An Array of Shrubs

The Plants
Some plants get all the attention; others are a bit like wallpaper; achieving coverage but considered background.  That seems to be true for many of the shrubs that fill, sometimes threaten to smother, our Panhandle wetlands.  I’m thinking, specifically about four woody plants classified in different genera (Cliftonia, Cyrilla, Clethra, and Itea) that share enough characteristics so as to be potentially confusing. The four plants in our Apalachicolan flora have been well-known to science for over two centuries.  Two were included by Linnaeus in the 1st edition of Species Plantarum, while the other two were published later, but still in the 18th century.  Over the decades, these four horsemen have confused botanists as to relationships and classification, a puzzlement that continues to the present.

Each is a widespread native that is the sole representative of its genus in the Florida flora.  Each produces spires of small white 5-merous sepals, petals, and stamens, against a background of simple green, alternate leaves.  Two (Cyrilla and Cliftonia) are evergreen, while the other two are deciduous.  The same two are in the same family, and grow larger, with the potential of reaching small tree size.

Here in the Panhandle, the different plants flower somewhat sequentially the earliest being Cliftonia monophylla, also the only one of our four plants solely native to the Southeast.  Cliftonia is a dense evergreen shrub that sends out racemes of small white flowers in March.  By the time the others are showing flowers, Cliftonia fruit are already developing.  And it is by their fruit ye shall know them, for the fruit are the best giveaway to identity, each being a small, half centimeter (quarter inch), 3(2-4)-winged leathery yellow structure called a “drupe” in Flora of Florida and “samaralike” in Flora of North America.

Cliftonia fruit are unmistakable, looking to me like super-miniature Carambola.  But to most folk, they resemble the angular (more-to-less winged) fruit of Eriogonum (Buckwheat), which gave rise to the common name, Buckwheat Tree. Enigmatically, though the plant seems to conquer ground effectively, descriptions tell us the fruit are often seedless.  I could find nothing in literature available to me about dispersal or germination, though Cliftonia is evidently very successful in establishment and conquering new territory.  

The related Cyrilla racemiflora follows in flowering, along with the very different Itea virginica.  Even from a distance you’ll not confuse these two, the thickly-branched Cyrilla, bearing multitudinous clusters of swooping and arching racemes, is something of a beast.  Commonly called Titi, Cyrilla racemiflora deserves its reputation as a problematic competitor in wet soils.  Exploring trails and roadways, you’ll encounter the plant hedging practically all wet edges, but it’s not just edges.  Titi seems to fill any wet swale in the forest, turning vast acreages into solid thicket, literally Titi Hell.  Hellacious, but also handsome, Titi generates nice foliage and quite a show of flowers. The North American Titi, which is recorded in every coastal state from Texas to Maryland, is one of as many as ten Cyrilla species varyingly accepted throughout the broad range of the genus into Central America and the Caribbean (some authors lump all species into the same variable taxon.)

Flowering about the same time, Itea virginica will not be confused with Titi. This plant, called Virginia Sweet Spire, is a far less-aggressive, shorter, more sparsely-branched, deciduous shrub that sends its flowering branches up and out. Itea, which Flora of Florida classifies in the Iteaceae (Saxifragales) is unrelated to Cliftonia, Cyrilla, and Clethra.  But Cyrilla and Itea both have 5 stamens, thus delving into taxonomic history, you’ll find that anciently, some botanists, from Swartz (1788) to L’Heritier, to Lamarck (1789) were so stricken by similarites as to consider Cyrilla racemiflora a species of the Linnaean genus Itea.  

In the field, there’s no confusing the two. The solitary racemes of Itea, encrusted with white flowers terminate short branches, projecting straighter than the clustered, multiple, swooping inflorescences of Titi.  The 2-chambered Itea fruit develop into dry capsules that split open at maturity, dispersing several small seed.  Stalks of dry fruit linger, even through the following year, such that the plant often can be identified by fruit alone.  

A clear component of the ancient North Temperate Flora, our Virginia Sweet Spire is an outlier; the bulk of Itea species are native to Southeast Asia.  If you search the internet to learn more, you’ll discover Itea fruit and seed (along with leaves and pollen) are of significance in the fossil record.  Hermsen (2013) tells us: “The fossil record of Itea begins about 50 million years ago in the Pacific Northwest of North America, and is sporadic throughout the Paleocene, when fossil occurrences are recorded only from North America and Europe.  The fossil record suggests that Itea invaded Europe over the North Atlantic Land Bridge no later than the late Eocene and spread from the northwest toward the south and east…..Given the ecological and morphological similarities of I. virginica and European Itea fossils and the evidence suggesting that I. rhamnoides (native to Africa) most likely arose from extinct Cenozoic European Itea, one might predict that I. rhamnoides and I. virginica are sister taxa showing a transatlantic disjunction….”  Given the Asian-centered diversity of Itea (18 of 20 species), it’s fascinating that the North American and African outliers are more closely related to each other than to their Asian congeners.

The last of these four to flower around Apalachicola is Clethra, a stiffly upright, root-sprouting, thicket-forming woody plant with leaves so much like those of Alnus that its scientific name makes that point twice – Clethra coming from the Greek klethra for alder, and alnifolius telling us the leaves resemble those of alder. 

Rapid, stiffly-vertical spring-growth terminates in rising racemes of small white flowers.  Though we record a single species in Florida, Clethra alnifolia is one of over 80 species recognized across a broad distribution, from North and Central America, the more montane areas of South America, and Southeast Asia, andinto tropical Macronesia.  The only other genus classified in Clethraceae, Purdiaea, was just recently realigned with the family (Anderberg and Zhang, 2002), having previously been classified with Cliftonia and Cyrilla. Thomas treated the genus as Cyrillaceae in his 1960 monograph of that family.  

Identification

Though similar in many regards, these four white-flowered shrubs can be readily distinguished at all stages, though you may have to get samples in hand.  We begin in the winter, with Cyrillaceae, comprising the two evergreen shrubs, Cliftonia and Cyrilla.  Left to develop without clearing or fire, both grow to become towering densely-branched shrubby trees, somewhat similar vegetatively to a host of small-leaved evergreen shrubs that populate the forests (Ilex, the Ericaceous shrubs.)

When I first encountered flowering specimens of Cliftonia, the plant proved surprisingly difficult to identify using keys, which was astonishing considering its omnipresence in wet soils.  In early Spring, Cliftonia is a prominent white-flowering shrub around Apalachicola, hedging  drainages and borrow ditches, and rising cloud-like in wet swales throughout the pine flat woods.  Left alone, Cliftonia becomes a small, densely-branched evergreen tree.  Given leaves and branches alone, the plant doesn’t readily stand out among associates in the same habitats.  It can be just another shrubby understory evergreen with small, simple, somewhat leathery, green, glabrous leaves – mingling with Cyrilla, Ilex, and Lyonia. With familiarity, the differences become apparent, but to a casual observer, Apalachicola’s evergreen shrubs are a vegetative morass.

Flora of Florida describes Cliftonia leaves as “elliptic to oblanceolate, 2.5-10 cm long, 1.2-1.8 cm wide, coriaceous, evergreen, the apex acute to obtuse or marginate, the base cuneate, the margin entire, the upper surface glabrous, the lower surface glabrous, glaucous, the petiole relatively short or absent.”  Comparing this description to that of Cyrilla, the only distinction of note comes with the term “glaucous”, which tells us the Cliftonia lower leaf surface should be conspicuously white or blue due to a waxy cuticle.  However, in living material I’ve examined in our area, that glaucous character is just barely perceptible.  It appears to show better in dried herbarium specimens; indeed, consulting photographs in ISB Plant Atlas, Cliftonia specimens from Mississippi are markedly glaucous abaxially. 

Examining the scan of leaves I posted above, you’ll see that the two top sets are Cyrilla (left) and Cliftonia (right).  To me, the most obvious differences in living material are that Cyrilla leaves show more conspicuous petioles, while the bases of Cliftonia leaves are more attenuate, Cyrilla leaves show greater variability in size on a single twig, and the Cliftonia leaves are somewhat rhombic to obtrullate. The far-right Cyrilla leaf and the three far right Cliftonia leaves show abaxial surfaces. Once again, in specimens from the Apalachicola area, Cliftonia fresh leaves, though somewhat paler, are not particularly glaucous.

Contrasts with Itea and Clethra, both of which are deciduous, are profound.  Itea leaves (the bottom right cluster in the group photo), though technically in the same size range as the other three species, are usually over 5 cm long and always broader than either Cliftonia or Cyrilla leaves.  The texture, shape, and margins are different, Itea leaves being chartaceous (papery) in substance, subtly bullate (such that the pinnate secondary venation is obvious), elliptic in outline, bearing finely serrated (serrulate) margins, and invested with some pubescence abaxially.  Itea leaves show a prominent petiole.  Though Itea leaves are stipulate, the stipules are small and “caducous”, which means they abscise early.

Clethra is the odd man out, identifiable immediately by its characteristic stems and foliage, which are marked by pubescence.  Texturally, the leaves are papery, not leathery at all.  In shape, they are variably obovate, somewhat cupped, with margins that are conspicuously serrate near the apex.  Abaxially, the leaves are light grey, covered densely with a mat of what appear to be shiny, stellate hairs, the texture being marked by a strong pattern of embossed pinnate venation.

Flowers of the four species are distinctive, but they’re all white, about the same size, with 5 sepals, 5 petals, and either 5 or 10 stamens (Cyrilla and Itea bear 5 stamens, Cliftonia and Clethra bear 10). 

Next season I plan to capture comparable photographic close-ups of the four species, so as to show them side-by-side. The record is incomplete for the moment.  A couple of general observations, however:  the individual flowers of Cliftonia show yellow anthers atop white filaments that stand together making a loose collar around the ovary, having the same effect as the stamens of Citrus flowers. 

Clethra forms its ten stamens in two unequal whorls of 5 each.  The anthers, once inverted, show distinctive appendages. 

Cyrilla flowers show obvious basal thickening in the petals, with alternating stamens and a white ovary topped by short, thickened bifid (sometimes triffid) styles. 

The petals of Itea flowers are comparatively attenuated, opening to reveal a white ovary that tapers to a thick, capitate stigma.

All of the different flowers produce small, superior multi-carpellate ovaries – 2 carpels in Itea, 3 carpels in Clethra, and 2-4 (5) carpels in Cyrilla and Cliftonia.  The fruit of Itea and Clethra mature as dry, dehiscent capsules, while Cyrilla and Cliftonia form leathery indehiscent fruit described as drupes. You might infer that I disagree with the use of the term, “drupe” in for these plants. It’s perfectly useful term for 1-seeded fruit that have a detectable endurate endocarp (a hard, inner layer to the fruit that encloses the seed).  Floras tell us that both Cyrilla and Cliftonia frequently produce seedless fruiting structures. Despite that deficiency, there seems to be no shortage of viable seed in the habitats.

The Back Story
Two of our four plants (Itea and Clethra) were included in the first edition of Species Plantarum (1753), and a third was published by Linnaeus a few years later. Here is the line-up:

• Itea virginica Linnaeus other species are Asian (One US species) Species Plantarum vol 1, pg 199
• Clethra alnifolia Linnaeus – 86 species, mostly Micronesia and subtropical to tropical Asia 1753 Species Plantarum vol 1, pg 396
• Cyrilla racemiflora L. – 10 species, North and South America (one US species) Syst Nat ed 12 1767, pg 182
• Cliftonia monophylla (Lamarck) Britton ex Sargent – monospecific, therefore (one US species). based on Ptelea monophylla Lamarck, Table Encycl 1, 1792 pg 336

Linnaeus’s “system” outlines plants based on combinations of stamens and pistils, thus making for some odd bedfellows.  Itea, with 5 stamens and one ovary, falls into his Class V. PENTENDRIA, Order MONOGYNIA. 

Five is not a lonely number in the plant world, such that PENTANDRIA MONOGYNIA covers over 80 pages in Species Plantarum, and other than Itea, includes  many widely-known and unrelated plants, for example: Heliotropium, Lithospermum, Primula, Azalea, Phlox, Convolvulus, Campanula, Lonicera, Verbascum, Solanum, Rhamnus, and Vitis. 

Clethra, with ten stamens and a single ovary, is classified as Class X. DECANDRIA, Order MONOGYNIA, a grouping with 25 pages of entries, including another odd assortment: Sophora, Cercis, Cassia, Melastoma, Schinus, Rhododendron, Kalmia, and Arbutus.  

With his system, Linnaeus demonstrated the convenience of knowing readily where a plant should be pigeonholed, but browsing through one of his “artificial” categories quickly demonstrates the absence of context.  It’s like browsing a Library in which books are shelved based on size, efficient for storage but having little to do with subject matter.   Grouping such obviously unrelated plants as Primula, Verbascum, and Vitis doesn’t bring extra value, while alienating Azalea from Rhododendron is actually unhelpful.
For a few decades, taxonomists followed Linnaeus’s classification system. The unrelated Cyrilla and Itea were placed in Class V, while Clethra and Cliftonia, with ten stamens, would have been in Class X.  That’s not so useful, implying Cyrilla is more related to Itea than to Cliftonia. By the 19th century, Linnaeus’s scheme was obsolete, replaced with a succession of natural systems. Cyrilla, Cliftonia, and Clethra were soon recognized as related to the Ericaceae, which remains true today, though placement isn’t absolutely resolved, even now.  Those three genera are kept at arms-length, in two small families. Cyrilla and Cliftonia are the only genera in the Cyrillaceae; Clethra is one of two genera in the Clethraceae.  Those families are considered allied to the Ericads, and are thus classified in the order Ericales.  Otherwise, there’s divergence among taxonomists regarding their evolutionary relationships and species delimitation.  Flora of Florida accepts Cyrilla as a widespread, variable species, C. racemiflora, while Kew’s Plants of the World OnLine (POWO), accepts ten Cyrilla species, most of which are native to Central and South America.  In his 1960 monograph of Cyrillaceae, Joab Thomas followed precedent by including Purdiaea as one of three genera in the family.  The 2002 publication by Anderberg and Zhang, realigned Purdiaea with Clethra, while the authors concluded that Cyrilla and Cliftonia are more closely related to the Ericaceae than is Clethra, even though Clethra does that cool Ericad-thing with its stamens (See Magical Stamens below).

Lumped in with Rhododendron and Kalmia by de Jussieu (1789), Itea was long ago segregated from the Ericales (an order in the galaxy of Asterids), but still has its own problems.  Considered part of the ancient and diverse “Saxifragalean stock”  (which are more related to Roses than to Asters) since forever, the genus is segregated to its own family, the Iteaceae, with or without Pterostemon, a small genus of shrubs native to Mexico. 

It’s amusing to me, viewing the evidence as an outsider, that these four plants, with relatively simple characteristics (woody stems, simple leaves, flowers with 5 sepals, 5 petals, 5 stamens, and a rather straight-forward ovary) seem challenging to place.  But where you start makes all the difference in how the trip might go. Early taxonomists often worked with plants that don’t represent the core diversity of a plant group, which means sequence of discovery, that is so important to taxonomy, will be prejudicial.  The genera Clethra, Itea, Cyrilla, and Cliftonia were first published from Eastern North America, though the bulk of diversity for both Itea and Clethra is in Asia, while for Cyrilla, the bulk of diversity lies in Central and South America.  This makes for an interesting circumstance. Each genus is typified based on a species that is something of an outlier to the main center of variability. That means our understanding of a group of plants is subtly hamstrung by sequence of discovery. That’s especially true for those of us based in North Temperate English-speaking communities. The iron fist of historical nomenclature is one reason some biologists dream we would dispense with binomial nomenclature altogether.  

Reflecting on conventions for naming, it’s well-accepted that Cliftonia is monotypic, which tells us there’s only one species in the genus, i.e. Cliftonia monophylla.  It would seem a bit odd that a plant, alone in its genus, would be christened with the specific epithet “monophylla” when its closest relatives (Cyrilla) are also simple-leaved.  But there’s history to consider. Lamarck described the species as Ptelea monophylla in 1792, believing it was a unique simple-leaved member of a plant group noted for its handsome compound leaves, which makes his choice of names more sensible, if odd in more recent context. 

Banks (cited in a publication by Gaertner, 1807) is credited with the genus Cliftonia, an entry that also included Gaertner’s description of the species C. nitida.  Two years later, Willdenow published a description of the same species as the monotypic Mylocaryum ligustrinum, which was transfered to Cliftonia a few years later.  Thus Chapman, in his Flora, used the name Cliftonia ligustrina.  In 1889, Britton noted (Bulletin Torrey Bot Club) that in keeping with Sargent’s “principle of maintaining the oldest specific names”, one would want to recognize Lamarck’s Ptelea monophylla as having priority. The result was transfer of the species name to Cliftonia in Silva (1891), which means that since 1891, we’ve come to accept both C. nitida and C. ligustrina as synonymous to Cliftonia monophylla.  
Another modest mystery surrounding Cliftonia is the eponym itself — the person on whom the generic name is based.  Contemporary botanical sources indicate the genus honors  William Clifton, a British lawyer who served the government in Georgia and West Florida (the short-lived entity from Apalachicola westward) and sent plant specimens to John Ellis.  In Silva, Sargent tells us the eponym honors Francis Clifton, a physician and correspondent of Hans Sloan.  Perhaps Banks’ records and correspondence have shown this references William Clifton.  
Notes on Magical Stamens:  More than their value in identification, Clethra stamens are noted for a special nature (see Hayden, 2015), reported first by C. R. Barnes in 1880.  Barnes explains that as the flower begins to open, the filaments that had been constrained by the petals elongate rapidly, springing the anthers beyond the corolla to a horizontal position, a reaction followed by “obversion”, which (within a few minutes) reorients anther openings such that pollen exits toward the flower center rather than outwardly toward the petals.  This “acrobatic” behavior appears to characterize the entire genus of more than 80 species.  

Many Thanks to the following for assistance with permission to study and voucher plants on public properties:

References:
Thomas, Joab L.  1960.  A Monographic Study of the Cyrillaceae, Contributions from the Gray Herbarium of Harvard University, No. 184.  “Front Matter.” Contributions from the Gray Herbarium of Harvard University, no. 186, 1960. JSTOR, http://www.jstor.org/stable/41764649. Accessed 31 July 2023.
Dute, Roland R., Debbie R. Folkerts, James E. Watkins, Milam E. Saxon, and Robert S. Boyd, 2004.  Floral Anatomy and Pollination Biology of Cyrilla racemiflora (Cyrillaceae), Southeastern Naturalist 3: 309-326.  Dute, Roland R., et al. “Floral Anatomy and Pollination Biology of Cyrilla Racemiflora (Cyrillaceae).” Southeastern Naturalist, vol. 3, no. 2, 2004, pp. 309–26. JSTOR, http://www.jstor.org/stable/3878109. Accessed 31 July 2023.
Vijayaraghavan, M. R., 1969.  Studies in the Family Cyrillaceae. I. Development of Male and Female Gametophytes in Cliftonia monophylla (Lam.) Britton ex Sarg., Bulletin of the Torrey Botanical Club 96: 484-489 Vijayaraghavan, M. R. “Studies in the Family Cyrillaceae. I. Development of Male and Female Gametophytes in Cliftonia Monophylla (Lam.) Britton Ex Sarg.” Bulletin of the Torrey Botanical Club, vol. 96, no. 4, 1969, pp. 484–89. JSTOR, https://doi.org/10.2307/2484067. Accessed 31 July 2023.
Gärtner, Carl Friedrich von, 1772-1850 and Gaertner, Joseph, 1732-1791,   1807.  Supplementum carpologiae : seu continuati operis Josephi Gaertner de Fructibus et seminibus plantarum  DOI  https://doi.org/10.5962/bhl.title.59684  Holding Institution  Missouri Botanical Garden, Peter H. Raven Libraryhttps://www.biodiversitylibrary.org/page/39371370
Sargent, Charles Sprague, 1841-1927, 1891.  The silva of North America – a description of the trees which grow naturally in North America exclusive of Mexico, volume ii.  Illustrations by C. E. Faxon, Engravings by P. and E. Picart.   DOI   https://doi.org/10.5962/bhl.title.14668  Holding Institution:  Missouri Botanical Garden, Peter H. Raven Libraryn   https://www.biodiversitylibrary.org/page/16051597
Britton, N. L., 1889.  Index to Recent American Botanical Literature, in Bulletin of the Torrey Botanical Club, Holding Institution: Missouri Botanical Garden, Peter H. Raven Libraryhttps://www.biodiversitylibrary.org/page/10415387
Barnes, C.R., 1880.  The anthers of Clethra, Botanical Gazette, Vol 5-6: 104-105  https://www.biodiversitylibrary.org/page/5173558
Hayden, W. John, 2015.  Upside-down Anthers of Clethra Stand Out, Sempervirens – The Quarterly of the Virginia Native Plant Society
Anderberg, Arne A. and Xiaoping Zhang, 2002.  Phylogenetic relationships of Cyrillaceae and Clethraceae (Ericales) with special emphasis on the genus Purdiaeae Planch, Organisms, Diversity, & Evolution 2: 127-137.
Anderson, Jill T., Alicia A. Landi, and Peter L. Marks, 2009.  Limited flooding tolerance of juveniles restricts the district union of adults in an understory shrub 9Itea virrginica; Iteaceae), American Journal of Botany 96:1603–1611  https://doi.org/10.3732/ajb.0800366
Mazzeo, Peter M. and Donald H. Voss, 1996.  Itea ‘Beppu’: The Return of the Native, Arnold 56: 21-25.  https://www.jstor.org/stable/42954493
Hermsen, Elizabeth J., 2013.  A Review of the Fossil Record of the Genus Itea (Iteaceaed, Saxifrages) with Comments on its Historical Biogeography, Bot. Rev. 79:1-47   DOI 10.1007/s 12229-0 12-9114-3   https://www.jstor.org/stable/41809867

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