The Mysterious Primula of Omta Tso (Bhutan)

Omta Tso, Bhutan

In November, 2016, I visited the herbarium at the Royal Botanic Garden Edinburgh to study non-type herbarium sheets which are unavailable online. One of the species I wanted to investigate is Primula tsariensis which was described from a collection made in 1936 by Ludlow & Sherriff in the Tsari valley, SE Tibet  28°41'29.98"N 93°23'57.85"E.

It was initially compared with Primula griffithii and P. tanneri but it was distinguished by leaves which are rounded at the apex, bluntly crenulate at the margin and a calyx which is sparsely farinose and glandular. The flowers are described as rich purple.

Primula tsariensis

Going through the herbarium specimens, I came across a folder labelled "P. tsariensis yellow form". What is that?!! All of the collections of this form were Ludlow & Sherriff's from the area near Omta Tso  27°42'54.88"N 90°17'17.96"E in the upper Richen Chu valley, Bhutan. In 1937, Sherriff explored this place and so I traced his 1937 route using his diary and field notes. Sherriff mixed up the names of the two lakes Omta Tso and Thita Tso, so his initial collection #3383 is from the hillsides and shores of Omta Tso (not Thita Tso as stated in his notes). Sherriff gave it the name "Primula strumosa var. nana floribus maximus". Later, W.W. Smith & Fletcher gave it the name "Primula strumosa var. perlata" and in the Flora of Bhutan it is included by that name as a synonym under P. tsariensis. In the genus Primula, there are a number of species which have both purple and yellow forms, so the idea of a yellow P. tsariensis is acceptable.

Primula species of Omta Tso, Bhutan

Continuing to trace Sherriff's 1937 route, I found that he took a two day side trip (July 13-14) from Omta Tso, westwards along the ridge to Chore (I don't have an exact location for Chore, but it would seem to be a yak herder hut at the base of a cliff SW of Omta Tso) and then followed the main ridge south until descending to Maruthang via a steep gully. Sherriff said "I have never seen so many alpines out together as on this march. In places the hillsides and cliffs were just covered with them and the variety was great."

Omta Tso area, Sherriff 1937 - hybrids found along the orange route

Between Omta Tso and Chore, Sherriff came across a little grassy hollow 50 ft. x 50 ft. where there was a swarm of primulas of at least eight shades of colors including yellow, white, purple and blue. These same colors are seen in hybrids where P. calderiana crosses with P. strumosa. Essentially P. strumosa and P. calderiana are yellow and purple forms of the same species, though they have different geographical areas and P. strumosa grows at higher elevations.

Color variations of P. calderiana X P. strumosa

Sherriff knew these were hybrids but couldn't decide which species were the parents. To further complicate matters, Sherriff also found that the yellow, white and blue-purple flowered plants grew in separate masses of one color only. However, the white masses included a few variations of yellow tinged, or blue tinged plants. Sherriff collected what he said was true P. calderiana (L&S 3437) and that it was distinct from the others and not involved in the hybrids. In the field notes, someone (perhaps W.W. Smith) later wrote in the margin "I believe these are normal hybrids of P. calderiana and P. strumosa" and included a reference to hybrid L&S collections from Weitang and Pangotang (Tsampa), Bhutan in 1949, which are unfortunately I haven't seen.

So are the plants at Omta Tso P. tsariensis or are they P. strumosa?

The most obvious differences between the two species are that P. calderiana / P. strumosa smells "disagreeable", and has farina present on the bud scales, and on the upper scape, calyx and pedicels whereas P. tsariensis is not noted for its smell, has efarinose bud scales and only trace amounts of farina on the pedicels and calyx.

Bud scales - (L) no farina, P. tsarieneis, (R) with farina, P. strumosa

Sherriff was limited in his ability to document these plants but certainly more images can shed light on this problem. If you travel to this area and encounter these plants, please take lots of images of the flower color variations, showing the calyx, the basal bud scales, and leaves. And take a sniff!

Pam Eveleigh © 2017