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

by H. B. Guppy

Sophora.

In this genus, as in Erythrina and Canavalia, we have a littoral species, Sophora tomentosa, that ranges over the tropical beaches of the globe, including most of the islands of the Pacific, but does not occur in Hawaii, where the genus is represented by an endemic inland species, S. chrysophylla. Here also we find the shore-species with seeds capable of floating for months on account of their buoyant kernels, and the inland species with seeds that sink even after years of drying (see [Note 56]). Unless other inland species of Sophora have recently been described from the tropical Pacific, the Hawaiian species is the only one of its kind known from this region.

But the problem wears a different aspect in the case of this genus, since the endemic inland species of Hawaii is a tree of the mountains where a temperate climate prevails, whilst Sophora tomentosa is a shrub of the tropical beach that only at times extends into subtropical latitudes. The Mamani tree, as the Hawaiians name S. chrysophylla, extends up to 9,000 or 10,000 feet above the sea, forming, with Myoporum sandwicense and one or two other trees and shrubs, the highest belt of the forest in the larger islands. It is in the open woodland between 6,000 and 7,000 feet that it is most at home, and here it attains a height of 20 to 30 feet. It descends in places to as low as 2,000 feet above sea-level; but here is living under uncongenial conditions, and, like Myoporum sandwicense, becomes dwarfed and shrubby. The climatic conditions under which S. chrysophylla thrives in the Hawaiian mountains are therefore those of the temperate zone. From the data given in [Chapter XIX.], the mean annual temperature at an elevation of 6,000 to 7,000 feet would probably be about 55°, the average temperature of New Zealand.

We must therefore look to the temperate and not to the tropical zone for the home of the parent species of Sophora chrysophylla; and if it was originally derived from a shore-plant dispersed by the currents, the widespread S. tomentosa could scarcely have been the species concerned. But this strand-plant is disqualified for another potent reason, since it belongs to a different section of the genus. Whilst S. tomentosa belongs to the section possessing smooth pods, S. chrysophylla is referred to the section Edwardsia having four-winged pods, which comprises about ten species found in Chile and Peru, Hawaii, New Zealand, Further India, and the Isle of Bourbon. What strange principle in distribution, we may fitly ask, has linked together in this odd fashion the continents of the Old and New World and the islands of the Indian and Pacific oceans?

Yet, discredited as Sophora tomentosa is as a possible parent of the Hawaiian mountain species, it may yet afford us a clue. It is significant that the distribution of this wide-ranging beach-shrub in the tropics of the southern hemisphere is almost coterminous with that of Sophora tetraptera, a species widely spread in the south temperate zone from Chile to New Zealand and extending towards the tropics as far as Juan Fernandez in lat. 33° S. and to Easter Island in lat. 27° S. Though not strictly a beach-plant, S. tetraptera is a plant of the sea-border; and it is remarkable, but not surprising, how in New Zealand, one of its principal homes, its behaviour in respect of its vertical distribution presents a great contrast to that of S. chrysophylla in the tropical latitudes of Hawaii. We have seen that, in Hawaii, S. chrysophylla, which thrives as a tree 20 to 30 feet high in the mountains, becomes shrubby when it descends to the lower levels. In New Zealand, S. tetraptera is, as we learn from Kirk, a prostrate shrub in the mountains, whilst in the lower elevations towards the sea it becomes a tree 30 and even 50 feet in height. It can scarcely be doubted that, if we exchanged the habitats of these Hawaiian and New Zealand species, each would to a great extent take up the other’s station and the other’s habit.

The whole problem of the dispersal of Sophora was brought immediately to my notice at Corral, in latitude 40° S. on the coast of Chile. Here a small tree of the section Edwardsia was growing in fruit on the lower slopes of the hills, becoming bushy when descending to the beach. Specimens of its four-winged pods have been identified at the Kew Museum as those of Sophora tetraptera; and, as far as the pod is concerned, I cannot distinguish between my specimens of the Hawaiian S. chrysophylla and the Chilian species. Subsequently I found the buoyant seeds of the same plant amongst the stranded beach-drift at Bahia San Vincente, nearly 200 miles further north. This led to my experimenting on the capacity of the plant for dispersal by the currents, and as a result it was ascertained (see [Note 56]) that whilst, as in the case of S. chrysophylla, the pods floated only one or two weeks, the seeds on account of their buoyant kernels floated for several months in sea-water, retaining their power of germination. The Chilian plant thus differs significantly in its capacity for dispersal by currents from the Hawaiian species, the seeds of which sink in sea-water even after years of drying.

The Mamani tree in Hawaii had always been an object of great interest to me. I was attracted by the mystery surrounding its origin and had long suspected that the clue was to be found in the non-buoyancy of its seeds and in the absence of a littoral species of the genus. When in Fiji it was to the littoral Sophora tomentosa that I looked in vain for a solution of the riddle, and seven years afterwards on the coast of Chile a solution of this enigma of the Hawaiian mountains presented itself in the form of an argument somewhat in this shape.

On account of the elevated station of the Mamani tree (S. chrysophylla) in Hawaii it is to be inferred that the original species was a plant of the temperate regions or of the uplands of some tropical mountains. If it has had its origin in some shore-plant dispersed by the currents, that species can only now be found on the coasts of extra-tropical regions. Such a maritime plant had buoyant seeds; and plants of this type are presented by Sophora tetraptera and its allied species that are at home in the cool latitudes of the southern hemisphere, as in Chile and New Zealand. No difficulty, as I argued, could be connected with the loss of buoyancy of the seeds of the Hawaiian mountain species, since it follows the general principle (laid down in [Chapter II.]) that in the same genus coast species have buoyant seeds or fruits, and inland species those that sink; and in support of this view it was recalled that this is what happens to the seeds of Cæsalpinia bonducella and Afzelia bijuga when the plants extend inland in the Pacific islands. It was held, in short, that the original form of Sophora chrysophylla in Hawaii was a coast plant with buoyant seeds, and therefore indebted for its presence to the currents. Hailing from an extra-tropical region, it abandoned the beach and found suitable conditions of existence in the mountains, where it underwent specific differentiation. Such was the explanation that presented itself to me on a Chilian beach.

The first objection that offers itself against this view is that Sophora chrysophylla is one of several species characterising the antarctic element of the mountain flora of Hawaii, and that many of these plants, such as those of the genera Astelia, Coprosma, Gunnera, Myoporum, &c., could only have reached these islands through the agency of frugivorous birds (see [Chapter XXIII.]). There is, therefore, something to be said for this mode of dispersal; but though one can understand how hard seeds and the “stones” and crustaceous pyrenes of fleshy fruits might be transported unharmed in a bird’s stomach half-way across the Pacific Ocean to the distant group of Hawaii, it is difficult to understand how Leguminous seeds, except in such cases as Tephrosia piscatoria, could be ejected unharmed by a bird after an ocean passage of some 1,500 or 2,000 miles. Yet evidence pointing to such a possibility is not lacking. It was pointed out by W. O. Focke (Nat. schaft. Ver. zur Bremen, Abhandl., Band 5, 1876) that for many Leguminosæ we are driven to the agency of birds in order to explain their dispersal. In this connection he mentions the case of a pigeon killed by some beast of prey that he found in his garden in the early winter. In the following spring he noticed numerous seedlings of Vicia faba sprouting up from amongst the feathers that alone remained of the bird. In this observation he detected the normal method of the dispersal of the Leguminosæ by birds, the seeds not being ejected by the bird but being set free by its death. It is well known that Darwin had this idea in his mind when he conducted his experiments on the dispersal of seeds; and reference may here be made to one that is recorded in More Letters of Charles Darwin (i., 436). Out of a number of seeds left in the stomach of an eagle for eighteen hours, the majority were killed; but amongst the few that germinated afterwards was a seed of clover (Trifolium). If such a bird had carried a Sophora seed to Hawaii, this would have involved a continuous flight of, on the average, 100 miles per hour for a period of fifteen to twenty hours. This would just come within the limitations laid down by Gätke as regards length and velocity of flight—a subject discussed in [Chapter XXXIII.]

We will now turn to the Sophora seeds themselves for evidence of their capacity of surviving the perils of such a journey. The seeds of Sophora chrysophylla, which are about a quarter of an inch (6 to 7 mm.) in length, possess unusually hard coverings for the order, and in that respect appear fitted for dispersal by animals. Indeed, in the large island of Hawaii wild pigs and sheep feed on the pods, and no doubt aid in the distribution of the plant over the island through the germination of ejected undigested seeds. But since the species is found on most of the larger islands, it is apparent that to birds we must look for the explanation of its inter-island dispersal. Mr. Wilson, in his Aves Hawaienses, remarks that one of the Hawaiian finches (Loxioides) feeds on the seeds of this tree, which probably, he adds, also serve as the food of Chloridops kona, another big finch; and it is to be inferred from the observations of Mr. Perkins, quoted by Mr. Evans in his book on Birds, that the Drepanididæ, a family peculiar to Hawaii, are in the habit of splitting the pods of trees like Acacia koa and Sophora chrysophylla to obtain the seeds. It would, however, seem that the agency of birds confined to these islands does not carry us very far when we wish to explain the original transport of the seeds over a breadth of ocean of some 1,500 miles and more. Yet we know that this must have happened with some of the Hawaiian plants, such as Osteomeles anthyllidifolia and Nertera depressa, that are not confined to these islands and possess fruits that would attract frugivorous birds. But whether it has occurred with the dry beans of the Hawaiian species of Sophora is another matter.

On the whole I am inclined to the view, bearing in mind the general indications of the Leguminosæ in the Pacific, that S. chrysophylla originally reached Hawaii as a littoral plant through the agency of the currents. Many points still need investigation; but it may be pointed out that South America probably received Sophora tetraptera from New Zealand by the West Wind Drift Current.