In the scientific name, barbatus (bearded) is straightforward, but the etymology of Penstemon is obscure. Robert Nold, Penstemons (Portland: Timber Press, 1999), pp. 58-59, lucidly traces the complicated history of the generic nomenclature:
The generic name Penstemon was first proposed by the Virginia botanist John Mitchell; he was probably referring to P. laevigatus, which was not actually described until 1789. When Linnaeus published his Species plantarum in 1753, he referred to two species of penstemon as Chelone: C. hirsuta (P. hirsutus) and C. pentstemon, disregarding Mitchell's new genus, and also, apparently, assuming that Mitchell's name "penstemon" was derived from the Greek pente ("five") and stamon ("thread"), since Mitchell gave no explanation. To Linnaeus, in other words, Mitchell's plant was just another turtlehead.Straw (1966) is a reference to Richard M. Straw, "A redefinition of Penstemon (Scrophulariaceae)," Brittonia 18 (1966) 80-95, which I have not seen.
Mitchell republished his description in 1769, in Nova Genera Plantarum Virginiensium, reiterating his spelling and again giving no derivation. In the meantime, Johann Schmidel, in Icones Plantarum (1763), had mentioned a plant "recently named by the English" (a resident of Virginia was still English at the time), answering the description of Penstemon laevigatus. Some botanists prefer to give Schmidel priority, although he did not describe the genus; credit for naming the genus is usually given to Mitchell, even though his first description was pre-Linnaean.
In the nineteenth century, many botanists adhered to Linnaeus's spelling, "pentstemon," or even "pentastemon"; it was not until Pennell's work (1920, 1935) that Mitchell's original spelling became the preferred spelling, although not among horticulturalists until quite recently. The incorrect spelling is therefore the fault of Linnaeus,and it should not be considered an alternative to "penstemon."
One reason the current spelling was not accepted was the implicit idea that Mitchell had conceived the name as referring to the number of stamens—not exactly an accurate name since the fifth stamen is not a stamen at all, but rather a sterile filament resembling a stamen. An alternative derivation, from the Latin paene ("almost") and Greek stamon ("thread") was suggested by Straw (1966). This explanation makes considerably more sense and is now universally accepted by authorities in the genus. The name penstemon ("almost a stamen") is in the masculine gender and requires agreement in the specific epithet: the endings -us, -is, and -cola (as in rupicola) are all appropriate.
English words derived from Greek πέντε (pente = five) include pentagon, pentagram, pentameter, Pentateuch, pentathlon, and Pentecost. I am aware of no words where pente has been shortened to pen- in combination. English words derived from Latin paene (almost, nearly) include peneplain, peninsula, penultimate (plus penult and antepenult), and penumbra. I know of no words where the final -e of paene has disappeared before a following consonant in combination.
Eric Thomson kindly took time to respond to my questions about possible sound changes in the derivation of penstemon from either pente or paene. In the following excerpt from Eric's email, "Stockwell and Minkova" refers to Robert Stockwell and Donka Minkova, English Words: History and Structure (Cambridge: Cambridge University Press, 2001):
Penstemon seems ab initio a flawed coinage. The combinatory form penta- can't easily be shorn of its root [t], though if you do omit the vowel, as you are not strictly entitled to do if the root is consonant initial, the [t] would indeed be swallowed up ("Total assimilation occurs most frequently in borrowed words in which prefixes ending in consonants are attached to roots beginning with a non-identical consonant. To highlight the transparency of the root, its initial consonant was was probably pronounced with sufficient force to trigger regressive, right to left assimilation" is how Stockwell and Minkova put it. One of their examples is ad-string-ent > astringent, where the consonant cluster is also [st]). What might have motivated Dr Mitchell to remove the vowel in the first place is the location of the stress, since penta- would shunt it to the left, thereby taking away prominence and transparency from the base, i.e. pentástemon like pentágonal. The prefix homophony with pen(e)- (< paene) would then make it vulnerable to reanalysis but, as with penta-, there is no hiatus at the morpheme boundary to resolve by elision, so if pen- did derive from paene, the correct form would be 'penestemon'.Britton & Brown III, 182 describe the genus:
Perennial herbs, mostly branched from the base only, with opposite or rarely verticillate leaves, or the upper occasionally alternate, and large, usually showy, blue purple red or white flowers, in terminal thyrses, panicles, or racemes. Calyx 5-parted, the segments imbricated. Corolla irregular, the tube elongated, more or less enlarged above, the limb 2-lipped; upper lip 2-lobed; lower lop 3-lobed. Stamens 5, included, 4 of them antheriferous and didynamous, the fifth sterile, as long as or shorter than the others; anther-sacs divergent or connivent. Style filiform; stigma cap1tate. Capsule ovoid, oblong, or globose, septicidally dehiscent. Seeds numerous, angled or even, wingless.There is no description of the species in Britton & Brown. Although Penstemon barbatus is a plant native to America, the fullest description of the species I could find was in Stuart Max Walters and James Cullen, The European Garden Flora: A Manual for the Identification of Plants Cultivated in Europe, Both Out-of-doors and Under Glass (Cambridge: Cambridge University Press 1984), VI, 293:
Plant to 1.8 m, hairless or minutely hairy at base. Leaves 5-15 cm; basal leaves lanceolate to spathulate or ovate; stem leaves lanceolate to linear. Inflorescence long and open. Calyx 6-10 mm, lobes lanceloate, acute to shortly acuminate with membranous margins. Corolla 3-4 cm, red, tinged pink to carmine, strongly 2-lipped, gradually inflated, upper lobes projecting, lower lobes reflexed and yellow-hairy at base. Staminode included, pale yellow, hairless.Related posts: Butterfly Milkweed; Blanket Flower; Golden Alexanders; Narrow-Leaved Purple Coneflowers.