The nature of the sorting-process is especially well shown in some of the families, as for instance with the Labiatæ. Let the reader put on one side the four species with buoyant fruits, namely, Lycopus europæus, Mentha aquatica, Scutellaria galericulata, and Stachys palustris, and on the other side all the species with non-buoyant fruits, such as Salvia verbenaca, Thymus sp., Calamintha officinalis, Nepeta glechoma, N. cataria, Prunella vulgaris, Stachys arvensis, S. betonica, S. sylvatica, Galeopsis tetrahit, Ballota nigra, Lamium purpureum, L. album, Teucrium scorodonia, and Ajuga reptans, and he will at once perceive that he has separated the regular water-side plants from those growing in drier stations.
If he does the same with the Umbelliferæ he will find that when he is separating Hydrocotyle vulgaris, Cicuta virosa, Œnanthe crocata, and Angelica sylvestris from Æthusa cynapium, Pastinaca sativa, and Chærophyllum sylvestre, on account of their buoyant fruits, he is also distinguishing them on account of their stations. On the other hand, there are apparently weighty exceptions to this rule in the non-buoyancy of the fruits of the three British species of Apium (graveolens, nodiflorum, inundatum), which grow in streams and marshes. Or, again, if we look at the sea-coast representatives of the family, we find that whilst the fruits of the Samphire (Crithmum maritimum) float buoyantly for months, those of Eryngium maritimum seemingly set the law at defiance, and all sink in less than a week or ten days, even after months of drying. To regard these as exceptions, however, is to miss the essential point of the principle concerned. It is not thereby implied that all water-side plants, whether by the sea or by the river or by the pond, have buoyant fruits or seeds, but that nearly all plants with such fruits or seeds have been gathered at the water-side. It will be shown in the next chapter that several other influences go to determine the station of a plant on a beach or by a river. This is true of the Compositæ, which, if we except our two species of Bidens (cernua and tripartita), come under the play of other determining causes, as indicated by the little or no buoyancy displayed by the fruits of Aster tripolium, Senecio aquaticus, and Carduus palustris.
Within the limits of a genus we can, however, point to other examples of this principle. Take, for instance, Convolvulus arvensis, the common weed of our fields. Its seeds, whether fresh or dried for months, have no buoyancy. On the other hand, those of Convolvulus soldanella float unharmed in sea-water for half a year and more. Its seeds have come frequently under my notice among the stranded drift of the Devonshire beaches, and also on the coasts of Chile; whilst Sernander includes them amongst the drift of the beaches on the Norwegian coasts. It is remarkable that Convolvulus sepium, which accompanies C. soldanella over much of its great range, has seeds that are sometimes able to float unharmed for long periods, even for years (Notes [13], [41], [49]). Though not strictly a water-side plant, it grows commonly over other plants on the banks of the Thames; and when it fruits its seeds occur typically in the floating drift of that river. According to Gray, it is almost a river-side plant in the United States, where it is found “especially on the moist banks of streams.” Not all the seeds of C. sepium, however, are buoyant; and in its varying behaviour in this respect it resembles the inland species of Ipomœa, which are referred to in the previous chapter.
The British species of Euphorbia also seem to behave in accordance with the principle that when a genus has littoral and inland species, the first-named alone possesses buoyant fruits or seeds. Thus, whilst the sound fruits of E. helioscopia and of another species found commonly as a garden weed are non-buoyant, those of E. paralias, the familiar beach-plant, float for several weeks, and are to be noticed among the stranded drift of the coasts frequented by this plant. (See [Note 90] for later results.)
The structural characters connected with the buoyancy of the seeds or seedvessels of some of the British plants are dealt with in [Chapter XII.] Here it may be remarked that this capacity is often associated, as with the Pacific island plants, with a “buoyant” tissue, that is either absent or less developed in the case of the non-buoyant group.
Enough has now been said to show in a general fashion how Nature through the agency of buoyant seeds and fruits has affected the stations of plants of the British flora. Allowing this line of inquiry to develop itself as the work proceeds, we will here pause and close the chapter with a reference to some of the principal points that have been brought into prominence.
(a) The proportion of flowering plants of the British flora that possess buoyant seeds or seedvessels is very small, probably not more than 10 per cent.
(b) In so far as buoyancy is concerned, Nature has for the most part ignored the dehiscent fruit with small seeds, such as we see in the Cruciferæ and the Caryophyllaceæ, and has chiefly endowed with floating power the dry indehiscent fruit, such as we see in the Umbelliferæ and in the Labiatæ.
(c) In the great sorting-process that has been in operation through the ages, nearly all the plants with buoyant seeds or seedvessels have been located at the water-side, principally by ponds and rivers, but also on the sea-beach. On the other hand, the great majority of the plants with seeds or seedvessels that sink have found a home in drier stations.
(d) The character of the operation is well displayed in certain genera possessing species of the water-side and species of drier situations, and in the case of genera having both coast and inland species. In both instances the species by the water-side possesses buoyant seeds or fruits, whilst that of the station in a drier locality or removed from the coast has seeds or fruits that sink.