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

by H. B. Guppy

(a) Those of the Pacific and Atlantic coasts of tropical America (including the West Indies) and of the West Coast of Africa. They include mostly plants of the mangrove-swamps and their vicinity, such as Anona paludosa, Avicennia tomentosa, A. nitida, Conocarpus erectus, Laguncularia racemosa, Rhizophora mangle, etc.

(b) Those of the Old World excluding the African West Coast and extending from the East Coast of Africa eastward to the Pacific islands. This is much the largest group and comprises many of the plants named in the list given in [Note 35] under Old World species. One may cite as examples of plants ranging almost all over this area, Barringtonia speciosa, B. racemosa, Bruguiera gymnorhiza (in its most comprehensive sense), Carapa moluccensis, Derris uliginosa, Guettarda speciosa, Hernandia peltata, Heritiera littoralis, Pemphis acidula, Rhizophora mucronata, etc. Plants of the mangrove-swamp and of the beach are, therefore, here included.

(c) Those occurring all around the tropics and including many of the plants mentioned under [Note 35] as Pacific island shore-plants found also in America. Most of them belong to the Leguminosæ, and there may here be mentioned Canavalia obtusifolia, Cæsalpinia Bonducella, Entada scandens, Gyrocarpus jacquini, Ipomœa pes capræ, Sophora tomentosa, and Vigna lutea.

(d) Those confined to a portion of the two great regions, such as Nipa fruticans in the Old World, and the Manchineel (Hippomane mancinella) to tropical America.

It is to be noted that the ubiquitous species do not include any of the mangroves. Each of the two regions has its own species, none being common to both the American and Asiatic regions, although, as is shown in [Chapter XXX.], the American species of Rhizophora is now seemingly breaking its bounds and intruding into the Pacific islands. On the other hand, some of the mangrove genera, Avicennia, Carapa, and Rhizophora, are found all round the globe, whilst others are restricted to one or other of the two regions, Bruguiera, Lumnitzera, and Sonneratia, for instance, to the Old World region, and Laguncularia to the American and West African region.

For convenience we may designate the two great regions of tropical strand-plants, with buoyant seeds or fruits, the American and the Asiatic regions, remembering that the first includes both coasts of America as well as the African West Coast, whilst the second extends from the East Coast of Africa to Polynesia. Excluding the ubiquitous species, these two regions are well distinguished from each other. If we look at the chart of the currents we perceive the reason of the American region including the West African Coast, and we see why none of the indigenous plants of this region occur on the African East Coast. So also with the Asiatic region, a glance at the chart will show that all the portions of its area are in connection with each other directly or indirectly through the currents, and that only time is required for the transport of buoyant seeds over most of the region.

Hitherto I have mainly followed Professor Schimper in this matter; but since my visit to Ecuador and the Panama Isthmus some further considerations have presented themselves to me. If the reader will look again at the map of the currents, he will observe that there is little reason for supposing that the Asiatic region can lend its littoral plants to the American region. On the other hand there are greater facilities, as far as currents are concerned, for America supplying the Asiatic region, namely by means of the great equatorial currents that course westward across the Pacific to the tropics of the Old World.

It would therefore seem that the American region can receive nothing by the currents from the Asiatic region. If accordingly it gives but gets nothing back, we are compelled to assign an origin in the American region to all littoral plants dispersed by the currents that are found in the tropics around the globe. This is what we have already regarded on other grounds as possible for nearly all the littoral plants of the tropical Pacific with buoyant seeds or seedvessels that are found in America. These plants are practically the same as those distributed around the tropical zone which are enumerated in the list given under [Note 35], b. With their home in America, by crossing the Pacific they would ultimately arrive at the East African coast, where their course westward would terminate; whilst commencing their journey from the east side of the American continent they would reach the West African coast; and their distribution around the tropics of the world would be explained. There follow from these considerations the corollaries that a tropical strand-plant dispersed by the currents which has its birthplace in Asia could never reach the American region, and that American strand-plants are for the most part native-born, excepting those, if there are any, that hail originally from the African West Coast.

It is necessary in passing to explain the similarity of shore plants on the Pacific and Atlantic coasts of Tropical America. For the mangroves and their accompanying plants inter-communication between the two coasts is now impossible; and a communication between the two oceans must be postulated within the lives of the existing species. For the plants like Entada scandens and Ipomœa pes capræ, which occur inland as well as at the coast, it is easy to show that in the case of the Panama Isthmus, their seeds could be readily carried into the Atlantic and Pacific Oceans by rivers draining the opposite slopes of the same “divide,” so that the dispersal of the same species from a common centre into two oceans may be seen in operation in our own day. My observations on this subject are given in [Chapter XXXII.], to which the reader is referred.

I have now gone far enough to indicate the place that America holds with regard to the distribution of tropical shore-plants dispersed by the currents and with regard to the currents. There is every probability, as I venture to think I have shown, that the Pacific islands have derived most of their ubiquitous shore-plants with buoyant seeds or fruits from America. But one of the results of our discussion of America in this double aspect was that excepting in the case of the African West Coast it gives but does not receive plants from the Old World. We apply this test, with perhaps a little hesitation, to the shore-plants of the Pacific islands that are dispersed by the currents; and we find, as will be seen below, that it is responded to in a remarkable manner.