Pinguicula esseriana — Vorstman's Guide to the Carnivorous Plants

Pinguicula esseriana — the trap in close-up — **The trap in close-up** · Photo: incidencematrix in cultivation — CC BY 2.0 · Wikimedia Commons

Pinguicula esseriana — the flower — **The flower** · Photo: incidencematrix in cultivation — CC BY 2.0 · Wikimedia Commons

Identification

Two guises in one year. Through the dry months there is a compact winter rosette of thick, fleshy, spathulate leaflets overlapping like roof tiles, grey-green to red-purple; that bud is barely larger than a fingertip and carries almost no sticky glands. As soon as the rains come the summer rosette unfolds: thinner, flatter leaves, given in the original description as up to about 18 mm wide at the tip, with a glistening glandular coating. The flower sits on a slender scape about 8 cm tall and is pale to dark violet with a paler throat; the lower lip is strikingly broad, about 18 mm, and the slender spur projects clearly backwards. The plant readily makes new rosettes from shed leaflets, so you often find dense clumps of dozens of buds together.

Similar species

The nearest relative is Pinguicula ehlersiae, which grows on the same gypsum soils in San Luis Potosí and has even been treated by some authors as a variety of P. esseriana. P. ehlersiae has a somewhat flatter winter rosette with a distinct pale mark on the leaf and a flower with a hooked spur. P. debbertiana from the same region is also very similar; it has a smaller flower and a shorter, rather thicker spur. Out of flower the three are barely separable in the field, and the literature itself admits the boundaries here are blurred.

The trap and how it works

A flypaper trap that works only part of the year. Only the summer leaves carry the full glandular coating: stalked glands with a sticky droplet and, between them, sessile glands releasing digestive enzymes as soon as prey lies on them. The winter rosette is largely glandless and serves to survive drought, not to catch. With small flat leaves the catch is correspondingly small: springtails, midges, small flies and whatever stirs in the gypsum hills when the dew falls. In this species the leaf margin rolls over only a little, enough to keep the digestive fluid around the prey.

Habitat and ecology

The gypsum and limestone hills of the dry interior of San Luis Potosí, in thorn scrub with agaves, hechtias and cacti (R2, D4). It is an emphatically inhospitable site: hot summers, a short rainy season, almost no rain outside it, and heavy nocturnal dew that carries the plants through the drought. The rosettes sit on north-facing slopes, in crevices and at the foot of gypsum banks where moisture lingers, often among mosses and Selaginella. Gypsum is an impossible soil for most plants — too little nourishment, too much sulphate — and it is exactly there that this species keeps competition at bay.

Distribution and elevation

A Mexican endemic, known from the dry part of San Luis Potosí, with the classic sites around El Huizache and Guadalcázar. Our species list gives only 'Mexico'; the literature is more precise and points to San Luis Potosí. Elevation lies roughly between twelve and seventeen hundred metres, on the tableland east of the Sierra Madre Oriental. The area is small, but within it the species can be locally numerous on suitable gypsum banks.

Recorded (one-degree cells)

Distribution based solely on records at GBIF (Global Biodiversity Information Facility), not on TDWG regions — those are often far larger than the actual range. The light halo is the area around a record, the dark core the record itself. The darker cells are 8 one-degree squares — roughly a hundred and ten kilometres — in which the species has been recorded, from all GBIF records. This is coarser than for most species because hardly any record of this plant carries an accurate coordinate: the locality of poached species is obscured at source. At this scale that obscuring disappears into the noise. The cells show where it has been seen, not where it occurs: an empty square means nothing.

Forms and variation

No wild varieties are distinguished, but flower colour clearly differs between populations: from pale lilac-violet to a deep, saturated violet. How far the winter rosette flushes red also varies, and that depends mostly on exposure: plants on an open gypsum bank turn purple, plants in the shade of an agave stay grey-green. The boundary with P. ehlersiae is, as noted, disputed; several authors regard the two as forms of one variable species, and on some hills there are plants falling exactly between them.

Where and when to see it

Go in the rainy season, June to September, when the summer rosettes are open and the flowers stand above them; in the dry months the buds are practically invisible among the grit. Search the north and east sides of gypsum hills, low down, in the lee of hechtias and agaves. Get on your knees: the plants are smaller than the pebbles around them, and only with your nose to the ground do you see the glands glisten. Watch for dew in the early morning; the rosettes are then dark and damp and stand out best against the pale gypsum.

Conservation status

Not assessed for the global IUCN Red List, and not listed in CITES. The known range is small and confined to an unusual soil, which makes the species vulnerable by definition to gypsum quarrying, road building and overgrazing — all three common operations in this part of San Luis Potosí. Against that, the plants can stand in large numbers on the gypsum banks themselves, and the species is very easily propagated in cultivation, so the trade has no reason to draw on wild plants. Without a formal assessment this remains a file of scattered observations with no coherent picture of the trend.

Scientific name

Pinguicula esseriana | Kirchner, 1981

Described by Bernd Kirchner in 1981, in Willdenowia 11(2): 317–319. The type area lies in Mexico. The exact origin of the species name esseriana (likely honouring a person named Esser) could not be established with certainty.

World Checklist of Vascular Plants (WCVP), via Plants of the World Online (POWO), Royal Botanic Gardens, Kew.