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INTRODUCTION 



As regards duration, there are 52 British trees, 210 shrubs (129 being Rubi or 

 Brambles), 7 climbing shrubs, 1020 perennials (130 being Hieracia), 76 biennials, 

 350 annuals (25 being Characeae (Cryptogams) ). 



In this work all or most of the truly native plants are included, save varieties 

 and forms ; also the better-established aliens, colonists, and denizens. The newer 

 aliens and a few doubtful species are not described. 



In the London Catalogue of British Plants, Edition 10, 1908, the number of 

 species enumerated is 1977, including some colonists, denizens, and aliens. Pro- 

 fessor Henslow's Catalogue of British Plants, 1835, gave the number as 1381 

 species (native), 57 naturalized, 56 doubtful, or 1494; and the varieties as 1650 

 (native), 62 naturalized, and 58 doubtful. 



From these totals it may be gathered what advance has been made during 

 the last half-century or more in the domain of British systematic botany. Between 

 the totals given by Henslow and Druce there is a difference of some six hundred 

 species, let alone varieties. Such increase may be due to several causes, of which 

 the main are: 



(a) Division of aggregates into segregates. 



(b) More systematic exploration of the flora, county by county. 



(c) Annual introduction of plants from elsewhere. 



(d) Better knowledge of species and varieties. 



It is necessary to define the accepted terms used to differentiate between the 

 status of different plants, and for this purpose no better definitions can be had than 

 those proposed by Watson, the pioneer of British plant geography. 



Watson used six grades to distinguish plant-nativeness or other degrees of 

 status, of which the first four are universally accepted. 



1. NATIVE. 1 Apparently an aboriginal British species; there being little or no 

 reason for supposing it to have been introduced by human agency. Examples: 

 Corylus, Calluna, Bellis, Teesdalia. 



2. DENIZEN. At present maintaining its habitats, as if a native, without the 

 aid of man, yet liable to some suspicion of having been originally introduced. 

 Examples: Aconitum, Pceonia, Viola odorata, Impatiens noli-me-tangere. 



3. COLONIST. A weed of cultivated land or about houses, and seldom found 

 except in places where the ground has been adapted for its production by the 

 operations of man; with some tendency, however, to appear also on shores, land- 

 slips, &c. Examples: Adonis, Papaver, Agrostemma, Melilotus. 



4. ALIEN (OR CASUAL). Now more or less established, but either presumed 

 or certainly known to have been originally introduced from other countries. 

 Examples: Semperuivum, Mimulus, Hesperis, Camelina. 



5. INCOGNITA. Reported as British, but requiring confirmation as such. Some 

 of these have been reported through mistakes of the species, as Ranunculus 

 gramineus. Others may have been really seen in the character of temporary 

 stragglers from gardens, as Gentiana acaulis. Others cannot now be found in the 

 localities published for them, as Tussilago alpina and other species reported by or 

 from Mr. George Don ; though it is not improbable that some of these may yet be 

 found again. A few may have existed for a time, and become extinct, as Echino- 

 phora spinosa. 



6. HIBERNIAN (OR SARNIAN). Native, or apparently so, in Ireland, or in the 

 Channel Isles, though not found in Britain proper. It is obvious that, in so 



1 Of endemic species (confined to the British Isles) it has hitherto been considered that only some 147 species 

 can be regarded as such, mainly Rubi and Hieracia. 



