Chapter XII, are so various, that it appears at first sight impossible

to connect them with the xerophilous or hygrophilous organisation of a plant, or, in other words, with any structural characters associated with particular stations; yet behind all lies the general principle that, given a plant of the buoyant group, if it is a Xerophyte it finds its way to the coast, and if a Hygrophyte it makes its home by ponds and rivers. In the case of a tropical littoral flora, such as we find in a Pacific island, the large proportion of plants with buoyant fruits or seeds gives so much prominence to the subject of their distribution by currents that the question of “station” is often masked. On the other hand, in the shore-flora of a temperate region like that of Great Britain, the plants with buoyant seeds or fruits are in the minority, and the question of “station” is the first to obtrude itself.

In establishing the principle that most of the plants with buoyant seeds or fruits have been gathered at the water-side, it was never implied that all the plants by the river or by the pond or at the coast are thus characterised. There is much to learn from the circumstance that whilst nearly all plants with buoyant seeds or fruits are placed at the water-side, not all water-side plants have buoyant seeds or fruits. In the first place, it is to be inferred in the light of what has been said above that the first determining principle in the selection of a station is concerned not with the buoyancy of the seeds or fruits, but with the xerophytic or hygrophytic organisation of a plant. In other words, it is the fitness or the unfitness of a plant for living in situations where the loss of water by transpiration requires to be checked that primarily determines the station at the coast. We thus see in the internal organisation of the plant the primary determining influence on station. Buoyancy of seed or fruit comes subsequently into play, the Xerophyte and the Hygrophyte, thus endowed, ultimately finding their way, the first to the beach, the second to the bank of the river or to the margin of the lake or pond.

In the next place, when we regard the composition of the British coast-flora, and examine the distribution of the plants in other situations than on the beach, we obtain some interesting results. There is first a group of plants, including such as Armeria vulgaris, Artemisia maritima, Cochlearia officinalis, Erodium maritimum, Matricaria inodora, Plantago coronopus, Polycarpon tetraphyllum, Raphanus maritimus, Spergularia rubra, Silene maritima (see Note 15), and others, all of which occur not only at the coast and on the adjacent hill-slopes, but also often far inland, and sometimes at considerable elevations in mountainous districts, as in Central Europe. It is on this occurrence of certain shore-plants in alpine regions that Prof. Schimper lays much stress in his memoir on the Indo-Malayan strand-flora (p. 28), and in his later work on Plant Geography (Engl. edit., p. 716), when pointing out that here temperature does not play a determining part, and that in both stations, whether on the sandy beach or on the mountain-top, the same xerophilous organisation is needed to obviate the risk of impeded water-supply. He quotes in this connection the observation of Battandier that many alpine species from the Atlas Mountains occur on the Algerian beaches, but not in intervening regions. Mr. Druce, in his discussion of the British species of Sea-Thrifts and Sea-Lavenders (Armeria, Statice), brought the subject of the occurrence of maritime plants on mountain summits again to the front; but he did not advance any general explanation, and seems to regard it as the result, as it doubtless is, of the recurrence of suitable stations (_Jour. Linn. Soc. Bot._, Dec. 1900).

Very few of these plants have any capacity for dispersal by currents, a subject dealt with in Note 16. Several of them have dehiscent, small-seeded fruits which, as pointed out in the previous chapter, hardly ever come into the buoyant category. I have experimented on the greater number of them, and in only one species, Matricaria inodora (var. maritima), do the results indicate a capacity for dispersal over wide tracts of sea.

If we look again at a list of British shore-plants, we find another group of plants frequenting salt marshes and muddy shores, and found also often far inland, as in the saline plains of Central Asia. Here we have such plants as Aster tripolium, Glaux maritima, Plantago maritima, Salicornia herbacea, Salsola kali, Samolus valerandi, Scirpus maritimus, Suæda fruticosa, S. maritima, Triglochin maritimum, T. palustre, &c. It becomes in this connection a subject of peculiar interest to the student of plant-distribution when he reads in Mr. Hemsley’s paper on the flora of Tibet (_Jour. Linn. Soc. Bot._, vol. 35) that amongst the British shore-plants above-named the two species of Triglochin and the same species of Glaux and Salsola occur in the salt marshes of the Tibetan uplands at elevations of 15,000 to 16,000 feet, Scirpus maritimus also being found in the swamps of the lower levels. We have the same thing, affecting much the same plants, illustrated in America. Thus we learn from Asa Gray that Salicornia herbacea, Scirpus maritimus, Triglochin maritimum, &c., which are common in salt marshes on the coast of the United States, occur also in the interior of the continent in the vicinity of salt-springs.

Facts of this sort are well known, and I merely refer to them here in order to emphasise the importance of this little group of British littoral plants, those of the salt marsh. Their very wide distribution is connected with the frequent recurrence of suitable conditions, not only in space, but what seems of greater import, also in time. One can scarcely doubt when the Saltwort (Salsola kali) is seen on the Devonshire coast, on a beach in Chile, and in the elevated regions of Central Asia that here a very ancient type of plant finds its still more ancient conditions of existence. In the capacity which most of the plants of the salt marsh possess of germinating in sea-water, this group of littoral plants is sharply distinguished, as far as my observations show, from the other groups of British shore-plants. For instance, in my experiments the seeds of Aster tripolium, Salicornia herbacea, and Triglochin maritimum germinated freely in sea-water, whilst those of Spergularia rubra, Cakile maritima, Convolvulus soldanella and others failed to do so (see Note 19). It will also be noticed with respect to this group of littoral plants that, except in the case of Scirpus maritimus, the seeds or fruits have little or no floating power, the exception offered by Salsola kali being not very striking. This feature is brought out in the Table given in Note 10; but some of the details of my observations are given in Note 17.

There yet remains a third group of the British shore-plants, namely, that comprising the plants that rarely stray far from the beach and often possess seeds or seedvessels that will float for months. Here we have such species as Arenaria (Honckeneya) peploides, Beta maritima, Cakile maritima, Crambe maritima, Crithmum maritimum, Convolvulus soldanella, Eryngium maritimum, Euphorbia paralias, Glaucium luteum, Lathyrus maritimus, Polygonum maritimum, &c. The seeds or seedvessels of quite half of these species will float for months unharmed in sea-water, but in a few, as with Cakile maritima and Eryngium maritimum, they float for only a week or two, whilst in others again like Glaucium luteum they have no buoyancy. (Some details of the buoyancy experiments on these plants are given in Note 18; and the long list in Note 10 may be first consulted.)

It is not necessary to enter here into more detail with respect to British shore-plants. Enough has been said to disclose cleavage-lines in what might have appeared as a homogeneous plant-formation. We can thus discern the elements of at least three groups amongst the plants of our beaches, each group bearing the impress of an independent history:—

(_a_) The plants of the beach and of the inland plain or of the distant mountain peak, excluding those of the salt marshes. Armeria vulgaris, Silene maritima, and Spergularia rubra may be taken as examples. The currents here as a rule take little or no part in their dispersal.

(_b_) The “saline” group, including the plants of the saline plains and the salt marshes of the interior of continents. Of these Glaux maritima, Salsola kali, and Triglochin maritimum are examples. The capacity of germinating in sea-water is a distinguishing character of most of the plants; and but few of them possess seeds or seedvessels that are markedly buoyant.

(_c_) The true beach plants that rarely stray far from the beach, of which Arenaria peploides, Cakile maritima, and Convolvulus soldanella are examples. Many of them have buoyant seeds or fruits capable of dispersion over wide areas through the agency of the currents.

The reader will be able to extend this subject for himself if he is so inclined, but we have gone far enough together to learn that the plants with buoyant seeds or fruits are in the minority on our beaches, scarcely a third of the total being fitted for dispersal by the currents over broad tracts of sea. The British strand-flora thus differs strikingly from the littoral flora of a Pacific island, or indeed of any ordinary tropical coast, and in this respect it is to be regarded as typical of the temperate regions. It has been remarked before that on a beach in the tropics we would expect to find that quite three-fourths of the plants are provided with buoyant fruits or seeds distributed far and wide over the tropical seas by the currents.

We pass on now to briefly discuss from the same standpoint the British plants that find their homes on the borders of rivers and ponds. It is here that the hygrophytes with buoyant seeds or fruits gather together, just as the xerophytes with similar seeds or fruits collect on the beaches. We have seen before that only a portion of the beach plants belong to the buoyant group, and the same applies to the plants at the edges of rivers and ponds. The plant-formation is no more homogeneous there than it is in the case of the strand-flora. Let us see if we can discern some lines of division there also, or in other words let us endeavour to connect the absence or presence of floating power in the fruits and seeds with some variations in the placing of the plants. We still pursue the clue to the study of the complicated problems connected with plant-stations by taking the floating seed as our guide.

We will carry ourselves in thought to the Thames-side between Teddington and Twickenham at the end of August, 1892. The river is at the high-water level, and we see flourishing at the margins, sometimes a little above the water and sometimes a little within its reach, Ranunculus repens, R. sceleratus, Spiræa ulmaria, Lycopus europæus, Scutellaria galericulata, different species of Rumex, Alnus glutinosa, Iris pseudacorus, Sparganium ramosum, and different species of Carex, with several other plants, all contributing their seeds or fruits to the drift that floats in the river from the autumn to the spring.

But besides these plants there are a number more or less submerged in the stream, including Nasturtium amphibium, N. sylvestre, Stellaria aquatica, Myosotis palustris, and Veronica beccabunga; and as the water falls other plants still more submerged come into view on the exposed flats, such as Nasturtium officinale, Apium nodiflorum, and Polygonum hydropiper. None of these plants are represented by their seeds or fruits in the floating river-drift. Several of them possess dry dehiscent fruits with small seeds, such as Nature ignores in the matter of buoyancy, and the small fruits of Myosotis, Apium, and Polygonum have little or no floating power.

We have thus here a clear dividing line between the plants with buoyant seeds or fruits that were more or less exposed above the high-water level, and those that were more or less submerged at that state of the tide. That which occurs in the Lower Thames twice in the day within the reach of the tide represents what happens in the higher part of the river during the seasonal floods, but in the last case the effects cannot be so readily distinguished. We thus perceive that the buoyant seed or fruit is as a rule only characteristic of the plants of the river-side that grow more or less exposed above the water, whilst those plants liable to periodic submergence have seeds or fruits that sink.

In this connection it is of especial interest to observe that as a general rule the truly aquatic plants of English rivers contribute little or nothing to the floating seed-drift. I pointed this out several years ago, in my paper on the Thames, as an agent in plant-dispersal, and it has been already noticed in this work (page 30). We look in vain amongst the floating winter drift of our rivers for the seeds or fruits of Ranunculus aquatilis, Nuphar luteum, Nymphæa alba, and of the species of Myriophyllum, Limnanthemum, Callitriche, Ceratophyllum, Zannichellia, and of several of the Potamogetons, all of which give character in summer to the aquatic vegetation of the river. In their place we find only the seeds and fruits of the plants growing on the banks.

There is, however, another small group of river plants, which in their structure and habits and in the behaviour of their floating fruits come between the true aquatics and the plants of the river-banks. They belong mostly to the Alisma family, and Alisma plantago and Sagittaria sagittifolia may here be specially mentioned. Their fruits display great variation in their floating power; and on this point M. Kolpin-Ravn, writing to me in 1895, made the following interesting suggestion, that since these plants approach true aquatics in structure they may be also regarded as approaching them in the inconstancy of the buoyant capacity of their fruits, those of aquatics having typically little or no floating power.

Seed-buoyancy, however, does not play quite such an important part in the plant-economy of a river as the examination of the floating drift would lead one to expect. Only a portion of the bank-plants have buoyant seeds or fruits, whilst amongst the true aquatics, the semi-aquatics, and the plants periodically submerged, the rule of non-buoyancy prevails. And, indeed, when we look at all the possible stations for the plants of the British flora, we discover that seed-buoyancy can rarely be connected with station. It is, however, in those few stations that plants with buoyant seeds have mainly gathered. There it is, probably, that the remnants of a past floral age find a refuge, since it would seem likely that the tendency has been in the course of geological time for the development of dry stations for plants at the expense of the wet stations.

The following is a summary of some of the points discussed in this chapter:—

(1) In the case of the strand-flora of a Pacific island, and indeed in that of an ordinary tropical region, the large proportion of plants with buoyant seeds or fruits tends to mask all other issues, and we are seemingly only concerned with dispersal by currents.

(2) But in the British strand-flora where plants with buoyant seeds and fruits are in a minority, constituting less than a third of the total, it is seen that the issue is primarily an affair of station, an inference that may be applied generally to temperate regions.

(3) All British shore-plants may be regarded as owning certain characters in common which may be collectively designated the xerophilous habit, and we may extend this view to other temperate strand-floras.

(4) But this xerophilous habit is also characteristic of inland plants in certain localities, as of those of the steppe, the desert, the rocky mountain-top, and of other exposed situations, in all of which checks to the loss of water by transpiration are required. Whilst the risks of drought are thus guarded against in the case of plants stationed in arid localities, the risk of injury to the plant from the accumulation of salt in the tissues is obviated in the instance of the plants of the coast.

(5) On the other side we have the hygrophilous habit characteristic of plants living under conditions where checks to transpiration are relatively little needed. All the plants of the margins of rivers and ponds belong here, and indeed all plants living under moist conditions.

(6) This distinction between the xerophilous and hygrophilous habits penetrates deeply into all questions connected with stations, and lies behind all matters relating to the buoyancy of seeds or fruits. It is the fitness or unfitness of a plant for living in dry situations that primarily determines the station. If a xerophilous plant has a buoyant seed or seedvessel it finds its way ultimately to the coast; if it is hygrophilous and its seeds or fruits can float, then it is finally established on the side of a pond or river.

(7) The composite character of the British strand-flora is to be explained on the above principles. We have in the first place the plants confined to the sandy beach, many of which possessing buoyant seeds or fruits are dispersed by the currents. Next come the plants of the sandy beach which are found also far inland in open plains and on mountain-tops; and afterwards come the plants of the salt-marsh and mud-flats of the coast, which appear again in the saline plains and swamps in the interior of the continents.

(8) The plant-formation of the river’s border displays also lines of division, and is by no means homogeneous; and indeed other factors besides those connected with seed-buoyancy have here been in operation.

(9) In only a few of the possible stations of British plants can a direct connection be traced with seed-buoyancy. Yet it is at these few stations, such as at the coast and by the pond or river, that the plants with buoyant seeds and fruits have mainly gathered.

(10) The plants now frequenting wet stations may often be regarded as the remains of an age when moist conditions for plant-life prevailed.