(e) Yet it is necessary to remember that the principle involved is not that all water-side plants have buoyant seeds or fruits, but merely that plants thus endowed gather at the water-side. There are many plants with non-buoyant seeds or fruits on our beaches and beside our ponds and rivers.
(f) We have now learned from the British flora that the “locating” of plants with buoyant fruits or seeds on the beaches of the tropical islands of the Pacific, and indeed of tropical regions generally, is but a part of a much wider principle by which plants thus endowed are placed at the water-side, whether by a river or a pond or by the sea.
(g) It is with this distinction between a fresh-water and a salt-water station that we shall be occupied in the next chapter; and it is of great interest, since it leads us to discover that the wider principle is in its turn part of a far larger scheme.
Note.—It must be clearly understood that by water-side plants the true aquatic plants, such as the Water-lilies, the Myriophylls, the Potamogetons, &c., are not implied. It will be seen from the list in [Note 10] that in most cases the seeds or fruits of aquatic plants have little or no floating power. This is true, for instance, of Ranunculus aquatilis, Nymphæa, Nuphar, Myriophyllum, Ceratophyllum, Callitriche, Naias, Zannichellia, Ruppia, and half the Potamogetons.
CHAPTER IV
THE LESSON OF THE BRITISH FLORA (continued)
The choice of station of the water-side plant possessing buoyant seeds or seedvessels.—Determined by its fitness or unfitness for living in physiologically dry stations.—In the internal organisation of a plant lies the first determining influence of station.—The grouping of the British strand-plants.—Whilst the Xerophyte with buoyant seed or fruit finds its station at the coast, the Hygrophyte similarly endowed makes its home at the river or pond side.—The grouping of the plants of the river and the pond.—Summary.
By following up the clue supplied by the floating seed, we have arrived at the conclusion with respect to the British flora that plants with buoyant seeds or fruits gather at the water-side. But we have yet to inquire why some of these plants are “located” at the sea-coast and others on the borders of ponds and rivers. Mere buoyancy aided by chance has not determined the choice. There are definite principles at work in the economy of plant-life that make the selection for each plant.
Rivers in all parts of the world carry to the sea in great abundance the seeds and fruits of the plants that are stationed at their borders; and such seed-drift is found in quantity washed up on the beaches in the vicinity of the estuary. One finds, for instance, on such beaches in the South of England the stranded fruits and seeds of Bidens cernua, Alnus glutinosa, Sparganium ramosum, Iris pseudacorus, &c., mingled with those of true beach plants like Cakile maritima, Convolvulus soldanella, Euphorbia paralias, &c. Yet we would be much surprised if either the Bidens or the Alder or the Sparganium were to establish itself on the sandy beach, even though they have had through the ages innumerable opportunities of doing so. We thus see that mere buoyancy of fruit or seed cannot determine a station on a sea-beach, and that some other factor makes the choice. The nature of this factor I will now endeavour to explain; but in so doing it will be necessary to employ a few technical terms, which it is not easy to dispense with altogether.
It may be doubted whether Professor Schimper could have conferred a greater benefit on the student of plant-distribution than in his clear delineation of the connection between the habit or organisation of a plant and its station. Nature has imposed an important structural distinction between plants that have been endowed with the means of checking excessive transpiration or water-loss in stations where there is risk of drought, as in deserts and in similar arid localities, and those that live in stations where such safeguards are not needed. Hence arises the distinction between Xerophytes on the one hand, and Hygrophytes on the other. This contrast is shown not only in minute structural features, but also, as my readers are aware, in the external characters, as in hairiness, succulency, a leathery cuticle, the occurrence of thorns, and in several other characters of the plants of the steppe and the desert. This important subject is dealt with by Professor Schimper in his recent work on Plant-Geography; but it was from his earlier work on the Indo-Malayan strand-flora that I learned this valuable lesson in plant-distribution.