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. 



