x erraticum Brügger, 1882
Parentage : Sedum annuum x S. boloniese (sexangulare) - see Note below.
Type : Siegfried 1872, collected at Mendrisio, ca 400 m asl., southern Switzerland.
Description (translation of the German protologue) :
The subterraneous creeping stem emits one to several ascending, simple or branching flowering stems 8 - 15 cm tall, leafy above, cymose in the upper third, but no or only exceptionally few 2 -3 cm long, loosely leafy sterile stems (of 22 plants only one had 3 sterile stems).
Leaves linear-terete, obtuse, spreading, 4 - 5 mm long, distinctly spurred (spur obtusish, sometimes acute).
Cyme very floriferous with 2 - 6 up to 5 cm long rather lax branches with 18 - 50 flowers.
Flowers 5-merous, sessile, 7 - 8 mm wide, petals whitish-yellow (4 x 1 mm), lanceolate-acute, twice as long as the oblong-terete, obtuse, not spurred sepals.
Regarding habit, inflorescence and leaves the plant is most similar to Sedum boloniense (S. sexangulare), however it is differing clearly in its smaller and paler flowers, the longer and much more floriferous branches of the inflorescence, similar to S. annuum, and the absence of creeping sterile stems, also corresponding to S. annuum.
S. x erraticum seems to represent an intermediate form between two distinct species otherwise not connected by transisional taxa. And its rudimentary capsules strongly suggest hybrid origin even though there is (yet) no strict prove that the parents are co-occurring at the locality it has been collected.
S. x erraticum has been found growing on erratic terrain along a mountain rivulet carrying stones and other material from higher up in the mountains to the much lower regions of southern Switzerland, and soil particles of the collected S. x erraticum plants contained numerous quarz crystal grains and minute pieces of glimmer.
Both, Sedumannuum as well as S. boloniense (S. sexangulare), are preferably associated with granite and gneiss, too, however the latter - though widespread in Canton Tessin (southern Switzerland) - is out of the question here [because it does not occur at Mendrisio]. As far as S. annuum is concerned, though belonging in the alpine flora associated with granite, it is not rarely found growing down to 240 - 300 m on the southern side of the Alps.
Note :
The introductory text to the descriptions of S. engadinense and S. erraticum calls these two taxa "transitional forms which very likely have to be considered hybrids" (Aufzählung neuer Pflanzenbastarde der Bündner- und Nachbarfloren (Enumeration of plant hybrids of Grisons and adjacent regions), compiled by Chr. G. Brügger (Jahresbericht der Naturforschenden Gesellschaft Graubünden's 25, 1882).
While in Brügger's list of hybrids (p. 57-58) the parentage of S. erraticum is indicated as S. annuum x boloniense, the respective description (p. 97) states that in habit, inflorescence and leaves S. erraticum resembles most S. boloniense, but explicitely adds that the latter – though not uncommon in Tessin – cannot possibly be one of the parents of S. erraticum. That means the second parent of S. erraticum is unknown.
Praeger – obviously either ignoring the description of S. erraticum or failing to correctly understand it - indicated the parentage of the latter as annuum x boloniense. And it seems that Fröderström did not check the German text either but perpetuated Praeger's error by simply copying it.
't Hart & Bleij (IHSP 237, 2003) consider both names as of unresolved application.