MIMULUS HYBRIDS IN BRITAIN

The hybrid M. guttatus X luteus has been synthesized and shown to be identical with naturally-occur­ ring plants, which have been found to be widespread in Britain. Plants of the putative hybrid M. guttatus x cupreus have also been found to occur in several northern localities. INTRODUCTION According to Loudon (1872) species of the genus Mimulus were first brought into the British Isles just over 200 years ago. They were introduced from the American continent and became popu1ar as border and pot plants. A few of them became naturalized: M. guttatus DC. , said to have been introduced in 1812, was already established as an escape by 1830 (Warburg 1962); it is now widely distributed on the banks of streams and rivers, and is recorded from all but a dozen of the vice-counties in Britain. M. luteus L. and M. moschatus Dougl. ex Lindl. have also become naturalized, but far less commonly; and more recently a fourth species, M. cupreus L. , has been reported as a widespread escape in the English Lake District (McClintock 1957). M. guttatus is a ± pubescent perennial, often glandu1ar-pubescent, particularly on the upper part of the flowering stem, the pedicel and the calyx. It has a bilabiate yellow corolla, marked with small, red spots in the throat, and with the lower lip longer than the upper. It is a native of western North America, where it is very variable, being made up of many polymorphic populations growing in wet places (Vickery 1959). M. luteus on the other hand is completely glabrous except inside the calyx and corolla, the latter usually marked with large, reddish-brown spots on the lobes, in addition to small ones in the throat. The lips of the corolla, moreover, are nearly equal. It is a smaller, more prostrate plant than M. gzlttatus and is a native of Chile. M. cupreus is described as being very similar to M. luteus, differing from it only in its compacter habit, smaller leaves and copper-coloured flowers, which mayor may not have small, dark spots in the throat. Like M. luteus, under which it was formerly placed as a variety (Bailey 1927), this also is a native of Chile. M. moschatus is a densely and viscidly-hairy plant with a much smaller, pale yellow, normally unspotted corolla, and smaller leaves of a paler green. It originates from western North America. THE NATURALIZED PUTATIVE HYBRID Despite the descriptions given in the floras many plants of Mimulus have proved difficult to identify and have often been misrecorded. For example, at several places in Caernarvonshire plants were found which cou1d not satisfactorily be placed under either M. guttatus or M. luteus; they showed considerable variation in pubescence and corolla marking, many of them being tall, robust plants, with large, heavily-blotched corollas. In 1960 the intermediate character of a plant growing at Harlech, Merioneth, aroused the suspicion that it might be of hybrid origin-a view which was further strengthened when its pollen was found to be highly sterile. Consequently, similar plants from as many other localities as possible were examined. In many of these places the plants were so uniform as to suggest that they were, in fact, clones, and that propagation by seed did not occur there. Altogether, plants with highly sterile pollen from seven localities were taken into cultivation and, during the summer of 1961, pollen of four flowers from each of them was examined and an estimate made of the percentage of full pollen grains. This was done by mounting