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Images taken with Nikon D-90 and 60 mm Micro-Nikkor AF-S F 2.8G ED Macro Lens with Phoenix Ring Flash
© W.P. Armstrong 24 May 2010
San Diego Thornmint
(Acanthomintha ilicifolia)

The rare San Diego thornmint (Acanthomintha ilicifolia) is native to clay lenses on the north slopes of Emerald Heights in the City of Escondido. Geologically, this plateau is composed of gabbro rock and is a southern extension of the Merriam Range. Thornmint is an unusual member of the mint family (Lamiaceae) because it produces conspicuous spines on the floral bracts. It is included in the California Native Plant Society (CNPS) Inventory of Rare and Endangered Plants on list 1B.1 (rare, threatened, or endangered in California and elsewhere). It is listed by the state of California as Endangered (January 1982) and by the Federal Government as Threatened (October 13, 1998).

View from Emerald Heights looking southwest toward Escondido. San Diego thornmint occurs in the clay lens clearing (white arrow) within the chamise chaparral.

Lamiaceae: Acanthomintha ilicifolia (San Diego Thornmint)

  San Diego Thornmint Photographed In Spring 1978  


Other Plants Associated With Thornmint At Emerald Heights

Plantaginaceae: Plantago rhodosperma (Red-Seeded Plantain)


This species is appropriately named.

  Red-Seeded Plantain At Lake Calavera  
Size Of Sewing Needle In Above Image


Apiaceae: Lomatium dasycarpum ssp. dasycarpum (Woolly-Fruit Lomatium)


Boraginaceae: Cryptantha clevelandii var. clevelandii (Cleveland's Cryptantha)

Magnified view of green nutlets of Cryptantha clevelandii. The nutlets are smooth and shiny. They do not have tubercles, although some have slightly roughened areas. Some of the nutlets are also mottled. Their length is about 1.5 mm.

Three additional species of Cryptantha in the southern Merriam Mtns: A. Cryptantha intermedia (Nievitas Cryptantha), B. C. muricata (Prickly Cryptantha), and C. C. micromeres (Minute-Flower Cryptantha).

Tubercled nutlets of the above three species of Cryptantha: A. C. intermedia, B. C. muricata and C. C. micromeres. The cuboidal grain of table salt (NaCl) is about 0.3 mm on a side.


Boraginaceae: Harpagonella palmeri (Palmer's Grappling Hook)

1. Hitchhiker fruit of Harpagonella palmeri resembling a miniature grappling hook. 2. Squamulose thallus of the soil lichens Placidium or Psora on reddish gabbro soil. 3. Seed capsules of Plantago erecta.

The fruits of Harpagonella palmeri resemble miniature grappling hooks. The numerous barbed projections make them very effective hitchhikers. In fact, they have a SRDU rating of 10 (sock removal difficulty unit), the second highest rating on the official Wayne's Word Top 17 Hitchhiking Plants (see following link).

  Wayne's Word Top 17 Hitchhiking Plants  


Soil Lichens Forming A Cryptobiotic Crust

Ascomycetous Soil Lichens: Placidium (Brown Earthscale) & Psora (Brown Scale Lichen)

This chunk of clay supports two similar-appearing genera of brown squamulose soil lichens: Placidium and Psora. The Placidium is similar to P. lacinulatum). The Psora resembles P. globifera or P. californica. Although both genera have a thallus composed of brown, scale-like lobes (squamules), their spore-bearing bodies (ascocarps) are quite different. The squamules of Placidium have spore-bearing perithecia embedded in the thallus. Each perithecium has a small circular opening on the upper side of the thallus. Squamules of Psora have one or several globose apothecia on the upper side, appearing like dark, dome-shaped bodies.

Magnified view of two species of squamulose lichens crowded together on the gabbro clay substrate. Although the squamules are similar, their reproductive (spore-bearing) bodies (ascocarps) are very different. Placidium has flask-shaped perithecia embedded in the thallus. Each perithecium has a small circular pore on the upper side of the thallus. Psora has conspicuous mound-shaped apothecia on the upper side of its thallus. Magnification 10x.

  Brown Earthscale In Desert Cryptobiotic Crust  

Reddish-orange squamules of the widespread soil lichen called "blushing scale" (Psora decipiens).


Ascolichens: Diploschistes (Cowpie Lichen) & Collemia (Black Soil Lichen)

A. Diploschistes (cf. D. muscorum). B. Collema (cf. C. coccophorum)

Diploschistes muscorum, a crustose soil lichen that is very similar to D. scruposus. It typically grows on soil (terricolous) and may even grow over mosses (muscicolous) and other lichens (lichenocolous). The grayish-white thallus contains numerous, crowded, cup-shaped apothecia. The brown muriform spores are about 35 micrometers long, four spores per ascus. D. scruposus may have 4-8 spores per ascus and typically grows on rock (saxicolous). The apothecia of D. scruposus are variable, from urn-shaped (urceolate) to slightly perithicioid (perithecia-like). The phycobiont of both species is Trebouxia, a unicellular green alga found in many species of lichens. Both D. muscorum and D. scruposus are K+ yellow to red and Cl+ red. They both contain diploschistic, lecanoric and orsellinic acids. View of spore and algal cells taken with Sony digital camera through compound microscope at 1000x.

Minute globular lobes (isidia) and apothecia of black soil lichen (Collema) in between the light brown squamules of earthscale (Placidium). The isidiate thallus of Collema imbibes water and becomes gelatinous.

  Black Soil Lichen (Collema) In Desert Cryptobiotic Crust  


Crustose Ascolichen On Rock: Acarospora socialis (Yellow Coblestone Lichen)


Polygonaceae: Chorizanthe fimbriata var. laciniata (Laciniate Spine-Flower)

Another laciniate spine-flower from Scissors Crossing.


Liliaceae: Calochortus splendens (Mariposa Lily)


Gentianaceae: Centaurium venustum (Canchalagua)


Themidaceae: Bloomeria crocea (Goldenstar)


Asteraceae: Deinandra (Hemizonia) fasciculata (Tarweed)

The following paragraph is reprinted from Bobbie Stephenson & Tom Oberbauer (CNPS San Diego Chapter Newsletter May 2010). Back in the late 1990s, Bruce Baldwin, now of the Jepson Herbarium and Department of Integrative Biology at UC Berkeley, noticed common leaf characteristics between the California tarweeds and the Hawaiian silverswords. Through his research, he determined that a California tarweed gave rise to the exotic looking silverswords of Hawaii. Possibly just a single seed, transported on a bird's foot or Pacific Ocean currents, apparently reached one of the islands about six million years ago and took root. The "Hawaiian Silversword Alliance" exhibits the most outstanding example of adaptive radiation in the world. The silverswords have evolved drastic differences in growth form, including rosette plants, cushion plants, shrubs and trees. Using genetic analysis, Baldwin has been able to show that silversword's closest relatives are perennial tarweeds of California, namely the species Carlquistia muirii of Monterey County, Anisocarpus scabridus of Shasta County and other northern California Counties, and Kyhosia bolanderi of the high northern and eastern California mountains. Early crosses between some tarweeds and the Hawaiian silverswords were fruitless, but when Baldwin crossed a Hawaiian silversword with each of these three species, hybrids were produced and their close relationship was confirmed.

  Images Of The Hawaiian Silver Sword Alliance  

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