Brief visit to Palo Alto’s Gamble Garden this morning, to take in the riot of summer flowers in bloom (7-foot tall white foxgloves! sweet peas climbing on everything, with intensely colored blossoms you can see from across the garden! giant bronze fennel plants, with feathery, scented leaves! and more!). On entering the garden Juan and I came across Plant 1, which I recognized as being a fancy (but unfamiliar to me) relative of the weed Queen Anne’s lace. A bit later, in the South African garden section, a huge thick-stemmed plant  — Plant 2 — caught our attention, and Juan took it to be a monster relative of the artichoke (elsewhere in the garden, artichoke plants were yielding up their edible flower buds in profusion), but I guessed that the similarities were inconsequential and that Plant 2 wasn’t related to the artichoke plant (a huge cultivated thistle) at all.

Homology — similarity due to common descent — for Plant 1 and Queen Anne’s lace; but analogy — similarity resulting from convergent evolution, or just accident — for Plant 2 and thistles, including the artichokes.

Crucial leading ideas in evolutionary biology, and also in historical-comparative linguistics (though I’m not going to pursue the linguistic side of things in this posting).

The textbook example of analogy rather than homology in biology is the eye. From Wikipedia:

The evolution of the eye is attractive to study, because the eye distinctively exemplifies an analogous organ found in many animal forms. Complex, image-forming eyes have evolved independently several times. Probably, the complex, image forming kinds of eyes are at least five: Cnidaria [jellyfish, sea anemones, corals, etc.]- lens eye, complex eye; Molluscs – lens eye, everse retina; Annelids – lens eye, everse retina; Arthropods – facet eye; Vertebrates – lens eye, inverse retina.

Different vertebrates (from fish, snakes, and birds through apes) are assemblages of similar characteristic features (eyes of similar type, in pairs; ears in pairs; a spinal column, housing a nerve cord, with mouth at one end and anus at the other; and so on. Such assemblages are the hallmark of common descent.

Homology. Plant 1 and Queen Anne’s lace  are also assemblages of similar features, almost too many to name, including their umbel flowers, composed of 5-petaled florets (a shared characteristic feature of the Apiaceae — the umbellifers, aka the celery, carrot or parsley family). Queen Anne’s lace:

(#1) In Amherst (MA) State Park

The weed Queen Anne’s lace and the food plant carrot are in fact variants of the umbellifer species Daucus carota.

Plant 1 is Orlaya grandiflora ‘Minoan lace’. From Wikipedia:

(#2) O. grandiflora, white laceflower

Orlaya is a genus of flowering plants from Europe in the parsley family Apiaceae, with between 1 and 11 species. They are annuals with finely-divided leaves, and umbels of lacy pink or white flowers. O. grandiflora [aka Daucus grandiflora], white laceflower, is well-known and widely cultivated as an ornamental in the UK and elsewhere.

Analogy. Plant 2 and the artichoke also share some features, beyond their impressive size: notably, large pinnate leaves (compound leaves with leaflets arranged like the vanes of a feather, on either side of the stem, in pairs opposite each other), in a range of colors from blue-green to silver-green , with dentate (toothed) leaf edges. The artichoke:

(#3) Artichoke leaves, greener than most

Plant 2’s leaves are somewhat similar. From Wikipedia:

(#4) Giant honey flower leaves, more silvery than most; the flowers in front belong to a different plant

Melianthus major, the giant honey flower or kruidjie-roer-my-nie, is a species of flowering plant in the family Francoaceae. It is an evergreen suckering shrub, endemic to South Africa and naturalised in India, Australia and New Zealand. It grows to 2–3 m (7–10 ft) tall by 1–3 m (3–10 ft) wide, with pinnate blue-green leaves 30–50 cm (12–20 in) long, which have a distinctive odour. Dark red, nectar-laden flower spikes, 30–80 cm (12–31 in) in length, appear in spring, followed by green pods. All parts of the plants are poisonous.

The Latin binomial Melianthus major literally means “large honey flower”.

But that’s just the leaves. Otherwise, the plants have little in common; in particular, their flowers are totally different:

(#5) Globe artichoke in bloom: a really really big thistle (a composite)

(#6) Melianthus flower spikes

In fact, the two plants have very little in common, beyond their leaves — but pinnate compound leaves and dentate leaf edges are both incredibly common features in the plants of the world, so these similarities count for little.

A plant family note. The Francoaceae are a new plant family on this blog, #80. According to Wikipedia, “The Francoaceae … are a small family of plants in the order Geraniales, including the genera Francoa, commonly known as bridal wreaths, and Tetilla.”

Francoa sonchifolia is commonly known as bridalwreath. But the common name bridal wreath is most often used for totally different, unrelated plants in the genus Spiraea (spirea or bridal wreath), flowering shrubs in the rose family (with the typical 5-petaled flowers of Rosaceae plants, utterly different from the flowers in either #5 or #6).



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