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

by H. B. Guppy

When on Keeling Atoll in the Indian Ocean I collected, amongst the stranded seed-drift brought by the currents to those islands, the seeds of five or six species of Mucuna, two of which were identified at Kew as M. macrocarpa, Wall., and M. gigantea D.C. (see my paper on the dispersal of plants at Keeling Atoll). No plant of this genus appears up to that time to have been recorded from the Keeling Islands, so that at all events most if not all of the seeds had been brought by the currents from the Indian Archipelago, some 700 miles away. It may be added that amongst the drift gathered by me on the south coast of Java the seeds of three species of Mucuna were identified at Kew, including the two above-named species from Keeling Atoll.

These current-borne seeds of the Keeling beaches had probably performed an ocean journey of a thousand miles, since the route could scarcely have been direct. Yet their behaviour when placed eighteen months after in sea-water in a hothouse in England was most erratic. Of three seeds of Mucuna gigantea all swelled and sank within eight days. Two seeds of M. macrocarpa sank after floating from sixty to a hundred days; whilst of two seeds of another species both remained afloat after a year. In a sea-water experiment in England on five Hawaiian seeds of M. gigantea, under the conditions referred to in the Mucuna urens experiment, one sank within ten days, whilst three of them were afloat after twelve months, one of them subsequently germinating. This species, it may be remarked, is widely distributed as a coast plant over tropical Asia, Australia, and in Polynesia. It seems to take the place in the Old World which Mucuna urens takes in America, and it is curious that they meet in Polynesia, being sometimes associated as in Hawaii. In the chapter on my observations in Ecuador and in Panama it is remarked that Mucuna seeds are frequent constituents of river, sea, and stranded drift. I, therefore, have enjoyed the opportunity of observing the behaviour of the seeds of this genus in a variety of localities, namely, in the Keeling Islands, in West Java, in Fiji, Hawaii, and tropical America; and this may be pleaded as an excuse for entering into so much detail respecting them.

The large seeds of Strongylodon lucidum (S. ruber), a Leguminous liane that ranks with the species of Mucuna amongst the huge climbers of the forest of the Pacific islands, behaved in a similar way in my flotation experiments in sea-water. Though, as shown in [Note 3], these seeds can float for a year and retain their germinating power, some of them brought their buoyant capacity prematurely to an end by an abortive attempt at germination. These black rounded seeds form a common object amongst the river seed-drift stranded on some of the Fijian beaches in the vicinity of estuaries. They are so hard and durable that they are mounted in brooches in Honolulu. Yet these pebble-like seeds will sometimes begin to swell in a few days in sea-water. Out of five seeds placed in sea-water in England under warm conditions (the water temperature for the first few weeks ranging between 75° and 90° F.), one swelled and sank within ten days, another did so after two months, whilst the other three were afloat after twelve months, and one of them subsequently germinated. There is some disagreement amongst botanists as to the limits of the specific characters of the plants of this genus (see [Note 39]); but the plan seemingly most in accord with the fundamental principles regulating plant-distribution in this region of the Pacific is to regard the forms found in Hawaii, Tahiti, and Fiji, as referable to one species. In addition to the Polynesian forms there are only two or three species, found in the Philippines, Madagascar, and Ceylon, and it is with the species from the last-named locality that the Polynesian species is by some identified.

The seeds of several other Leguminous climbers would probably act in a similar way, for instance, those of Entada scandens; but the seeds of this plant experimented on by me were too few to enable an opinion to be formed. Of four seeds of Dioclea violacea from Fiji that were subjected to the same experiment as those of Strongylodon lucidum, all floated in sea-water after a year, with the exception of one that did not swell and sink until after ten months. On the other hand, in my experiment in Fiji on the fresh seeds of Canavalia obtusifolia, a plant found on tropical beaches all round the globe, seventy per cent. sank in the first six or seven weeks, swelling and displaying the first signs of germination, but quite ten per cent. were afloat after three months.

My experiments on the foregoing and other littoral species of the Leguminosæ merely indicate that under the ordinary temperature of tropical currents a portion of the seeds will probably sink owing to abortive attempts at germination. It is likely that if in the experiments in England a constant temperature of 85° to 90° F. had been sustained throughout, most if not all of the seeds would have swelled and sunk within a month or two. The temperature of the experiments in Fiji and Hawaii did not exceed that of many tropical currents; but there are areas of superheating in equatorial seas, which I think would prove insurmountable barriers in the path of most drifting Leguminous seeds, a subject to which further reference will be made.

Coming to the Convolvulaceæ, my experiments show that the buoyant seeds often lose their floating powers from the same cause. Those of Ipomœa pes capræ may be taken as an example. I was surprised to find when experimenting on the buoyancy in sea-water of these seeds in Fiji and Hawaii that a considerable proportion, about a third, sank in the first two months, swelling and sinking to the bottom. That this swelling represented the early stage of germination was well brought out in parallel experiments in fresh water and sea-water made in England on the buoyant seeds of the British littoral species, Convolvulus soldanella. A good proportion of the seeds in the first part of the experiment absorbed water, swelled, and sank, those in fresh water proceeding at once to germinate healthily at the bottom, whilst those that sank in sea-water merely decayed. Of the survivors about fifty per cent. in either case floated after six months. It may be added that the seeds of other tropical littoral species, such as those of Ipomœa glaberrima and I. grandiflora, behaved in the same way.

It would appear from my experiments, and it is a result that we should expect, that buoyant seeds of the Leguminosæ and Convolvulaceæ would often float for much longer periods under cool than under warm conditions. There must be areas of high temperature in mid-ocean that would prove much more fatal to the chances of a drifting tropical seed than the icy waters of a Polar current. In my paper on Keeling Atoll I have described how I procured the germination of a seed of Ipomœa grandiflora, Lam., after a year’s flotation in sea-water in London, which included a period of three weeks when the water temperature was at or about 32° F. These seeds from this point of view would be exposed to much more risk of sinking through abortive attempts at germination when drifting across some parts of the Pacific Ocean. It would appear from the Admiralty Chart of Surface-Temperatures, published in 1884, that such an area with a surface-temperature of 83° to 86° throughout the year extends north and east of New Guinea well into the Pacific, reaching in the first half of the year as far east as the Tahitian region. It would seem highly probable that the immersion of Leguminous or Convolvulaceous seeds for many months in these tepid waters would in most if not in all cases induce incipient germination which would lead to the sinking of the seed. There are, however, exceptional cases, as that of Cæsalpinia bonducella, which, as my experiments recorded in [Chapter XVII.] indicate, appear to be quite proof against any conditions of temperature such as are likely to be found in tropical seas in the present day.

There are a few general considerations arising out of the foregoing observations to which reference may now be made. The study of the behaviour of the floating seed or fruit often carries us, as I have before implied, to the borderland of vivipary. When from a canoe on a Fijian river we lift up the germinating fruit of Barringtonia racemosa from amongst the drift floating past in the stream and pull down from the branches overhead the seedling a foot in length of Rhizophora, we hold in our hands the two extremes of the series of vivipary. With many of the plants of the mangrove-formation there is a fine adjustment with respect to the germinating capacity of the seed, or in other words a delicate balancing of organisation on one side and of physical conditions on the other. A slight disturbance of the equilibrium would produce great results in plant distribution. Thus, an elevation of the temperature of the sea-water in the tropics to 90° F. would, I apprehend, produce the abortive germination of nearly every floating seed and fruit in equatorial seas, even of those of the beach-trees like Barringtonia speciosa and Terminalia littoralis that are regarded as proof against such risks under existing conditions where the surface-temperatures would average 78° to 80°.

There would thus be a barrier to the dispersal of plants by currents as effective as that of a frozen ocean. In the warm, humid climates of the early geological ages, seed-transport by currents may have been often impossible, since the seeds that did not begin to germinate on the plants of the swamps would probably do so in the tepid water of the sea. Viviparous plants would, however, be placed at no greater disadvantage than they are at present, since the genera Rhizophora, Avicennia, and others are now only dispersed by the floating seedlings. But such an increase of temperature at the present time would mean the death in the current of the floating seeds and fruits of nearly all non-viviparous shore-plants. As a rule every Leguminous and Convolvulaceous seed would swell up and go to the bottom; whilst fruits like those of Barringtonia racemosa and Carapa obovata, that often germinate afloat in tropical estuaries, would invariably do so under the changed conditions, and the seedlings not being adapted for ocean transport would perish.

Yet we know that with the seeds of many inland plants temperature has seemingly very little to do with starting the process of germination. We are familiar with the fact that the seeds of many plants that fail to germinate in the summer of their production habitually germinate under apparently less favourable conditions of temperature in the following spring. This is attributed by botanists to the immaturity of the seed on first falling from the plant, a further period of maturation being necessary before, under any conditions, germination is possible.