Aloes

Aloe ferox

The orange-red inflorescence of an Aloe ferox.

The recent prolific mid-winter flowering in a range of Aloe species had piqued my interest again in this mostly African genus of plants.

There’s been at least half a dozen species flowering and after a good soaking this year most have been throwing out plenty of flowers in response to just a bit of mid-winter sun.

Aloe arborescens

Aloe sp.

Aloe sp.

Aloe ferox under a Dragons Blood tree (Dracaena cinnabari)

Aloe vera

Aloe sp.

 

The popularity of Aloe vera gel for skin and digestive health and repair has grown around the world, though it seems many people don’t really know where the plant originates and how large the Aloe genus is. There are 516 species recognised currently according to The Plant List.

Aloe ferox is one the primary sources of an old medicine, Aloes, often called Cape Aloes. The dried yellow exudate from just beneath the skin of the leaf. The golden orange-brown colour being due to the characteristic presence of the anthraquinones known as aloins (aloe-emodin, barbaloin and isobarbaloin), and this thin layer of latex producing cells just below the skin is a characteristic of all Aloes.

Aloe ferox – cut leaf with orange brown latex

Aloe ferox – leaf with anthraquinone bearing latex cells

Anthraquinones are quite well-known and found in various forms in a range of other long utilised herbs, such as rhubarbs (Rheum), docks (Rumex) and knotroots (Polygonum, Fallopia), Cassia and Senna, buckthorns (Rhamnus) and madders (Rubia). Mostly they are from a few specific and smaller families. The main action of anthraquinones in humans, long known across many cultures, is as a laxative or in higher doses as a purgative.

Cape Aloes once held a significant place as a herbal medicine, though it is less known, used or available in the herb trade of contemporary times. There does still seem to be some small industry mostly in Africa. Once widely used in low doses in many patent and over the counter remedies, or in bitter tonics, it has fallen largely out of use. The FDA withdrew its status as GRAS (generally regarded as safe) in the US in 2002 after no product sponsors came forward to provide supporting evidence for a review, or perhaps no sponsors had any significant products containing it at the time. It seems that the less concentrated and potent Senna or Cassia products are readily available and preferable agents for most these days.

Aloe socotrina – another historical source of Aloes latex

The anthraquinones themselves, from Aloe and other plants, continue to be investigated for a range of pharmacological activities aside from their laxative action. With anti-inflammatory, anti-fungal, anti-cancer and even anti-malarial research using anthraquinones. Of course with such strongly active compounds there are some issues when it comes to chronic consumption, though this applies to Senna and other anthraquinone based laxatives too. We don’t absorb them very well into our circulation generally, they act directly on the large bowel for the laxative action by decreasing water and electrolyte reabsorption.

The strength of some anthraquinones as pigments also saw them utilised in dyeing of materials. With the anthraquinone alizarin from madder (Rubia tinctorum) the first chemically synthesised dye, turkey red, that was widely available.

Aloes were considered members of that diverse tribe of plants the Liliaceae, the Lilies. Though it’s not really until they flower that many would see the relationship with what most would think of as Lilies.

The diversity of the Liliaceae seems to have all been a bit of taxonomic nightmare for a while, so it was interesting to see how a few different sources classified Aloe even now. For those readers with little knowledge of the complexities of plant taxonomy, the confusion probably starts here.

I found a few of the sources I would regard as the best that at first glance had a slightly different take on the taxonomy of Aloes.

The Plant List places them in the Family Asparagaceae, alongside a large bunch of varied plants from Asparagus and Aspidistra to the Butchers Broom, Ruscus.

Whilst African sources (eg PlantzAfrica and Flora of Mozambique) seem to still have them in a family all their own, the Aloeaceae.

Xanthorrhoea australisvia Wikipedia

A review in the Annals of Botany (pdf) and Angiosperm Phylogeny  site has them in the Order Asparagles, family Xanthorrhoeaceae, sub-family Asphodeloideae. Which is kind of interesting from an Australian perspective given the widespread endemic occurrence of Xanthorrhoea, grass trees, here. Not an immediately obvious relationship to most I’m sure. It is around a 180 million years since Africa and Australia were connected, via Antartica, as part of the Gwondana supercontinent. Though phylogenetic studies suggest that Xanthorrhoeaceae is only from 50-90 millions years old, which is evidently a long time after Africa and Australia were separated. The Aloeaceae and Asphodelaceae are evidently synonyms for some members of the this family.

It’s easier to see a relationship with our local Bulbine lillies (Bulbine bulbosa and others) that are considered members of the sub-family Asphodelaceae as well.

Bulbine bulbosa

The type species of the Asphodelaceae, Asphodelus, is named after the mythical meadows of Asphodel a kind of middle of the road domain of the underworld where the dead ate flowers, apparently for lack of much else to do. However the brilliant show from the Aloes seems more appropriate for the everlasting reverie of the Elysian fields. Perhaps that is how our local honey eaters (birds) have felt at least, with the winter long supply of nectar from these colourful Aloe flowers that they’ve been enjoying.

Aloe ferox

Further reading:

Chen, Van Wyk, Vermaak and Viljoen, Cape Aloes – A review of the Phytochemistry, Pharmacology and Commercialisation of Aloe Ferox, Phytochemistry Letters, vol 5: 1-12, 2012

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