimlessly swaying, hence the common name of ponytail palm. As a digression, my family was fortunate enough to befriend a miniature pony named Bonita, whom we would later adopt and bring home. A little less than fondly, I remember unsuccessfully dodging a few kicks in order to steal hairs to make a genuine ponytail paintbrush. Little did I know that the real treasures came from the backside of a camel instead.
Like all other monocots, the ponytail palm (incidentally not a palm at all), lacks true secondary growth. Its increase in girth results from cellular divisions of a secondary thickening meristem (STM). Note that this isn’t a true secondary meristem, as monocots lack these (such as the vascular cambium or the cork cambium). This is akin to the true palms, such as the wax palm, which produce width by diffuse secondary growth where cells in older parts of the stem undergo division and primary tissue becomes lignified (also see Anatomy of Monocot Stems via Wayne Armstrong). Other monocots, such as the banana, have many layers of leaf bases and give the appearance of a thick trunk, but only have a primary thickening meristem. For more reading, here is an interesting article regarding tissue development in monocots.