Since I know I wouldn’t be surprising anyone to reveal that this photo isn’t of a tortoise shell, the best I can add is that it is also not the bark of a tree. If you’re not familiar with Dioscorea mexicana or other similar species of Dioscorea (like me before writing this entry), you might not be able to guess what this photo depicts. This structure is the dome-shaped caudex of Dioscorea mexicana, covered in woody polygonal plates.
Commonly known as the tortoise plant or Mexican yam, Dioscorea mexicana is one of around 600 species of Dioscorea, including several of which are culinary yams. The Mexican yam’s range spans from northern Mexico south to Panama; its preference is dry environments. Like all caudiciforms, its caudex base is a modified stem (or rootstock) designed to hold water and nutrients in dry environments where such resources can be hard to come by. This woody caudex can reach 90cm (3 ft.) in diameter and 25cm (10 in.) in height. The furrows between each polygonal plate get deeper with age, in similar fashion to annual growth rings (I assume).
Vines as long as 9m (30 ft.), with large heart-shaped leaves, emerge from the centre of the caudex. The vines typically die back in the winter (such as it is), as the plant usually goes into dormancy. On rare occasions, a second vine will grow alongside the first. This species is dioecious, so each plant bears either female or male flowers. The inconspicuous male flowers hang in clusters from the vine, each with dark purple centres (see photos). Female flowers are greenish-yellow and also very small.
Smithsonian Gardens has an informative page on Dioscorea mexicana titled “I Yam Not a Tortoise but a Plant”; it includes tips on how to grow it. They also mention that this species was originally grouped in a different genus, Testudinaria (named after Testudo, a genus of tortoises).
Compounds in Dioscorea mexicana and other species of Dioscorea facilitate the synthesis of hormones like progesterone and cortisone with the steroid diosgenin. This was known by indigenous peoples of the region, who use/used the species for birth control and treating sore joints. Chemist Russell Marker synthesized progesterone from Dioscorea mexicana‘s naturally produced diosgenin, leading to the production of affordable pharmaceutical birth control.
Dioscorea mexicana was also featured with a brief write-up a decade ago on Botany Photo of the Day.