Aspects of plant life; with special reference to the British flora

by R. Lloyd Praeger

In the first place, it must be recalled that the British Isles are situated on a broad shelf which extends into the Atlantic on the western edge of Europe. In comparison with the depth of the adjoining ocean, this shelf is but little below sea-level, and a slight elevation of the land—much smaller than those which have occurred over and over again in recent geological times—would join our islands to Germany, Holland, Belgium, and France. The British Isles are geographically and biologically by no means a separate area, and they have derived their population, both plant and animal, by immigration at various periods of time from the great land area to the eastward. Our present flora proves the truth of this as a general assertion; a study of its constituents shows that it is essentially a reduced continental European flora. As we step from France across to England we lose a number of plants familiar on the French side. As we step again from England into Ireland a further number of plants disappears; and these losses are no doubt due either to an unsuitability of climate on the insular areas, especially the absence of a hot summer, or to the inability of the plants to cross the barriers of sea which have now existed for some time. If the whole of the flora fitted in with this idea of mere reduction of the Continental flora by elimination, the problem would be much simplified. But there are other elements in it which do not harmonize with this conception of simply a general western migration, and which give rise to very interesting problems.

Let us first consider the main mass of our flora, which is closely akin to that of the adjoining parts of the Continent. When we say that it represents a reduced Continental flora we do not imply that it is therefore uniform in its composition throughout the British Isles. We know, on the contrary, by everyday observation, that it varies much in its constituents. The principal general change is noticed if one travels from the south of England to the north of Scotland. Great Britain extends in this direction for 700 miles—far enough to allow climate to have a marked effect as between its extremities. The flora of Hampshire is very different from that of Caithness or the Orkneys. But both represent in the main the vegetation of that part of the Continent which lies in the same latitude, the Hampshire flora being akin to that of Northern France, the Caithness flora to that of Southern Scandinavia. The likeness is in each case heightened by the fact that the rocks of the respective areas correspond, producing similar soils, which tend to support similar floras. The soft Secondary and Tertiary deposits of Southern England are repeated in the Paris basin and surrounding area, while the ancient gneisses of Scotland are akin to those of Norway. To quote a few instances of this north and south difference coupled with east and west similarity: the Small-flowered Crowfoot (Ranunculus parviflorus), White Bryony (Bryonia dioica), Water Violet (Hottonia palustris), Yellow-wort (Blackstonia perfoliata), and Black Bryony (Tamus communis), all widely spread throughout England and Wales, die out in or about the Lake District, and are absent from Scotland; the Scale Fern (Ceterach officinarum) gets farther north—about half-way up Scotland—before it disappears; other plants again, widespread in the south, die out before the Mersey-Humber line, or even the Severn-Thames line, is crossed. On the Continent, the plants enumerated are mainly southern in their range. All occur widely in Central and Southern Europe, but from Scandinavia most are absent, and the rest are rare. On the other hand, some characteristic Scottish species cease as we come southward—the little Primula scotica, for instance, is confined to the northern extremity of Scotland; the Chickweed Wintergreen (Trientalis europæa) ranges only as far south as Yorkshire; and the beautiful Globe Flower (Trollius europæus), so characteristic of northern pastures, creeps southward as far as the Severn. The first of these is on the Continent confined to Scandinavia; the others, though found in France, etc., are characteristic of the hilly regions there, and are much more abundant farther northward.

Next to this north-and-south change, due to climate, we may notice an east-and-west change, due partly to climate, but more perhaps to elimination, for in passing from France to Ireland we have to cross two barriers of sea. The climatic change is not unlike that experienced in going from south to north. We leave a dry climate (rainfall under 25 inches at year) for one of increasing wetness, a warm for a cool summer, a colder for a milder winter.

The chief difference between the extreme west of the British Isles and the extreme north lies in the warmer winter of the former, frost being almost unknown in the milder spots. But the general similarity of northern and western conditions as opposed to eastern and southern leads to a fusing of the northern and western plant groups, so that on a map designed to show the distribution of our species analyzed according to their general range in Europe, the grouping of plants in the British Isles will be found to be roughly north-western as opposed to south-eastern. The further change due to elimination of species has been already referred to. Most plants no doubt have spread in our islands as far as prevailing climatic and soil conditions allow, but in other cases the sea-barriers seem to have put a period to their natural advance. Considering the wide range of conditions of climate and soil under which, for instance, the Hairy Crowfoot (Ranunculus sardous), the Common Rock-rose (Helianthemum Chamæcistus), the Needle Furze (Genista anglica), and the Small Marsh Valerian (Valeriana dioica), occur in England, Wales, and Scotland, it is difficult to impute their absence from Ireland to climate.

Thirdly, we find (as we have already seen in the first chapter) varying conditions of soil intruding themselves and producing such local changes in the grouping of the plants as may quite obscure the broader differences just dealt with. Were our islands a plain formed of uniform materials, the gradual changes from south to north or east to west might be traced step by step. But their surface is most diversified; their rocks contain an epitome of the whole geology of Europe; the soils are consequently various: from the point of view of the plant world the area is an archipelago: for some plants a desert with occasional oases, for others an oasis enclosing occasional deserts. Certain species are confined to the Chalk—for instance, the Box (Buxus sempervirens) and the Stinking Hellebore (Helleborus fœtidus)—while to others a limy soil is a barrier comparable to that formed by the English Channel. It will be seen, then, that when we speak of the flora in general being a reduced Continental one, many considerations, geographical, climatic, and edaphic, must be duly taken into consideration if we are to understand the composition and distribution of our vegetation.

But making all allowance for these various disturbing influences, there are found in our flora certain plant groups which will not fit in with this general conception of immigration from the east. Let us take a few examples. In fir woods in Dorset, until some forty years ago (when it was exterminated), grew a slender little plant allied to the Lilies, too little known to have a popular English name, and called by botanists Simethis planifolia or S. bicolor, the latter name having reference to the fact that the flower is purple on the outside, white on the inside. This plant is unknown elsewhere in Great Britain, and was at first set down by H. C. Watson, the leading British plant geographer, as an alien or denizen, not a true native; but the fact that it grows over a considerable area of very wild ground in Kerry (its only Irish station), far from possible sources of introduction, and undoubtedly native, indicates a strong probability of the plant’s having been indigenous in Dorset also. It is not present on the adjoining parts of the Continent, but turns up again in the Pyrenean region, some 500 miles to the southward, and may be traced thence into Italy and North Africa. Did this instance of an apparent migration from the south stand alone, it might not excite much attention, and we should probably be inclined to attribute the plant’s peculiar and discontinuous distribution to the extinction, perhaps by human agency, of intermediate stations. But it stands by no means alone. In Cornwall two

FIG. 28.—GREAT BUTTERWORT (PINGUICULA GRANDIFLORA).

[To face p. 173.