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Excerpts from Jim Conrad's
Naturalist Newsletter
Entry dated January 12, 2024, issued from near Tequisquiapan; elevation about 1,900m, (6200 ft), ~N20.57°, ~W99.89°; Querétaro state, MÉXICO
COULTER'S MOCK ORANGE
Amid a 2.5m-high (8ft) jumble of spiny trees, bushes and weeds mounted up against one of the village's lime-washed cinder-block walls encircling a home, I was astonished to see the above flowers in full bloom. We're in the midst of a severe, two-year-ongoing drought, and hardly anything is flowering nowadays.
Blossoms tended to grow in three-flowered, raceme-type clusters on stiff, slender stems arising at right angles to the main stem. Flowering stems gave the impression of being semi-viny, the ends drooping and the flowers themselves nodding downward.
Inside the flowers, insects gathered among maybe 35 stamens, which surrounded a fleshy 4-branched style.
Four fleshy sepals subtended the corolla, and were covered with short, soft hairs leaning forward.
A dissected blossom made clear that we had an inferior ovary, with corolla and stamens arising above the ovary, not below. The stalked, four-branched style was impressive.
The ovary seen in cross-section consists of four compartments, or locules, each locule containing numerous ovules, which will become seeds.
Above, we see the plant's capsular fruit, strikingly similar to a pecan nut. The two silhouetted objects above the fruit are two sides of a fruit which has split open, releasing its seeds. These were the only fruits on the whole large plant.
Leaves usually appeared opposite one another in pairs. A strong pair of secondary veins arose near the midrib's base. Leaf margins bore tiny, widely spaced tooth-nubbins on otherwise smooth or entire margins. Bases of larger blades were broadly rounded.
Young stems, petioles and leaf blades bore soft, white hairs. Only a small part of the plant was flowering and fruiting, that being limited to the east-facing, sunny wall. Around the wall's corner, on the usually-shaded north side, the plant grew up against the wall like a hedge, not bearing a single flower:
Above, the yellow blossoms belong to a drought-stunted Tithonia tubaeformis, surely flowering because just up the street there's a municipal water pipe which breaks and leaks frequently enough to cause out-of-season flowering. The same leak may explain why a small part of our plant is flowering, right around the wall's corner from the Tithonia. It's possible that the above picture shows our plant as a single organism, for the Flora del Bajío describes this species as a semi-climbing shrub with slender, inclined or hanging stems up to 10m high (33ft).
Our plant has been photographed in detail because the flowers indicated that we had a mock orange of the genus Philadelphus, which is home to some very well known and much planted ornamental mock-orange bushes. However, this plant's vegetative parts were unlike any mock-orange species I've seen. The genus embraces maybe 70 accepted species, so possibly here we had something unusual. I was unsure whether it was a planted ornamental from Eurasia or North America, or a native species.
In the end, all the details documented above indicate that it could be both native and planted. It's PHILADELPHUS COULTERI, in English often called by gardeners Coulter's Mock Orange. It's endemic just to Mexico's Eastern Sierra Madre mountains from near the Texas border south to about here in central Mexico. However, in Querétaro state I find it documented as growing in the wild only in Querétaro's mountainous northeastern part.
My guess is that our plant has been transplanted from the mountains rising just to our east, to here on the semiarid altiplano. The Flora del Bajío describes the species as a calciphile -- prefers limestone -- while we're atop volcanic tuff. Also it's said that the plant occurs in humid oak and juniper-pine forests, and in secondary vegetation deriving from those forest types. Here, our plant is "out of habitat." The species occurs infrequently, but isn't immediately in danger of extinction.
The genus Philadelphus currently belongs to the Hydrangea Family, the Hydrangeaceae.