This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
In this volume are included the remaining- British species not described in Vols. II-V. They are also arranged in systematic order under each section based on broad ecological lines. To assist in the identification of plants in this and preceding volumes, a summary, first, of the natural orders and, secondly, of the genera is given in Vol. V. But, the work not being primarily intended to define the systematic characters of plants, reference to a work with analytical keys, such as Bentham and Hooker, may be made for that purpose. In the introduction to this volume some remarks are appended upon:
(a) The composition of the British flora.
(b) Status of some Britannic plants.
(c) Some further notes on species and varieties.
(e) Identification of species. (/) Nomenclature.
The composition of the British Flora, as at present understood, may be best gathered by a study of Mr. Druce's British Plant List, which is very comprehensive, so that there is no danger of erring on the side of under-estimation.
In this list are included 2958 flowering plants, excluding Gymnosperms and Cryptogams. Since it was published many more have since been added.1
Of native or well-established species there are some 1759. These, including some Gymnosperms and Cryptogams not embraced in that number, are made up as follows:
Species doubtfully native
Alien species now well established
Aliens more or less fugitive...
This includes 5 extinct species and 5 others doubtful. There are 1430 varieties and 259 hybrids, over and above 23 not included in these numbers.
Of the aliens 751 are natives of Europe, 30 are cultivated (in many cases natives of Asia and Africa).
1 See Adventive Flora of Tweedside and Mr. Druce's Alien Flora.
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. Professor 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, Calhma, 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, lmpatiens 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, landslips, etc. 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: Sempervivum, 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 peculiarly situated a group of islands as the British Isles, the flora must be very varied. The world trade of the British Isles brings to us many aliens, and the migration of birds, the lines of which lie across these areas, is responsible for the occurrence of many plants here that grow in far-distant regions; and, similarly, the Gulf Stream and prevalent winds may be the deciding factors in other cases.
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.
As to the general character of the flora and the status of many species, and the occurrence of endemic species, Mr. Druce in his report of the floristic results of the International Phytogeographical Excursion, 1912, writes: "Whereas many leading British systematists have been extremely reluctant to acknowedge that the British Isles contained endemic species . . . yet, as one would expect, a more minute and critical study of plant forms . . . has led to well-marked differences being established between many of our island species and their homo-logues on the mainland of Europe". Referring to some remarks by Dr. Graebner, on the difference between British and Continental forms of Sedam acre (the British type is distinguished as Sedum Drucei), Mr. Druce adds: "This statement is borne out by one's own experience; one sees that the common species of Jersey have a different facies from those of our Midlands, while those of the north Scottish coast possess a distinct individuality from those of Devon and Kent. But it is only exceptionally that specific distinctions can be found. This range of variation, differing necessarily in degree, however, suggests that we may be unwise when working with critical forms to attempt to identify the microspecies of Geranium, Erodium, the Melanium Violas, and the critical species of Rosae, Euphrasia;, Hieracia, and Taraxaci, with Continental names. In many cases I strongly suspect that the British plants are sufficiently distinct to warrant them being described and named. Indeed, as will be seen, two of our British plants, Erigeron alpinus and Melampyrum pratense, should bear, Dr. Ostenfeld suggests, other names. And, if evolution be a fact, we might be prepared to expect that these plants, living for so long under different climatal conditions and geographical position, should have evolved a facies of their own."