Norman and Sernander (see p. 172) attribute considerable buoyancy to these fruits on account of the hollow cavity in each. I used to find them in England in floating river-drift in autumn; and Norman observed them on the Scandinavian beaches. They do not, however, float long, as the cavity is open; and in two sets of my experiments they sank within a few days.
NOTE 13 (page [29])
The Buoyancy of the Seeds of Convolvulus sepium
This plant seeded freely in 1893 in the Lower Thames Valley, as at Molesey. I kept some of the seeds afloat for thirty-three months, of which the first nine months were spent in sea-water and the rest in fresh-water. One seed, at the end of the period, germinated healthily in the fresh-water.
NOTE 14 (page [26])
Other Long Flotation Experiments
Whilst keeping my collections of Thames seed-drift in water from year to year, I obtained a number of records of long “flotations.” Thus in several cases, as with Bidens cernua and different species of Carex, germination of the floating fruit took place in the water after a period of two years. The same is also true of the seeds of Iris pseudacorus and of the drupes of Sparganium ramosum. The last-named remained afloat in the vessels, with the seed still sound, after four years; and the fruits of Carex paludosa germinated afloat after three years in water. Many drift fruits and seeds did not germinate freely in the vessels until the second spring, that is, after a lapse of eighteen months; and in those cases where the experiments were still further prolonged, a few germinated in the vessels in the third and sometimes even in the fourth year.
NOTE 15 (pages [33], [280])
The Occurrence Inland of Silene maritima
Prof. Schimper appeared to be in doubt as to the inclusion of this littoral plant amongst those found in elevated mountain districts. However, an interesting note on the occurrence of this plant on the summit of one of the inland Norwegian mountains is given by Sernander (p. 405), and is referred to by me on page [280] of this work.
NOTE 16 (page [34])
The Buoyancy of the Seeds or Fruits of the British Beach-plants that also occur Inland
My experiments in the case of Armeria vulgaris, Artemisia, Cochlearia officinalis, Plantago, the maritime forms of Spergularia rubra with and without winged seeds, and Silene maritima disclose little or no floating capacity even after prolonged drying. Thuret obtained similar results for the Spergularia. It is unlikely that other plants of the group possess any floating power worth speaking of. As indicated in [Note 71], the fruits of Raphanus maritimus float only for 7 to 10 days.
Nature disperses the fruits of Armeria vulgaris inclosed in the persistent calyx; but in this condition they float only for 2 to 4 days in sea-water, and the buoyancy of the capsule and seed is still more limited. They are sufficiently light to be blown some distance by strong winds, and the stiff hairs would cause them to adhere to a bird’s plumage in the case of gulls nesting where the plants grow.