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.
The influence of man over the character of the present flora has been emphasized in previous volumes. His relation to the status of plants, as defined by Watson, has been ably defined by the Rev. E. A. Woodruffe-Peacock in Natural Habitats and Nativeness, where he writes: " In relation to man as a predominating factor in the botanical problem, all we have to set ourselves to disentangle is, ' what is permanent, and what is transitory?' The former is natural, the latter accidental, semi-alien, or alien. No other criterion appears to be possible. It has the advantage of being simple and practical. It may be difficult to say whether a given species is ' native, denizen, colonist, or casual' under a certain environment, but it is easy enough to ascertain whether it is permanent or transitory. It is not difficult to demonstrate whether a plant is found in the majority of fitting situations, or whether it is peculiar to one locality, or at most a few, under suspicious circumstances, under the same conditions of growth.
"To help in this respect, and to escape from the difficulties and perplexities of the Watsonian system of terminology, I propose to class all species into various categories, as they stand in an intimate or more distant relation to man and his undertakings. Their position in a category or categories will at once settle their status. Samples only can be given here: Followers (1) of man, (2) of cultivation, (3) of commerce. Frequenters (1) of broken ground, (2) of waste ground, (3) of pasture, (4) of meadow, (5) of woodland, (6) of roadside hedges, (7) of field hedges, (8) of lakes, (9) of ditches, etc, are much more simply applied, even though the phrase lacks the sweet simplicity of the Watsonian word. Both should contain a distinct idea, or set of circumstances; unfortunately, in practice, this is what Watson's words do not express."
A further feature in the occurrence of plants of transitory type lies in the necessity of absence of competition which determines the floral cycle of a piece of ground. The last writer indeed says: " A very little observation, properly directed, will soon convince any unprejudiced mind, that freedom from competition is the chief influencing- cause of the appearance of annuals and biennials on freshly moved soils. It settles the question of the duration of their stay; and everywhere, under natural and artificial conditions alike, freedom from competition is the most potent influence in the distribution of transitory species.
"Man acts as a disturber of the floral cycles of nature, as a clearer and mover of the ground, as an interrupter of competition, as a finder of elbow-room for annuals, biennials, and the less powerful perennials. Directly or indirectly his influence is felt in many other ways as a modifying force. Some have hardly been fully appreciated yet. The one special manner in which every human being, by his actions or through his wants, influences plant life, is as an aider and abettor of the weaklings of our flora in their struggle for existence."
Whilst most of the alien plants found in the British Isles colonize ground which differs from their natural habitat in their country of origin, some may establish themselves here in closely similar habitats. In Mr. S. T. Dunn's Alien Flora of Britain, the natural habitat of the species is given, but some of the plants included in this work may, as the Rev. E. S. Marshall points out (The Status of Some Britannic Plants), be regarded as native in this country. The latter rightly emphasizes the importance of correctly defining the width of meaning to be attached to the somewhat loose term "waste places", and considers that in this we should include village greens, rubbish heaps, unoccupied land near towns and villages, and sandy commons, sea-shores, etc. The actual geographical distribution in Europe of many species is complicated also by the existence of outliers.
Of plants regarded as alien by Dunn, Mr. Marshall would exclude, among others: Aconitum Napellus, possibly Paeonia corallina (though Mr. Druce thinks it is not native), Lepidium Smithii, Sisymbrium Sophia, Viola tricolor, Ceraslium aroense, Malva rotundifolia (near the sea), Geranium pusillum, Medicago minima, Vicia lutea, Prunus insititia, Ribes Grossularia (in Yorkshire), Cotyledon Umbilicus, ^Ego-podium Podagraria, Anthriscus vulgaris (on the coast), Sherardia arvensis, Artemisia Absinthium, Chrysanthemum Parthenium, Lactuca Scariola, Matricaria inodora (vars.), Sonchus arvensis (seashores), Hyoscyamus niger (in woods), Veronica arvensis, Ajuga Chamcepitys, A triplex patula (by slow streams), Chenopodmm album (in marshes), Rumex pulcher (on the coast), Parietaria officinalis (on rocks and cliffs), Galanthus nivalis (especially in the west), Apera interrupta.
These are but a few of the plants whose nativeness has been questioned, that may be regarded as indigenous. It is important to remember that a plant may be native in one situation and casual in others. Thus it is necessary to make a careful rock-soil analysis of records of each species, and to determine where and when it is permanent, and when transitory.
In considering the meaning of the terms species, varieties, etc, it is necessary to admit the influence, not only of the inherent tendency to vary, but also of such physiological factors as pollination (whether self-pollination or cross-pollination), and of factors such as soil or climate.
Variation in some genera, even in the British Flora, is extremely marked, and whilst the normal number of species in a genus is four or five, there are some genera that include as many as 100 species (or sub-species or races).
The whole problem centres around the interpretation of terms, and the value attached to each. But whatever value be attached, the underlying- causes remain. The multiplication of forms may in some cases, as in Willowherbs, Willows, etc., be due to hybridization, in others to the normal causes of variation.