Anatomy: Protoyucca

Protoyucca

Protoyucca shadishii
Virgin Valley Formation
Miocene
Vya, Nevada
7cm Diameter

The picture above shows both the primary and secondary cylinder. You can clearly see the difference in the arrangement of vascular bundles. The vascular bundles in the secondary cylinder are much more orderly. The specimen below illustrates the transition between the secondary cylinder and the periderm. The secondary thickening meristem (STM) has eroded, but its former location is noted.

Joshua tree
Yucca brevifolia
I stand next to my grandmother Grace Furnish
My mom, Noni sits in our 1969 Datsun 510, while my dad, Wayne, takes the picture.

The Joshua tree forms scattered groves on arid mesas and mountain slopes. It is widely distributed in the Mohave Desert and is found in California, Arizona, and Nevada (Elias, 1980, p. 910). Two coevolutionary relationships have been important for the Joshua tree's reproduction, one for fertilization and the other for seed dispersal. During the last ice age Joshua trees were widespread across the American Southwest.

Gypsum cave in Clark County, Nevada holds clues to the past success of the Joshua tree. In the 1930's skeletons, hides, hair and dung of the giant ground sloth Northrotheriops shastensis were discovered in Gypsum cave. Layers of 13,000 year old dung revealed that the Shasta ground sloth's favorite food were the leaves, fruits and seeds of the Joshua tree. During the ice age giant ground sloths provided a method of seed dispersal, via their droppings, which positively influenced the geographic range of the Joshua tree (Shogren, 2008). This type of seed dispersal had the added advantage of providing a natural fertilizer.

Today the single surviving species of Joshua tree, Y. brevifolia, has a much smaller geographic range. Still, the tree depends on an animal for its reproduction, specifically for fertilization. The "Yucca Moth" Tegeticula synthetica has an obligate pollination mutualism with the Joshua tree. The white female moth visits the flowers when they open at night. She visits and collects pollen from one tree and then visits another. The female moth deposits her eggs inside the ovary of the flower. She then rubs the pollen on the stigma of the flower. After completing the necessary cross-pollination the seeds develop and some serve as food for the developing moth larvae (Bland & Jaques, 1978, p. 304).

Emily the Joshua Tree
Emily, then the largest Joshua tree in the world,
in 2005 with her friend Frank Daniels.
Photo by Frank Daniels, (c) 2006-2013

Bibliography

Bland, R.G. and Jaques, H.E. (1978). How to Know the Insects [3rd Ed.] Dubuque, IA: WCB McGraw-Hill

Elias, T.S. (1980). The Complete Trees of North America. New York: Van Nostrand Reinhold Company.

Shogren, E. (2008). Outlook Bleak for Joshua Trees. NPR
http://www.npr.org/templates/story/story.php?storyId=17628032

Tidwell, W.D. and Parker, L.R. (1990). Protoyucca shadishii gen. et. sp. nov., An Arborescent Monocotyledon with Secondary Growth from the Middle Miocene of Northwestern Nevada, USA. Review of Palaebotany and Palynology. 62, pp. 79-95.