Thursday, September 1, 2011

More corrections

Readers have alerted me to two more errors on PATSP, having to do with one of the profile posts and one misidentified plant. Then I ran into another thing all on my own, where something in one of the profiles needed a bit of clarification. So here we are again.

1. Ficus elastica and isoprene

In the Ficus elastica profile, I sort of implied that the people making natural rubber extract isoprene from the sap of Hevea brasiliensis and other plants, then polymerize it into natural rubber. This is not the case.

Model of isoprene. Black balls are carbon atoms, white are hydrogen, and the gray connectors represent pairs of electrons. This is my own photo: I would have used it in the Ficus elastica profile, except that I'd forgotten I owned a molecular model set.1

Straight-up, pure isoprene can and does exist. It's a colorless, low-boiling liquid which is mostly obtained either as a byproduct of oil and naphtha refining, or by heating natural rubber until it starts disintegrating into smaller molecules. The isoprene so obtained can then be polymerized into a substance with properties very close to natural rubber.2 Readers who collect oxymorons will be pleased to know that the term for this artificially polymerized, artificially obtained polyisoprene is "synthetic natural rubber."

However. Natural natural rubber polymerizes within the plant, forming small globs of polyisoprene that float around in the sap. When the sap is collected from the plant, these globs are then coagulated, washed, filtered, pressed, and stretched to form blocks of rubber. So it's still technically correct of me to say that isoprene is polymerized to form natural rubber, but the polymerization has already happened by the time the sap is collected.3

2. Persea americana toxicity

Next up, in the Persea americana profile, I devoted footnote 2 to the toxicity of the plant, saying that all parts of the plant except the fruit should be considered toxic, particularly to pets. Then Poor Richard's Almanac had to go and spoil that one for me by writing a post about the culinary use of avocado leaves.

My Persea americana, as of yesterday morning. I had two plants to begin with, and planted them together in this pot to try to make it fuller. That's mostly worked, but the two plants together seem to dry out a lot faster than they did when separated, so there have been more dropped leaves and

This sent me deep into the bowels of the internet to do research. I won't bore you with all the twists and turns, but the gist is:

Yes, people really do cook with avocado leaves.

No, not any avocado leaf will do: the variety used in Mexican cooking is a specific race of the avocado (according to some sources it's a separate species, Persea drymifolia, but others consider it just a race of P. americana).

Avocado leaves should still be considered toxic to all pets, especially especially especially to birds: ALLLLLLLLLLLL the bird-toxicity lists say Persea fruit or foliage is potentially lethal for at least some species of bird, and they say this over and over again, in extremely shrill and insistent language.

Humans don't seem to be as affected by the toxin in avocado leaves as animals are, and animals aren't all affected to the same degree. Wikipedia's article on persin, the actual toxic agent in Persea americana leaves,4 reports that consumption of avocado leaves produces a wide array of unpleasantness in a whole barnyard full of animals (cats, dogs, rabbits, birds, mice, cows, goats, horses, pigs, sheep, ostriches, chickens, turkeys and fish), ranging from reduced milk production all the way up to asphyxia and death. (Wikipedia's original source is a bit more detailed, q.v.)

The way to tell whether you have one of the cooking-type avocado leaves or the useless and/or poisonous avocado leaves: leaves which are okay to cook with will smell like anise, and the fruits will have much thinner skin. Generally, if you start an avocado plant from a supermarket fruit's seed, you're not going to end up with leaves you can cook with.

So I was partly wrong: a subset of avocado plants have leaves which are not toxic, at least not in the small quantities needed to flavor food. (I don't recommend sitting down and eating ten leaves in one sitting, even if it is the drymifolia variety.) In the context of a houseplant that was started from a supermarket avocado, though, you're better off treating it as toxic. Even if it doesn't hurt you, it's not likely to do anything for your food.

3. Aloe aristata isn't Aloe aristata

Finally, I've found out from Taylor Holzer (a reader) that I've been calling the below plant Aloe aristata when it is in fact a hybrid of Aloe aristata and something else.

My Aloe aristata x whatever, in January 2011.

The most likely candidate, I think, is Gasteria batesiana, which cross is called Gasteraloe x beguinii, but a similar hybrid is formed by A. aristata and A. variegata, and I'll probably never know for sure which cross I have.

A. aristata is apparently distinguishable by the dead tips on the ends of the leaves, and the thinner, less fleshy leaves. (I'd thought that maybe the differences just depended on cultural conditions -- since I'm not growing them outdoors, a lot of my plants don't look quite like they ought to. But sadly, no.) It's also reluctant to offset, is the rumor, whereas my plant offsets more or less constantly. My guesses about the actual ancestry are based both on Holzer's own guesses and on this discussion thread at

Aloe aristata hybrid (L), Aloe aristata species (R). Photo credit: Taylor Holzer. Used by permission.

I will go through the blog and change the name at some point, but I just finished a few rounds of name-changing, and am pretty sick of it, so it might take me a while. (It's bad enough that I have to go change two profiles.) Since some of you got offsets of this plant from me, though, I figured I should let you know that it was half-misidentified.


Photo credits: Mine except where noted.

1 Yes, I am that nerdy. In fact, it's actually quite a bit worse than you think, because I don't just have a molecular model set, I have six, which were purchased between my senior year of high school and my sophomore year of college. (The first one, in high school, was I think actually a birthday or Christmas present from Mom and Dad, but I think I bought the other five. It's been a while; some of the details are fuzzy.) So I can model anything up to about 80 carbon atoms. Here is a short segment of a natural rubber (cis-polyisoprene) molecule, for example:

2 It's not exactly the same as natural rubber because natural rubber contains impurities from the original sap: fatty acids, proteins, inorganic compounds ("ash"), that sort of thing.
3 (If you want to be even more technical: the plants don't polymerize isoprene, but its phosphorylated form.)
4 Regrettably, in the course of editing this post, I lost a "persin"/"person" pun I was very proud of. I could put it here anyway, but it was context-dependent and wouldn't be funny without the set-up. I just wanted you to know.


our friend Ben said...

Excellent, Mr.S., and thanks for your research! Sorry you lost your persin/person pun, though, but I know how that goes! At least you have your wonderful models; Im so jealous!!!

Pat said...

Do you have some sulphur model atoms so you can vulcanise that rubber?

Persinal variation could affect your response to avocado toxins.

Bom said...

No rush. Thanks for alerting to the corrections.

mr_subjunctive said...


In fact I do, but if memory serves, one's tetrahedral and the other is octahedral. (I'm also limited on phosphorous -- the models are different sets from the same manufacturer, and only one of the sets had a phosphorous atom. No diphosphates for Mr. Sunbunctive. :^( )

Anonymous said...

The isoprene correction is still not correct. Plants don't make natural rubber from isoprene, they make it from DMAPP and IPP which are close to isoprene but definitely different. A few plants make isoprene from DMAPP, but once it is in this form, it is stable there and cannot be used to make rubber.

mr_subjunctive said...


Of course I meant Subjunctive. I don't know who this Sunbunctive guy is. Though he's probably a lot more fun.


It's possible that you have PATSP confused with a chemistry blog.

I recognize that IPP and DMAPP are the biologically active forms of isoprene in the plant. I just don't see the distinction as being relevant for purposes of the post. As I tried to explain to you by e-mail, this blog is read by people with both scientific and non-scientific backgrounds, so I try to balance off accuracy with readability as best as I can. I don't see that anything is gained by acknowledging the diphosphate group in the discussion, since it isn't incorporated into the final product, and serves mainly to activate the isoprenyl part of the molecule and/or facilitate its transport around the plant.

The whole rubber = polyisoprene thing was already a digression about a digression (the actual topic of the post, remember, was Ficus elastica, with emphasis on its use as a houseplant). I don't feel compelled to dig ever deeper into the technical details and jargon until I'm explaining natural rubber in terms of quarks and the Big Bang. (Even my pedantry has limits.) I'm sorry if that disappoints you.

Pat said...

Perhaps Mr Sunbunctive follows the example of Heracles who wore nothing but a short lionskin. He once let a pair of thieves go because they amused him so much by their funny comments on his very tanned bottom.

mr_subjunctive said...


I have relented and included links to the Wikipedia pages for IPP and DMAPP in the new version of the post.