Sorbus species are unusual in that they can produce seed without the need for normal pollination and fertilisation of the flowers (apomixis). As a result, the UK has many endemic Sorbus species which have long intrigued plant taxonomists.
Some believe that they should be recognised as ‘microspecies’, while others contend that they are ‘full’ species. Matters are further complicated by the fact that the microspecies occasionally hybridise sexually, giving rise to further new species or microspecies. On the Isle of Arran there are unique Sorbus taxa resulting from crosses between Rowan (S. aucuparia) and rock whitebeam (S. rupicola). S. arranensis is an apomictic hybrid; and S. pseudofennica is a backcross (also apomictic) between S. arranensis and S. aucuparia.
It was our first Director, Humphrey Gilbert Carter, who started the interest in Sorbus here in Cambridge. His student, Warburg, also took a special interest and much of our collection dates from his work in the 1960s.