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Excerpts from Jim Conrad's
Naturalist Newsletter
entry dated April 28, 2023, issued from near Tequisquiapan, elevation about 1,900m (6200 ft), Querétaro state, MÉXICO
(~N20.55°, ~W99.89°)
MAT CHAFF-FLOWER
Now at the end of an especially rainless dry season, after last year's rainy season never developed normally, Tequisquiapan's large reservoir is completely dry, except for a narrow stream running through it, looking suspiciously like sudsy sewage. Nowadays the reservoir's flat, dry bed, normally submerged most or all of normal years, is partially vegetated with weeds. It's interesting to see which species invade such a sun-drenched, windy place. One species forming solid carpets in some places is shown above, two fingertips showing how small the individual plants are.
The white flowering heads are composed of several densely compacted flowers, with both the tepals (indistinguishable petals and sepals) and tight clusters of scale-like bracts below them, white and papery. It's not just coincidental that right next to this mat-forming plant grew a cudweed, Pseudognaphalium chartaceum, similarly bearing tiny flowers sheltered amid clusters of tough, white bracts. This broad reservoir bottom during the dry season is exposed to glaring sunlight and gusty, desiccating wind. The white bracts protect the flowers from environmental extremes, plus this plant's ground-hugging form avoids the wind.
Above, the leaves are somewhat banjo-shaped and only sparsely hairy. In the image's lower, right corner, a flower's cluster of minute, yellow, pollen-producing stamens can be seen nestled among the bracts. Stamens peep out here and there in other bract clusters, too.
Closer up, an open flower displays 5 stamens with their yellow anthers shedding pollen. Above and to the right of the open flower, white flower buds await their times to open. Below the flower cluster, white bracts form a collar amid long, cobwebby hairs. The nearby cudweed also bore cobwebby hairs, which insulate like feather down in a sleeping bag.
Above the plant's underside is seen, highlighting the hairy stem. A brown, wart-like item between the petiole bases is the beginning of a stem-root, an adventitious root to supply water directly into the wide-ranging stem.
Our plant's compact clusters of tiny flowers sheltering amid white, papery tepals and bracts strongly suggests the Amaranth Family, the Amaranthaceae, home to over 2000 currently accepted species, in about 165 genera. The family occurs worldwide, with many species especially adapted to salty and/or dry soils. Many species are edible. Here in Mexico, my granola contains plenty of popped amaranth seeds, which was an important ancient Mesoamerican food. Spinach, chard and beets now reside in the Amaranth Family.
Flowers of the Amaranth Family are so small that without magnification it's hard to see features determining which genus you have. I was lucky to have previously met a member of the genus our reservoir plant belongs to, the Yellow Joyweed, which wasn't at all mat-forming like our plant, but its flowers were tightly clustered in heads bristling with white tepals and bracts. The genus Yellow Joyweed and our unknown plant belong to the genus Alternanthera, whose species generally are known as joyweeds. Species of Alternanthera often are robustly weedy.
In our area several Alternanthera species can turn up as weeds, but by far the most commonly documented is our reservoir plant, ALTERNANTHERA CARACASANA. In English the plant is about equally referred to as Mat Chaff-flower, Khakiweed, and Washerwoman. In Spanish it's verdolaga de puerco, or "pig purslane." It's native to tropical and subtropical America, and occurs as an invasive weed in warm places elsewhere. The no-nonsense Flora of North America describes its habitat as gravel, sand bars and sidewalks. The Flora Fanerogámica del Valle de México says that it's very common in street cracks in Mexico City and other towns in the region.
One reason our Alternanthera caracasana may look so at home on the sun-scorched floor of a dry reservoir may be provided by the 2012 study by Maxim V. Kapralov and others entitled "Rubisco Evolution in C4 Eudicots: An Analysis of Amaranthaceae Sensu Lato." In that work a chart indicates that Alternanthera caracasana uses the C4 carbon fixation photosynthetic pathway, not the C3. Wikipedia has a page on C4 carbon fixation. There it's said, with citations, that "... C4 plants represent about 5% of Earth's plant biomass and 3% of its known plant species. Despite this scarcity, they account for about 23% of terrestrial carbon fixation." In evolutionary terms, the C4 pathway is a "modern innovation," with the C3 pathway being the primitive condition.
Amazingly, in a classic case of convergent evolution, C4 carbon fixation has evolved as many as 61 times in 19 different plant families, with the Amaranth Family being one of them. Within that family, as well as our genus Alternanthera, some taxa are C4, some C3. Our Alternanthera caracasana is C4. In the context of our bone-dry, sun-scorched reservoir environment, the significance of our plant being C4 is that photosynthesis takes place at higher rates and at higher temperatures, using less water, than in C3 plants. In moist, cool environments, C3 plants have the advantage, but in hot, dry ones, C4 plants do better.
With global warming, the low-profile, weedy little Mat Chaff-flower may be among those humble beings poised to inherit the Earth.