Poncirus trifoliata (Japanese Bitter Orange)

Against the sunny south facing walls of the Ruskin (Nash) Building, a few dozen small orange fruit lay on the ground but there are no fruit trees, as such, in the borders. The deciduous shrub that produces these inedible bitter fruit, that resemble oranges, is the Japanese Bitter Orange, Poncirus trifoliata. (Apparently they are good for making marmalade and preserves though)

During late spring to early summer the Poncirus produces a profusion of small white flowers with a sweet citrus fragrance that, when fertilised, develop in to these citrus like fruit. 

However, the shrub is covered in long, sharp spiky thorns that will protect its fruit from being harvested even by the most hardened of fruit pickers. (Make sure your body is covered in thick clothing, your hands by thick gloves, your eyes by protective glasses and a hat that protects your head, that should be sufficient to pick the fruit for those that want to make marmelade!)