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Excerpts from Jim Conrad's
Naturalist Newsletter
Entry dated February 18, 2024, issued from near Tequisquiapan; elevation about 1,900m, (6200 ft), ~N20.57°, ~W99.89°; Querétaro state, MÉXICO
AN ABERRANT LAGASCEA RIGIDA
On the edge of a small village, enmeshed in a 5m (16ft) tall hedge of spiny Desert Hackberry, Smooth Mesquite and weeds, the above semiwoody shrub's slender stems up to 2.5m tall (8ft) snaked through the brush. The stem-tip, branching, panicle-type inflorescence was composed mostly of almost mature fruiting heads; only the head appearing below still bore flowers.
With numerous florets packed into a head subtended by a green, scaly involucre, the bush is a member of the largest of plant families, the Composite/Aster/Sunflower Family, the Asteraceae. In that family, enormous diversity of flower structure and function is expected. However, when taking the above picture I never suspected that, because of a feature right there in front of us, it would end up the subject of intense speculation.
That's because the above flowering head is composed of two distinct floret types: disc florets in the center with corollas consisting of cylindrical tubes topped by five recurving lobes, and ray florets along the cluster's periphery with corollas consisting of flat ligules. The genus to which this plant belongs is not supposed to produce ray florets at all.
The above image also is jarring in that it doesn't show "normal Aster Family structure." At the left, the older, dried-brown disc floret arises from a somewhat scoop-shaped, green item which typically would be interpreted as one of the subtending involucre's scale-like phyllaries. However, on the right side of that floret we see the unopened, dome-shaped top of an unexpanded corolla of a second disc floret, whose base also is enveloped by a green item. And behind that unopened corolla, barely visible below downcurving corolla lobes, there's yet another green, pointed tip. Eventually it was realized that in this cluster of flowers each single floret was subtended by its own much-simplified involucre. The whole head comprised numerous such one-floret-bearing involucres.
Here we're seeing "secondarily aggregated flowering heads." Of the 1688 genera which as of January 2023 are accepted by the Plants of the World Online database as constituting the Aster Family, few develop such structures. This distinction of our bush enables us to be more certain of our plant's genus, and when we look up that genus, we're told its species aren't supposed to bear ray florets.
The above flowering cluster is broken open better to show that each floret has its own simplified -- but very hairy -- involucre. On each side of the cluster, at its base, there's a single ray floret. The ray florets are sterile, producing no fruits, so their involucres are much slenderer than the disc florets'.
Above, several much-simplified involucres bear inside dried-up and brown disc florets.
Above, two secondary heads each are subtended by modified leaves looking like involucral bracts, or phyllaries, below more typical flowering heads.
Simple leaves appear in pairs opposite one another on stems, and their margins are irregularly low-toothed or lobed. The upper blade surface is somewhat whitish with short hairs lying close to the blade's surface.
The blades' undersurfaces are densely soft-hairy. In the image it's hard to say whether the blade surface and hairs are glandular, but it seems that they could be.
At the right, some of the petiole's hairs could be tipped with small glands, but that's unclear.
Our bush can be determined to belong to the fairly large and commonly occurring Aster Family tribe, the Sunflower Tribe, the Heliantheae. In the 1987 work by Tod Stuessy entitled "Revision of Lagascea (Compositae, Heliantheae)," it's said that "Lagascea Cav. from Mexico and Central America is the only genus of the tribe Heliantheae with secondary heads of this type." He defines "this type" as meaning producing aggregated flowering heads which "... are composed of uniflowered primary heads and sometimes are surrounded by a series of secondary involucral bracts." On this basis, we can feel confident that our bush belongs to the genus Lagascea, and pictures of various Lagascea species support this. We've seen this genus before, but that species, usually called Silkleaf, was a low herb in the Yucatan.
The 2019 study by John Pruski entitled "Studies of Neotropical Compositae–XIII. Lagascea espinalii (Heliantheae: Helianthinae), a new species from dry canyons in Antioquia, Colombia, and two noteworthy range extensions," appears to be the latest study looking at all known Lagascea species, which number nine, counting Pruski's discovery. All Lagascea species are native Mexican, except Pruski's discovery in Colombia.
Pruski provides a key to all nine species. Here we see that if our Mexican bush is woody with white flowers and with most leaves less than 10cm long, as with our bush, we have one of only three species. However, naming our plant can't be a simple matter of choosing which of those three Lagascea taxa our bush might be. No Lagascea species is supposed to possess the ray florets so plainly visible in our pictures.
At this point, it was time to get some help. José Luis Villaseñor at UNAM, Mexico's main university, in Mexico City, is the author of the 2018 study Diversidad y distribución de la familia Asteraceae en México, and he continues his studies of the Aster Family in Mexico. He has expressed interest in seeing anything out of the ordinary in the family I photograph. The above images were sent to him.
He replied that he thought the species was LAGASCEA RIGIDA, but that by bearing ray florets our plant was an aberrant, atavistic individual expressing the genes of its ancestors, which bore ligulate florets -- "... es una condición atávica de que la especie guarda genes de sus antepasados con lígulas." He's seen such a thing previously in the genera Simsia and Viguiera, but only rarely.
Wikipedia's Atavism page explains that in biology "... an atavism is a modification of a biological structure whereby an ancestral genetic trait reappears after having been lost through evolutionary change in previous generations." Numerous examples of atavism are provided.
Lagascea rigida is endemic just to upland central Mexico. Its distribution is so limited and it's so hard to differentiate from other Lagascea species, that I find no literature referring to its uses or special features. In fact, the 2011 volume of the Flora del Bajío doesn't document its presence in Querétaro state, and elsewhere I find no documentation of its presence here. Mostly it occurs farther south in the Trans Mexico Volcanic Belt.