Some years ago the author made a number of experiments on the buoyancy of the acorns of Quercus robur, and he formed the conclusion that when freshly collected not more than 4 to 8 per cent. of mature fruits will float in fresh-water, and not more than about 10 to 12 per cent. in sea-water, but that in either case they all sink in a day or two. Immature acorns float much longer, and it is these that mostly figure in the drift. However, unlike most fruits of little initial buoyancy the mature fruits gain considerable floating power by drying. Of some that had been kept for seven months 20 per cent. floated after four weeks in sea-water and 15 per cent. after 10 weeks.... It may be added that, according to Thuret, the fruits of Quercus ilex have little or no floating power.
The buoyancy of the fruits of Quercus is due entirely to the cavity left by the shrinking of the kernel. I never remember to have found one with a sound seed amongst the drift in England and Sicily; and I should doubt much whether those in the tropical drift retain their germinating powers. But, apart from this, the genus Quercus finds in its own constitution or habit the greatest obstacle in most species to the adoption of a littoral station. However, there are exceptional tendencies displayed by the evergreen oaks; and this is very significant, since in their xerophilous leaves they possess the preliminary qualification for a station near the sea. Quercus ilex, it is well known, shows a partiality for the sea-air, and Q. virens, the “live oak,” flourishes near the sea in the southern states of America, a maritime variety being distinguished by botanists. One of the willow-oaks of America, Q. phellos, which grows in swampy land, also has a beach variety.
The Hazel-tree (Corylus avellana) must be placed in the same category with Quercus. I found the empty nuts commonly amongst the stranded drift of the Sicilian and English beaches. The fruits were also frequently noticed by Dr. Sernander in the Scandinavian sea-drift; but he says nothing of their empty condition. Mr. Darwin remarks, in the Origin of Species, that he found that fresh hazel-nuts sank, but that after drying a long time they floated for ninety days and subsequently germinated. The floating-power is no doubt due to the cavity arising from the shrinking of the kernel, and it is to this cause that Dr. Sernander attributed the slight initial buoyancy observed by him. However, the hazel, like the common oak, lacks the habit that would fit it for a station by the sea, and, whatever capacity its fruits may possess for dispersal by currents, it is quite useless for the spread of the species.
NOTE 49 (page [131])
On the Distribution of Ipomœa pes capræ, Convolvulus soldanella, and Convolvulus sepium
Whilst Ipomœa pes capræ is cosmopolitan in the tropical zones, Convolvulus soldanella is cosmopolitan in both the north and south temperate zones; but, as might be expected, the two species at times meet and their areas overlap. Thus, according to Mr. Cheeseman (Trans. New Zealand Inst., xx., 1887), they meet in the Kermadec Islands, in the South Pacific, in about latitude 30°. From my observations on the coast of Chile it would seem that C. soldanella in its northward extension fails somewhere between Valparaiso and Coquimbo, that is to say, between 33° and 30° S. lat. Gay merely refers to the plant as existing in North Chile, which in his time would include the coast between 33° and 24° S. lat. It intrudes within the “thirties” on the coast of California and is found in Madeira in about 33° N. lat. Ipomœa pes capræ in its turn extends into subtropical regions, being recorded from the Kermadecs, as above noted, and from the Bermudas in 32° N. lat. Owing probably to special physical conditions of the coast, which are referred to in [Chapter XXXII.], this plant is evidently limited to the tropics on the west coast of South America. It did not come under my notice on the beaches of North Chile, and it is apparently not mentioned by Gay in his work on the Chilian flora.
Convolvulus sepium, the frequent inland associate of the littoral C. soldanella over the temperate regions of the globe, belongs to the same section of the genus (Calystegia). Its extraordinary occurrence by itself in the island of St. Paul, in the Southern Ocean, about fifty yards from the shore (Bot. Chall. Exped., ii., 153, 264), almost suggests that we have here a dimorphic species with a littoral and an inland form; and its existence in the Azores is in this connection very remarkable. It may be here noted that of three plants raised from seeds found in the beach-drift near Palermo two had the foliage of C. sepium and one of C. soldanella. Perhaps one of my readers, in imitation of De Vries with Œnothera, might be able to settle this point by raising some hundreds of seedlings from the seeds of the beach species. It is possible that the relation between these two species of Convolvulus may be in some respects akin to that between Cæsalpinia Bonducella and C. Bonduc, two littoral plants that accompany each other over much of the tropical zone.
The student of dispersal will, however, find some curious gaps in the distribution of Convolvulus soldanella even in the temperate regions; and it will be curious to observe how they affect the distribution of C. sepium. He will have to answer the query of De Candolle:... “Admitting, if one wishes, that the currents have transported this marine species, how comes it that it chances to be in the Pacific and in Europe, without occurring on the east coasts of America and on the east and west coasts of Africa?” (Geographie Botanique, ii., 1056). He will have to explain why some botanists give C. soldanella a habitat in the tropics, as in the Indian region. Schimper, who investigated this point, says that he arrived at no certain result (p. 127). See Notes [13] and [41] and pages [29], [91], for further remarks on these two species of Convolvulus.
NOTE 50 (pages [79], [132])
On the Structure of the Seeds and Fruits of Barringtonia
As regards the fruits and their coverings, the littoral and inland species of Fiji evidently fall into different sections, the first named (B. speciosa and B. racemosa) being distinguished by their outer fibrous husk, to which the buoyancy is due, the last-named (B. edulis and an undescribed species) possessing a hard stone surrounding the seed, and here the fruits sink or float only for limited periods.
The fruits of B. edulis have an outer almost fleshy covering, a little fibrous at the outside, and the hard ligneous “stone,” containing an edible seed, requires a hammer to break it. They float heavily for three or four weeks, whereas those of the littoral species float for many months. In the case of another inland species found by me growing as a small tree 12 feet high on the slopes of Mount Seatura in Vanua Levu at an elevation of 1,000 feet above the sea, the seed was similarly protected by a hard “stone” that could only be broken with an axe, and the fruit was non-buoyant, with thin and perishable outer coats.