Of Shrimp Plants and Bear's Breeches

For today's semi-random post, I drew the plant subfamily Acanthoideae. As recognised by Scotland & Vollesen (2000), the Acanthoideae is the largest of the subfamilies of the Acanthaceae by a considerable margin, including about 95% of the family's 2500+ species. Though perhaps not hugely familiar to readers in more temperate climes, the Acanthoideae are one of the dominant groups of herbs and shrubs in tropical parts of the world.

Golden shrimp plant Pachystachys lutea, copyright Dryas.

The Acanthoideae have been recognised as a morphological group since the late 1800s and their integrity has been confirmed by more recent molecular studies. They are distinguished from related plants (within the Lamiales, the order that also includes such plants as the mints and snapdragons) by having capsular fruits that dehisce explosively when mature to scatter their seeds. The seeds are attached within the capsule by hook-shaped stalks called retinacula that presumably play a role in determining how the seeds are released. A classification of Acanthaceae published in 1965 by Bremekamp restricted the family to species with explosive fruits and retinacula, dividing them between two subfamilies, the Acanthoideae and Ruellioideae, based on the absence or presence, respectively, of cystoliths. These are outgrowths of the epidermal cell walls that are impregnated with calcium carbonate. They are visible in the stems and leaves, at least in dried specimens, as hard white streaks. As phylogenetic studies have supported division of Acanthoideae in the broad sense between a cystolith-possessing and a cystolith-lacking clade, the decision whether to recognise 'Ruellioideae' as a separate subfamily comes down to a ranking choice only. At lower levels, the classification of Acanthoideae is less straightforward. Over two hundred genera of Acanthoideae are recognised but just three of those—Justicia, Strobilanthes and Ruellia—account for about half the total number of species. Each of these mega-genera is morphologically diverse and likely to be para- or polyphyletic with regard to related taxa, raising the distinct likelihood of future revisions.

Spiny bear's breeches Acanthus spinosus, copyright Magnus Manske.
Economically, few of the Acanthoideae are of great significance except for a number of species being grown ornamentally. One such species is Acanthus mollis, which goes by the vernacular name of 'bear's breeches' (why, I have absolutely no idea). Acanthus was a popular decorative motif in classical Greece and forms the basis for the design of Corinthian columns. Its use as an ornamental has lead to it becoming regarded as an invasive weed in some regions, largely because this is one of those garden plants that Just Will Not Die, spreading easily from seeds and tubers. We've got some in a pot outside that is currently flourishing despite having been burnt down to a nub by the searing Perth summer sun, metaphorically shouting its defiance at an uncaring world.

REFERENCE

Scotland, R. W., & K. Vollesen. 2000. Classification of Acanthaceae. Kew Bulletin 55 (3): 513–589.

Sorting Monkeys: Dissecting Mimulus

New Zealand musk Thyridia repens, one of the species previously included in Mimulus section Paradanthus, copyright Jon Sullivan.

Long-time readers of this site will know that often, when I pull out the name from the virtual hat of my subject taxon for the week, I find that the greatest challenge lies in determining just what that name applies to. I knew that I was in for another one of these challenges as soon as I established that this week's post would be on the section Paradanthus of the plant genus Mimulus.

Mimulus, as it is most commonly recognised, is a genus of about 120 species found in many parts of the world but with by far the greatest diversity in western North America (where about three-quarters of the recognised species are found). They are commonly known as monkeyflowers or muskflowers in reference to the appearance and scent of the flowers of some species. Because of their diversity (in a well-studied part of the world), monkeyflowers have attracted a fair bit of interest as a model system for studying processes of evolution and speciation. Most Mimulus species are herbs though some are small shrubs. However, even shrubby forms do not have extensive root systems, and they are mostly annual or seasonal; perennial forms usually die off above ground over winter, growing back from the rootstock when conditions improve. These perennial forms may also propagate vegetatively through drooping stems putting down new rootstock where they contact the ground (Grant 1924). Mimulus species primarily grow in damp habitats; some will even grow in standing water.

Nepal monkeyflower Erythranthe nepalensis, copyright Qwert1234.

Until fairly recently, Mimulus was primarily classified in the family Scrophulariaceae. Members of this family (including Mimulus) were united by the possession of a flower type referred to as a 'scroph'; examples of plants with scroph flowers that may be familiar to you include snapdragons or foxgloves. Characteristic features of a scroph include having the calyx and corolla each fused basally. The corolla is hence more-or-less tubular at the base, then divided towards the top into five out-turned lobes corresponding to the petals (usually with two above and three below; the flower is therefore zygomorphic or bilaterally symmetrical). This flower type is primarily adapted to pollination by insects such as bees that use the lower lobe (or lip) as a 'landing ramp' when visiting the flower. After pollination, the flower develops into a dehiscent, many-seeded capsule. The advent of molecular phylogenetic analysis has established, however, that scroph-flowered plants belong to several lineages within the order Lamiales; the scroph has apparently evolved (and been lost) on a number of occasions. As a result, Mimulus is now placed in a separate family Phrymaceae with a handful of small, closely related genera.

Muskflower Erythranthe moschata, copyright Nick Moyes.

Mimulus was divided into two subgenera and ten sections by Grant (1924); with minor modifications, her system remained in place until recently. The two subgenera, Synplacus (or Mimulus proper) and Schizoplacus, were recognised based on whether the flower's placenta was united or divided, respectively. Four of Grant's sections were placed in the subgenus Mimulus; one of these was Paradanthus. Grant's system was not entirely phylogenetic as we would understand the term today: she did provide a diagram of suggested relationships between the sections but it did not cleanly separate the two subgenera. The section Paradanthus, in particular, was explicitly established as a convenient holding-place for a number of "small closely allied associations which, however, were not sufficiently distinct to warrant being placed in sections by themselves". Most had relatively unspecialised flowers, mostly (but not always) with funnel-shaped corollas and more or less equal lobes. Grant also somewhat unceremoniously dumped the majority of non-American Mimulus species into Paradanthus. She confessed that this section was "a collection of groups not necessarily related to one another and in all probability most of them have been derived from members of the other sections".

Phak taptao Mimulus orbicularis, one of the few true Mimulus species under the system of Barker et al. (2012), from here.

It therefore came as no surprise whatsoever when later phylogenetic analyses did not uphold the section Paradanthus as monophyletic. Instead, the primary division within Mimulus found by Beardsley & Olmstead (2002) was between a clade centred on western North America and one containing the majority of species from elsewhere in the world, with the Paradanthus species falling in either clade depending on their distribution (many of Grant's other sections, in contrast, do correspond to monophyletic groups). Perhaps more unexpected was the finding that Mimulus as a whole was not monophyletic. Instead, other genera of Phrymaceae were nested in Mimulus, including Phryma, a genus of one or two species found in eastern North America and eastern Asia in which the fruit is a single-seeded achene instead of a multi-seeded capsule, and Leucocarpus, a Central American genus in which the fruit is a fleshy berry.

The non-monophyly of Mimulus raised the question of whether these other genera should be subsumed within a broadened Mimulus (in which case the genus Mimulus and the family Phrymaceae would potentially become identical in content). An alternative tack was proposed by Barker et al. (2012) who divided the 'traditional' Mimulus into several genera corresponding to monophyletic clades separated by the previously recognised segregate genera. Of the 120 or so original species, only seven remain in the restricted Mimulus (species of Grant's section Paradanthus end up divided between no less than four genera). None of the western North American species remain in Mimulus; instead, species from this region are divided between the genera Diplacus and Erythranthe.

Only time will tell whether this proposed reorganisation will gain acceptance. I can see there being a lot of resistance to the idea that many of the most familiar 'Mimulus' species should no longer be included in Mimulus, particularly in non-academic circles. Nobody likes being made a monkey by monkeyflowers.

REFERENCES

Barker, W. R., G. L. Nesom, P. M. Beardsley & N. S. Fraga. 2012. A taxonomic conspectus of Phrymaceae: a narrowed circumscription for Mimulus, new and resurrected genera, and new names and combinations. Phytoneuron 2012-39: 1–60.

Beardsley, P. M., & R. G. Olmstead. 2002. Redefining Phrymaceae: the placement of Mimulus, tribe Mimuleae, and Phryma. American Journal of Botany 89 (7): 1093–1102.

Grant, A. L. 1924. A monograph of the genus Mimulus. Annals of the Missouri Botanical Garden 11 (2–3): 99–388.

Mintbush Genus Limits

Victorian Christmas bush Prostanthera lasianthos, copyright Melburnian.

Despite (or perhaps because of) the severity of Australia's climate over much of the continent, the country has become famed for its wildflower displays. At the right time of year, the otherwise bleak landscape becomes a riot of form and colour. The display shown above belongs to a species of the genus Prostanthera, an assemblage of about 100 species of bushy shrubs (very rarely small trees) known as mintbushes, endemic to yet ubiquitous around Australia (new species continue to be described at fairly regular intervals). As suggested by their vernacular name, mintbushes belong to the mint family Lamiaceae, the same family as many well-known garden herbs such as sage, rosemary or thyme. Like these relatives, mintbushes have strongly aromatic foliage, due to the presence of glands secreting volatile oils on the leaves. However, the edibility of most species is unknown; I did find a couple of references to culinary uses of the round-leaved mintbush Prostanthera rotundifolia though it is not common (and a couple of comments in this thread suggest that it may be a bit pungent for regular use). Certainly, to push the pun in this post's title far further than it deserves, there is no evidence of mintbush gin.

Prostanthera species are most readily recognised by their flowers (Wilson et al. 2012). The calyx at the base of the flower has the sepals fused so that it is shaped as two lips, an upper and a lower. The corolla of petals has five lobes, two in the upper lip and three in the lower. There are four anthers, which often (though not always) have a distinct basal appendage; it is this appendage that gives the genus its name (from the same Greek word that gives us the term 'prosthetic'). Different species have flowers in a wide range of colours, and many Prostanthera species have become popular ornamentals.

Flower of scarlet mintbush Prostanthera aspalathoides, copyright Patrick Kavanagh.

Prostanthera is a member of a tribe of Australian Lamiaceae known as the Westringieae, members of which have a dry fruit splitting into four sections (Conn 1984). The two-lobed calyx of Prostanthera separates it from most other genera that have been recognised in the Westringieae except for a small genus called Wrixonia. The only significant difference between Wrixonia and Prostanthera is that whereas the latter retains four fertile anthers, the former has one pair of anthers sterile and reduced. A molecular phylogenetic analysis by Wilson et al. (2012) found that Wrixonia species were nested within Prostanthera, raising doubt about whether Wrixonia should be recognised as a separate genus. Also of interest was the relationship between the two sections into which Prostanthera has been divided: section Prostanthera and section Klanderia. These sections differ in characteristics of their flowers. Prostanthera section Prostanthera has flowers that are white, mauve or blue, with a corolla in which the central lower lobe is longer than the others so the overall appearance is similar to an orchid (such as in the P. lasianthos at the top of this post). In section Klanderia, the flowers are green, yellow or red, and the two upper lobes of the corolla are the longest so the appearance of the flower is more tubular (such as in the P. aspalathoides just above). Some authors have regarded the difference between two sections as enough to warrant recognised section Klanderia as a separate genus (in which case it becomes known as Cryphia, because botanical nomenclature is complicated like that). The two sections differ in flower morphology because they differ in pollinator type: flowers of section Prostanthera are pollinated by insects, whereas flowers of section Klanderia are pollinated by birds. Again, Wilson et al. (2012) found that the larger section Prostanthera, which retains the ancestral pollinator type, is paraphyletic with regard to the derived section Klanderia.

REFERENCES

Conn, B. J. 1984. A taxonomic revision of Prostanthera Labill. section Klanderia (F.v.Muell) Benth. (Labiatae). J. Adelaide Bot. Gard. 6 (3): 207–348.

Wilson, T. C., B. J. Conn & M. J. Henwood. 2012. Molecular phylogeny and systematics of Prostanthera (Lamiaceae). Australian Systematic Botany 25: 341–352.