The last two Thanksgivings we spent down in St. George. This year we had to be back in town for the weekend*, and so just did a short trip down to Moab. We had a wonderful time, but the temps were absolutely frigid. 8F in the morning, and a high of ~24F by mid-afternoon. It was a little odd to be hiking in the desert more bundled up than I typically am when backcountry skiing.
*Because we were hosting a 50th birthday party for Vicente. And when I say “hosting”, that’s all we did. His lovely wife threw the party, but had it at our place as our house was better able to handle the crowd. So mid-afternoon Saturday, the caterer/sushi chef and bartender showed up, did all the set up, threw a fantastic bash, then completely cleaned up. Cost us no effort, no money, nothing. All throughout the evening guests kept coming up to us and thanking us, and we were like, “It was nothing. No really, it was nothing!” The next morning Awesome Wife said, That was fun- we should have more friends throw parties at our house.” So anyway, if you’re looking for a place to throw a party, our house is available. You just have to prep, set up, pay for everything, and clean up. Oh, and you have to invite us.
Side Note: If you do happen to find yourself in the Moab area in colder temps than you bargained for, and you need warm stuff cheap, head to Alco, on the South end of town on the East side of US191.
Thanksgiving morning we headed up to Arches National Park, just a 5 minute drive out of town. We drove up into the park, past Balanced Rock, the Windows section, the turn-offs for Delicate Arch and Fiery Furnace, before parking at the Devil’s Garden trailhead.
Tangent: I’m one of these backcountry snobs who routinely avoids national parks because of crowds, over-development, regulations, etc. But the truth is, every time I go to a national park I have a wonderful experience. No matter how popular the park, it’s possible- with a bit of planning, timing and/or effort, to get away from the madness and have a great time (as we just saw a few weeks ago down in the GC). I need to get over myself and start enjoying the parks more.
Devil’s Garden is series dozens of large, parallel Entrada sandstone fins, running Northwest to Southeast. We’ve come across Entrada before, down around St. George. It was laid down as bottom deposits in a shallow sea some ~160 million years ago. It erodes easily in smooth surfaces, waves, alcoves, and of course arches, but we’ll get to that in a bit. The Devil’s Garden trail winds through this area running alternately around, through, over, along and in between the fins, visiting a dozen + spectacular arches en route.
Setting out from the trailhead I noticed the (as I always do) the shrubs and stopped to check out one of the most interesting desert plants in the Western US: Four-winged Saltbush, Atriplex cansecens.
From a distance, FW Saltbush doesn’t look all that remarkable. In the Fall the female plants are easy to pick out. They’re the ones with branches bearing bunches of what look like little cornflakes, which are actually the fruits*, each of which bears 4 little, papery “wings” (pic left).
*Specifically achenes, which I described in this post.
Extra Detail: In spring, conversely, the males are easy to pick out, with their bunches of tiny yellow flowers. Female flowers are small, green and easy to overlook.
The other half of its name comes from its tolerance for salty, dry soils. FW Saltbush is widespread across Western North America, ranging from Baja to Alberta to Kansas, from sea level to over 6,000 feet. So it’s another wide-ranging drought-tolerant scrubby shrub. But A. canescens is both unusual and way cool in not just one, not just two, but three (yes, three!) ways.
First Cool Thing
First is its wood architecture. Long ago I described the basic wood architecture common to dicots, and- with some variations*- conifers. The main takeaway is that standard wood architecture is defined by an ultra-thin layer of undifferentiated stem-type cells called cambium, which then differentiate into phloem on the exterior, and xylem on the interior. Phloem carries the products of photosynthesis to the various parts of the plant, while xylem transfers water up from the roots to the branches and leaves. Oaks, maples, pines, sagebrush, spruces, roses- their wood all follows this basic architecture.
*Vessels in dicots are a good example; conifer wood doesn’t have them.
Some trees don’t share this standard wood architecture. “Woody” monocots, like palms or Joshua trees for example, have completely different wood architectures, which they evolved independently from their herbaceous (non-woody) monocot ancestors.*
*Since no monocot has the standard dicot/conifer wood architecture, it’s assumed that the common ancestor of all monocots was herbaceous.
FW Saltbush is a dicot (eudicot) and belongs to Chenopodiaceae, the Goosefoot family*, the family to which spinach and beets belong. Members of this family- I’ll call them “chenopods” for purposes of this post- have a distinctive wood architecture that is significantly different from the standard wood architecture. Specifically, rather than a single strip of cambium surrounding the stem, roots and branches, chenopods grow multiple, successive, concentric layers of cambium each of which produces its own “secondary” phloem and xylem. This architecture is called successive cambia. You can see successive cambia when you slice open a beet cross-wise (pic left, not mine***).
*Chenopodiaceae may sorta/kinda be obsolete now, as nearly all of it looks to be getting reclassified as the sub-family Chenopodioideae within Amaranthaceae, the Amaranth family. This sort of thing happens all the time these days**, with newer genetic data forcing researchers to re-think traditional taxonomic groupings. I stuck with the traditional taxonomy for this post, mainly because so many of the sources I used referenced it. And I’m lazy.
**But it doesn’t bother me, and here’s why: this kind of sorting and categorizing and re-sorting and re-categorizing happens all the time in sales (my trade) at a big company. If you’re not in sales, you might think that the sales process ends when the order is signed. But at a big company that’s just the beginning of a lengthy process of follow-up, paper-chasing and arguing with numerous internal bureaucrats over contracts, invoicing, commissions and credit. Here’s an interesting corollary: The bigger the company is, the less a salesperson’s success depends upon his/her personal client-facing sales ability, and the more it depends on his/her internal political and organizational abilities. Seriously. I’m not kidding, it’s true. This is just one of the countless, off-topic yet utterly brilliant insights you get from reading this blog.
***Is that the weirdest blog or what? Guess I shouldn’t talk…
The benefit of successive cambia isn’t exactly clear, but there’s some indication that it helps improve the flexibility as well as the strength when wood grows asymmetrically. Though successive cambia is unusual in shrubs and trees*, it’s very common in lianas**.
*None of which grow very tall.
**Woody vines rooted in the ground that grow up and all over trees- very asymmetrically- in tropical forests. They’re a form, not a closely-related group. Like a “shrub” or a “tree”, for example. Bougainvillea is an example of a liana. See the Phytophactor’s recent post and photo on lianas down in Costa Rica.
It’s not clear exactly what drives the evolution of successive cambia. One hypothesis is that the common chenopod ancestor was herbaceous*, and evolved wood all over again. If so, it would be another version of the monocot re-invention story, but within the eudicots, which is kind of cool.
*I only found this mentioned in one source, Hugh Mozingo’s Shrubs of the Great Basin. Finding no other mention of it, and seeing what appear to be many details in common with the standard wood architecture, I sort of doubt it.
Whatever the benefit, it must be significant. Successive cambia has evolved independently dozens of times, and not just within angiosperms. It’s actually common in the gnetales, including our own, close-to-home Mormon Tea (also all over the place in Arches) as well as the bizarre Welwitschia on the other side of the world.
Second Cool Thing
The second cool thing is that FW Saltbush is C4 photosynthetic. I described C4 photosynthesis in detail in this post*, and we’ve looked at several C4 plants before. But most commonly-known C4 plants, and most of those we’ve looked at, like corn and crabgrass, are monocots**. FW Saltbush is a eudicot C4 plant. C4 has evolved at least 20 times among the eudicots, 10 of which have been within the chenopods. One of those 10 times was in the genus Atriplex, whose common ancestor appears to have evolved C4 independently sometime in the last 8-12 million years. So these chenopods have this cool alternative wood architecture, and they’re the most C4-adept family of eudicots. Pretty cool family.
*Man, it is like I have a post for everything.
Side Note: Shadscale, Atriplex confertolia, is a close relative of FW Saltbush, and a super-common (if small and unimpressive-looking) shrub across the Southwest. (It’s in that category of so-common-I’m-embarrassed-I-haven’t yet-blogged-about-it.) In any case, Shadscale shares the first 2 cool things- successive cambia and eudicot-C4- as well.
Third Cool Thing
The third cool thing is its sex-determination system. As I already implied, FW Saltbush is generally dioecious, in that plants bear female or male flowers, but not both. But some of them- a small minority- do bear both male and female flowers, sometimes on the same branch, sometimes on different branches. And sometimes these “switch-hitters” switch back and forth from year to year. Hold that thought.
FW Saltbush exhibits all kinds of polyploidy*. There are diploid plants, with 18 chromosomes, tetraploids, with 36, hexaploids with 54, and even 12-ploids with 108! But here in Utah, and practically everywhere North of latitude 37N, nearly all FW Saltbushes are tetraploid. But there is a population of diploid FW Saltbush not far from here, growing along the edge of the dune fields at Little Sahara Recreation Area. And the interesting thing about these diploids is that all of them are male or female- there are no switch-hitters.
*I explained polyploidy in this post.
Extra Detail: The other interesting thing about them is their height- more than twice normal. A. canescens generally grows to about 4 feet in height, but the Little Sahara diploids grow up to 10 feet high. This was originally suspected to be a characteristic of the diploids, along with faster growth. The thinking was that the species was originally diploid, but that smaller, slower-growing tetraploid race had come to dominate Utah as the climate dried out. However, in the rapidly shifting landscape of the dune-margins, survival favored a fast-growing relict diploid population. But in the years since, plenty of regular-sized diploids have been found down in New Mexico and Texas.
Tangent: I’ve never seen the Little Sahara diploids, because in 15 years here in Northern Utah, I’ve yet to visit Little Sahara. I love dune fields, and have visited several across the West. But the state of Utah runs Little Sahara overwhelmingly as an OHV/ORV playground, which isn’t really my thing.
The explanation was this: Dioecious plants determine sex genetically, through specific sex chromosomes, analogous to mammals, where XX is female and XY is male*. And that’s what was going on with the Little Sahara diploids. But with polyploidy, new combinations become possible. The tetraploid plants might be XXYY and male, or XXXX and female. Or they might be XXXY and be either/or. (XYYY and YYYY combinations were/are(?) thought to be non-viable.)
*Just to make things more complicated, some plants, like strawberries, have evolved a system more analogous to that of birds (which I explained in this post), where the female form is expressed through the heterozygous form (WZ) and the male through the homogeneous (ZZ).
It was a neat, tidy hypothesis, but it’s since been undermined by the discovery of numerous monoecious, “switch-hitter” diploids down in Texas and New Mexico. A. canescens gender expression is now suspected to be influenced by both genetic and environmental factors. What’s cool about FW Saltbush sexuality is that on the one hand it’s this wonderful cross-kingdom example of convergent evolution of a chromosomal system of sex-determination. Yet at the same time there are hints that it may be more subtle and “plastic” than the binary on/off sexuality of mammals and birds.
Anywho, where was I? Oh, right the Entrada sandstone and the hike. Oh man, looks like I ran out of time on this post-long shrub-tangent. We’ll have to pick it back up tomorrow, but I promise I’ll quit dinking around and get to the hike!
Note about sources: Info on successive cambia came in large part from the work of botanist Sheldon Carlquist, including Successive cambia revisited: ontogeny, histology, diversity and functional significance, as well as from his wonderfully informative website. Additional successive cambia info came from Origin of successive cambia on stem in three species of Menispermaceae, Neusa Tamaio et al, and Xylem function of arid-land shrubs from California, USA: an ecological and evolutionary analysis, U.G. Hacke et al. Info on C4 occurrence and evolution in Chenopodiaceae and specifically Atriplex came from Phylogeny of Amaranthaceae and Chenopodiaceae and the Evolution of C4 Photosynthesis, G. Kaderet et al. Info on sex determination in A. canescens came from The Effects of Chromosome Number on Sex Expression in Atriplex canescens, Jerry R. Barrow, and Hugh Mozingo’s Shrubs of the Great Basin.
Note: The original version of this post contained a (bass-ackwards) error* in the the wood architecture recap. Many thanks to readers Sally and Ford for the catch.
*I make frequent errors in this project and welcome corrections. My hope is to leave this blog around for a while so that future “motivated amateurs” can get a head-start on some of the topics covered here, and the fewer bum steers I leave for them, the better.