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

by H. B. Guppy

The consideration of the occurrence of these plants in other tropical or subtropical oceanic islands need not detain us long, since, with the exception of the solitary Lobelia scævolifolia of St. Helena, they seem rarely to be found. This species, which is endemic, is a shrub growing on the upper slopes and summit of the island at elevations of 2,000 to 2,700 feet (Introd. Bot. Chall. Exped., p. 40, and Part ii. pp. 54, 76).

There are two herbaceous species of Lobelia in Juan Fernandez, of which one only, according to Hemsley, could be regarded as indigenous. This is a showy Chilian and Peruvian species (Lobelia tupa) noticed by Bertero as very common in 1829 (Bot. Chall. Exped., Part iii.). Since, however, it would belong to the present age of plant-dispersal in the Pacific, it does not require further mention here; and indeed it would almost appear, when we bear in mind the geographical position and the history of this island since its discovery in 1563, that even as a truly indigenous plant it is not above suspicion. Lobelias of this type are now amongst the commonest plants of the coast regions of northern Chile, where I noticed some as much as 9 or 10 feet high.

On the Capacities of Dispersal of the Lobeliaceæ of the Pacific.—Of actual observations, with the exception of the instance of birds pecking at the capsules of our garden Lobelias, I have come upon few that bear directly on this point. When writing of the flora of the Kermadec Group, many years ago, Sir Joseph Hooker referred (Journ. Linn. Soc. Bot., vol. i.) to the minute seeds of Lobelia as not adapted for transport unless their minuteness and number fit them for it; but since he associates in this connection the tiny seeds of Metrosideros, which is now represented by a species found all over the Pacific, it would seem that the difficulty in the case of Lobelia is not connected so much with the nature as with the suspension of these means of distribution during the later stages of the plant-stocking of the oceanic islands of the tropical Pacific. It will be gathered from the following remarks that the descendants of the early Pacific Lobeliaceæ are probably as well fitted for dispersal as their ancestors, and that the break in the communication is the ultimate subject for inquiry.

The fruits of the Hawaiian endemic genera are in four out of five cases baccate, with usually fleshy or pulpy contents. Such berries, which are generally yellow, but sometimes bluish in colour, vary in size from about half an inch in Rollandia and Delissea to an inch in Cyanea, and not infrequently to more than an inch in Clermontia. The fruits of Lobelia and Brighamia are capsular and dehiscent. With regard to the two genera of the Society Islands and Rarotonga, the fruits of Sclerotheca are hard-walled capsules, opening by two pores; whilst those of Apetahia are seemingly dry and indehiscent. I do not imagine, therefore, that the character of the fruit has determined to any important degree the distribution of these plants.

Nor is there reason to suppose that the fruits have acquired their baccate character in Hawaii, and that they were originally dry and capsular. Both types of fruit are found among the arborescent Lobeliaceæ of America, with which the Hawaiian genera have their affinities. Centropogon, for instance, which occurs in Central America and in the warm parts of North and South America, has, according to Baillon, a somewhat fleshy berry. It is noteworthy that a similar question is raised with respect to Cyrtandra as to the relation between fleshy fruits in the Pacific islands and dry or capsular fruits in the continental home of the genus (see [Chapter XXV.]).

The berries of the Tree-Lobelias would attract birds. We learn from Mr. Perkins that one of the Hawaiian Drepanids, the Ou, is very partial to the berries of some of the Tree-Lobelias and especially those of Clermontia, the seeds passing unharmed in the droppings. The mode of dispersal of the seeds of the dry-capsular fruits is not so apparent; but the fruits could scarcely be less inviting to birds than the dry capsules of Metrosideros, the small seeds of which have in some way or other been carried to almost every island-group of the Pacific. I have beside me the dark brown, smooth crustaceous seeds of a species of Clermontia. They measure ¹⁄₄₂ of an inch or 0·6 of a millimetre, and about 500 go to a grain. Mr. Wallace, in his book on Darwinism, advocates the paramount influence of winds over birds for carrying small seeds, like those of Orchis and Sagina, over tracts of ocean a thousand miles across. I am, however, not inclined to think that, except as regards the spores of cryptogams, winds have done very much for Hawaii. For small seeds we can appeal not only to the agency of birds and bats but also to insects (see [Chapter XXXIII.]).

Observations of this kind, however, merely indicate that these early Lobeliaceæ possessed the same capacities for dispersal that in the succeeding stages of the plant-stocking of the Pacific islands have belonged to Metrosideros, Cyrtandra, Ophiorrhiza, Freycinetia, and many other small-seeded genera. They go no way to explain why the same agencies which transported the minute seeds in a later age could not have been available for continuing the dispersal of the early Lobeliaceæ. To find an explanation we are compelled to go behind the mere capacities for dispersal and to appeal to the general laws of distribution in so far as our facts enable us to interpret them.

We have seen that the two principal components of the early Pacific flora, the Compositæ and the Lobeliaceæ, have American affinities. The plants of the later ages are mainly Old World in their connections. Though containing often endemic species in the various groups, the genera occur also outside each group. The stream of migration that came from America during the early age of the Compositæ and the Lobeliaceæ, when the islands of the Western Pacific were more or less submerged, was during the later ages (after these islands had re-emerged) suspended or diverted, giving place to a stream that brought plants in numbers from tropical Asia, Malaya, and Australia. The general dispersion of the Compositæ and Lobeliaceæ took place during the Tertiary submergence of the islands of the Western Pacific, including the island-groups of Fiji, Samoa, and Tonga. The migration from the west, mainly Indo-Malayan in character, occurred after the re-emergence of those archipelagoes. Thus we get to understand how genera like those of the early Lobeliaceæ and Cyrtandra, which possess, as regards the minute size of their seeds, closely similar capacities for dispersal, have such different distributions, the first confined to Hawaii and Tahiti and American in their affinities, the second widely spread over the Pacific with its home in Malaya.

We have yet to inquire whether this suspension of the means of transport in the later ages of the Pacific Lobeliaceæ is confined to the tropics or whether it extends to the colder latitudes in the southern hemisphere. The indications of the Lobeliaceæ of the “antarctic flora” go to establish that the dispersal of the order is still, or was very recently, in operation in these high latitudes. It is well illustrated, among other plants, by Lobelia anceps, which is found in extra-tropical South America, Australia and New Zealand, and South Africa. This, indeed, recalls Bentham’s view concerning the Compositæ, that whilst communication was broken off in the tropics, it was kept up in higher latitudes.

Here ends, therefore, our consideration of the Tree-Lobelias of the Pacific islands; but as it is not quite complete without a discussion of the remaining endemic genera of other orders than the Compositæ and Lobeliaceæ which also belong to the same early age of the Pacific floras, I will proceed at once to their consideration.