Page:VCH Surrey 1.djvu/80

 A HISTORY OF SURREY numerous introduced plants which have occurred in the county. Only a few which have taken a very firm hold, such as the balsam (Impatient fufoa) or Galinsoga parvifora, are mentioned. Such plants are denoted by an asterisk (*). The hybrids also are for the most part omitted. The study of hybridity among wild plants has been almost entirely neglected in this country until a comparatively recent date ; but con- siderable advances have been made, so that a very large number of hybrid forms are now known. In some genera which are more especially prone to crossing, such as the willow herbs (Epilobium) and willows (Sa/ix), the hybrids far outnumber the true species. In various others, such as the violets (Viola), docks (Rumex) and pondweeds (Potamogefon), they are more or less frequent. The few named are included either for their rarity or for the rarity with which the phenomenon of hybridity occurs in the genus to which they belong. Some hybrids, such as the thistle (Cnicus Forsteri), are probably rare simply because of the different time of year at which the parent species flower, one of them (Cn. pratensis) being usually seeding at the time that the other (Cn. palustris) is opening its first flowers. Others, such as the small willow (Sa/ix ambigua), do not occur at all in the county, although common in other parts of the kingdom. The reason is in this case, I believe, that -although the parent species are often found growing together, one of them (S. repens) flowers later than the other (S. aurita}. As we go further north this hybrid becomes more frequent, indeed in Shetland one does not ever see the two parent species growing together without the accompaniment of hybrid forms. The explanation no doubt is that as we advance towards more boreal regions the summers become more and more shortened, so that the flowering period of some species which do not bloom at the same time in the south becomes synchronized, and the opportunity of crossing occurs. The following summary of orders, etc., is drawn up in accordance with Hooker's Student's Flora of the British Islands, but the census of species is calculated according to the London Catalogue of British Plants (ed. 9). The result shows Surrey to possess, including both native and naturalized plants, 1,081 phanerogams out of a total of 1,861 found in Britain. Of these 830 are dicotyledons and 251 monocotyledons.