Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 2 / Plant-Dispersal

by H. B. Guppy

(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.