CHAPTER VIITHE HAWAIIAN STRAND-FLORA
Its poverty.—Its negative features.—Their explanation.—The subordinate part taken by the currents.—The Oregon drift.—The inland extension of the beach plants.—Summary.
Comparedwith the rich strand-flora of Fiji, that of Hawaii presents but a sorry aspect. In the number of species (30) it does not amount to half; whilst it lacks the great mangrove-formation and the luxuriant vegetation accompanying it that gives so much character to the shores and estuaries of Fiji. Strangely enough, it is also deprived of most of the familiar trees that, whether in foliage, in flower, or in fruit, form the chief attraction of the sandy beaches of the Pacific islands.
Neither the mangroves, therefore, nor the plants of the intermediate formation, are to be found in Hawaii; and when we reflect that the absentees from the beach formation include most of the trees, under the shade of which the visitor to the Pacific islands can nearly always find protection from the fierce rays of a tropical sun, it cannot be a matter of surprise that this littoral flora has such a poverty-stricken appearance. We look in vain for such shady beach trees as Barringtonia speciosa, Terminalia Katappa, and Hernandia peltata; and we are lucky if we find some small trees under which we can obtain a scanty shade.
I have been speaking, of course, of the indigenous shore-plants, those that have arrived at these islands without the assistance of man. Yet it must be added that the existing littoral flora does include some of the missing indigenous trees, though rarely in any number. There is, however, scarcely one of them that is regarded by Dr. Hillebrand as having formed part of the original flora. That botanist would indeed rob the present beach flora, scanty as it is, of most of its conspicuous plants, as far as their claims to beconsidered indigenous are concerned. Dr. Hillebrand indeed includes Calophyllum Inophyllum, Hibiscus tiliaceus, Thespesia populnea, Morinda citrifolia, Cordia subcordata, and Pandanus odoratissimus in the present Hawaiian flora, and nearly all of them are to be found at times at the coast as well as inland; but he regards all, excepting the last-named, as having been introduced by the aborigines. I was not inclined at first to go quite so far as Dr. Hillebrand in this direction; but he carefully considered the case of each individual plant, and, remembering his sojourn of twenty years in the islands, his authority cannot be lightly put aside. In the list of Hawaiian strand-plants given inNote 28there are several species not always littoral in the group, but typically littoral in other tropical regions. One species, Ipomœa glaberrima, Boj., has not been recorded before from these islands.
A strong reason in favour of the contention of this botanist is that all the trees above-named are useful in some way to the natives; and, indeed, when we look at the works dealing with the floras of the islands of the South Pacific, we observe that in almost all the groups one or other of these six trees bears the reputation of having been introduced by the aborigines. All of them in their turn lose their fame as truly indigenous plants in some group or other. The occurrence of two or three useless South Pacific beach trees, that are known to be dispersed by the currents, in the indigenous strand-flora of Hawaii, would go far to invalidate Dr. Hillebrand’s argument, since the six trees in dispute are also known to be dispersed by the currents. But such trees are not to be found; and we look in vain for trees like Cerbera Odollam, Guettarda speciosa, Gyrocarpus Jacquini, and Hernandia peltata, that are spread far and wide over the beaches of the South Pacific.
It is also of interest to notice how trees like Morinda citrifolia and Terminalia Katappa, concerning the non-indigenous character of which there can be but little doubt, are in our own day acquiring a littoral station. The second is not even regarded by Dr. Hillebrand as having been introduced by the natives, but is referred by him to the European epoch. After having been extensively planted, it is now, as I found, becoming a littoral tree on the coast of Oahu, and supplies its buoyant fruits in a regular way to the beach drift. Its native name of Kamani is merely that of Calophyllum Inophyllum. All the six trees in dispute are known in Hawaii by the names by which they are distinguished far and wide over the South Pacific, a fact of which the reader may satisfy himself by referring to my paper on Polynesian plant-names. TheHawaiians, when their ancestors abode in the South Pacific, must have been well acquainted with one or other of the prevailing names of Terminalia Katappa (Talie, Tara, &c.); but it had lapsed in the memory of the race when the Europeans introduced the tree into Hawaii.
It may be added in this connection that Dr. Hillebrand weakens his argument by regarding Pandanus odoratissimus as of pre-aboriginal origin or as truly indigenous. Like the other six trees in question, its fruits are known to be capable of dispersal far and wide by the currents; and if this species of Pandanus is indigenous, we are obliged to assume that its fruits were first brought by the currents. That being so, we cannot exclude the probability of the currents having been also effective with several of the other plants regarded by Hillebrand as of aboriginal introduction, more especially those with large fruits like Calophyllum Inophyllum, and Cordia subcordata, where the alternative agency of frugivorous birds would be impracticable, at least over a wide extent of ocean. Pandanus odoratissimus is, as I venture to think, a tree that was introduced ages since by the aborigines. Next to the Coco palm, few trees have been more utilised by island-peoples, more particularly perhaps in the ruder stages of their history.
This point has been discussed at some length, because on the correctness of Dr. Hillebrand’s view depends the explanation to be subsequently given of the origin of the shore-flora of Hawaii. Though differing in some details, my observations on the Hawaiian coast plants, which are given inNote 29, tend to strengthen his contention.
I now return to the consideration of some of the negative features of the Hawaiian strand-flora, and will allude first to the absence of the mangroves and of the numerous other plants that live in and around a mangrove-swamp. This cannot be connected with a total absence of suitable stations. Although it is true that there are but few large rivers and but few suitable localities, yet such localities exist. The shores of Hilo Bay might readily have been the home of a mangrove-swamp; and one can point to different places on the coast of Oahu, such, for instance, as Pearl Harbour, which in Fiji would have been occupied by a luxuriant growth of mangroves. The same argument applies to the missing beach trees, such as Barringtonia speciosa, Hernandia peltata, Guettarda speciosa, &c., that adorn the beaches of many a coral island or of many a coral-bound coast in the South Pacific.Although in a large island like Hawaii with its lava-bound coasts but few white calcareous beaches exist where we might expect to find such a flora, yet such beaches occur wherever the scanty coral reefs are found off the coast; and it is just in those localities, as is pointed out in the account of my observations inNote 29, that the “plantes madréporiques” of the French botanists, the plants of the coral atoll and of the reef-girt coast, make their best endeavours to establish themselves. In other islands like Oahu, where coral reefs are more developed, calcareous beaches are more frequent, and there the few “madreporic” plants of Hawaii make a home.
Nor can the deficiencies in the Hawaiian strand-flora be connected with climatic conditions. That its meagre character cannot be so explained is indicated by the manner in which the Indo-Malayan shore-plants have pushed their way northward on the western side of the Pacific to the Liukiu and Bonin Islands. Here in latitude 26-27° N. we find several Fijian littoral trees and shrubs, such as Hernandia peltata, Pemphis acidula, Pongamia glabra, Sophora tomentosa, Terminalia Katappa, Tournefortia argentea, &c., that do not occur in Hawaii, although this group is some degrees nearer the equator, namely, in latitude 19-22° N. They are accompanied by the mangroves (Rhizophora, Bruguiera, &c.) in strength as far as South Liukiu in latitude 25° N.; but we learn from Dr. Warburg that the mangroves thin off further north, though they reach to South Japan, where Döderlein found in latitude 32° N. solitary examples of Rhizophora mucronata. These interesting facts of distribution, which are taken from Schimper’s work on the Indo-Malayan shore-plants (pp. 85, 90), show us that we can scarcely look to climatic conditions for the explanation of the absence of mangroves and of many other tropical littoral plants from Hawaii. We form the same opinion when we regard the extension northward of the mangrove-formation on the American coasts of the North Pacific Ocean. According to the account of Dr. Seemann given in the “Botany of the Voyage of H.M.S.Herald,” the mangroves with the coco-nut palm, and many other littoral plants common on the western shores of tropical America, reach their northern limit a little north of Mazatlan within the mouth of the Gulf of California in latitude 24° 38ʹ N. The parallel of 25° N. latitude, as indicated in Drude’s Atlas, probably represents the extreme northern limit, which is thus five or six degrees north of the latitude of the large island of Hawaii.
Neither can the explanation be found in the deficient floating powers of the seeds or seedvessels of many of the “absentees.” Those of Barringtonia speciosa, Guettarda speciosa, Heritiera littoralis, the two species of Terminalia, &c., possess great buoyant powers equal to, and probably often exceeding, those of the plants that, like Ipomœa pes capræ, have succeeded in establishing themselves in Hawaii. One has only to look at the lists giving the results of flotation experiments in Notes2and3, in order to realise that there are very few of the “absentee” littoral plants, the non-existence of which in Hawaii could be attributed to deficient floating powers of the fruit or seed. Being able to float unharmed for months, and in several cases even for years, the seeds or fruits of the shore-plants unrepresented on the Hawaiian beaches have been carried far and wide by the currents over the tropical Pacific even to Ducie and Easter Islands, that is, as far as the islands extend.
The only plants about which one could express a doubt concerning their ability to reach Hawaii through the agency of the currents, and to establish themselves there, are the true mangroves of the genera Rhizophora and Bruguiera. Since germination takes place on the tree, it is only through the floating seedlings that they could reach these islands; but, as shown inChapter XXX., it is doubtful whether the seedlings would be in a fit condition for reproducing the plant after such a long oceanic voyage. If they had been as successful in establishing themselves in Hawaii as they have been in the Liukiu Islands, which lie in latitude a few degrees farther north, these two species through their reclaiming agency would alone have prepared the way for the whole mangrove formation. We have seen in the preceding chapter that the absence of the mangrove formation from Tahiti appears to be mainly due to the failure of the pioneer species of Rhizophora and Bruguiera to establish themselves there. This evidently also applies to Hawaii, the cause of their exclusion being connected neither with climate nor with station, but as in Tahiti with the general unfitness of the floating mangrove seedlings for crossing broad tracts of ocean without injury to the growing plantlet.
With regard, however, to the bulk of the “absentee” littoral plants, those of the beach-formation, no such incapacity on the part of the buoyant seed or fruit can be accepted. These plants, which have reached Tahiti in numbers, have in the mass failed to reach Hawaii. It will, therefore, be of interest to glance at thecharacter of the fruits of the “absentee” trees, which a traveller fresh from a visit to the coral islands and reef-girt coasts of the South Pacific sadly misses on the Hawaiian beaches. We notice in the first place that the absent trees, such as Barringtonia speciosa, Cerbera Odollam, Guettarda speciosa, Heritiera littoralis, Terminalia Katappa, &c., have large fruits which could only have been carried to Hawaii by the currents, the agency of birds being quite out of the question. On the other hand, almost all the littoral plants of Hawaii, whether trees, shrubs, or herbs, which are regarded as truly indigenous by Mann, Hillebrand, and other Hawaiian botanists, have only small fruits or seeds available for dispersal, from which the agency of birds cannot, on the point of size, be excluded. Amongst these shore plants possessing buoyant seeds or fruits are Cassytha filiformis, Colubrina asiatica, Ipomœa pes capræ, Scævola Kœnigii, Vigna lutea, and Vitex trifolia; whilst amongst the plants with non-buoyant fruits or seeds are to be reckoned Heliotropium anomalum, H. curassavicum, Tephrosia piscatoria, Tribulus cistoides, &c. The seeds or seedvessels of the plants of the buoyant group possess great floating powers; and it seems at first sight scarcely credible that the currents which have failed to establish Barringtonia speciosa, Guettarda speciosa, and the other trees that through this agency have often found a home on the remotest islands of the Pacific, should have succeeded in the instances of plants like Scævola Kœnigii and Vitex trifolia.
It would indeed almost seem that in nearly all cases where it would be impossible in point of size for a bird to transport the fruit or seed of a shore-plant to Hawaii, such a plant is not to be found in the strand-flora of that group, even though it is well adapted for dispersal by the currents. Many of the littoral trees missing from the Hawaiian coast-flora, having large buoyant fruits, come into this category; and grave suspicion is thus apparently cast on the agency of the currents in the case of the plants with small fruits and seeds that really compose the strand-flora, even when their capacity for sea-transport has been well established by observation and experiment. The efficacy of the currents would thus seem to be called into question for the whole littoral flora of Hawaii.
If, however, we were to adopt such a sweeping conclusion we should be led into an error. It is pointed out in the following chapter that nearly all these large-fruited beach trees that are found far and wide over the South Pacific, but are absent from Hawaii, do not occur as indigenous plants in America. If, therefore,the fruits of such Old World littoral trees as Barringtonia speciosa, Cerbera Odollam, Guettarda speciosa, Ochrosia parviflora, Terminalia Katappa, &c., that could be dispersed only by the currents, have failed to reach Hawaii, it is essential to remember that they have also failed to reach America. This suggests that Hawaii may have received some of its littoral plants from America through the agency of the currents; and it is shown in the following chapter that, as a rule, when a South Pacific plant with buoyant fruits or seeds is not found in America, it is equally absent from Hawaii. The question thus acquires quite a different aspect, and we shall accordingly have to regard tropical America in the next chapter as a possible centre of diffusion of littoral plants over the globe, a centre possibly as important as that connected with the tropics of the Old World.
Although, however, the currents have played a part in stocking the Hawaiian beaches with their plants, their share in the work has been unimportant, and the number of plants concerned is limited. If we take away the seven or eight littoral plants introduced by the aborigines, as well as the three endemic species as indicated in the list inNote 28, and then remove from the residue the plants with small fruits or seeds possessing little or no buoyancy, there remain only the following eight species, the presence of which in Hawaii might be attributed to the currents, namely, Cæsalpinia Bonducella, Cassytha filiformis, Colubrina asiatica, Ipomœa glaberrima, Ipomœa pes capræ, Scævola Kœnigii, Vigna lutea, and Vitex trifolia. Of these plants, three species, those of Cassytha, Scævola, and Vitex, possess fruits that would be likely to attract frugivorous birds, and are in some cases known to be dispersed by them (seeChapter XIII.); so that we are not in these instances restricted to the agency of the currents. With the other five the currents offer the readiest explanation, but, as is indicated in the cases of Cæsalpinia Bonducella and Ipomœa glaberrima (Chapter XVII.), it is quite possible that birds have occasionally intervened. Altogether we may infer that in stocking the Hawaiian beaches with their littoral plants the currents have taken a subordinate part.
Coming to the Hawaiian littoral plants having seeds or fruits that have no floating power, we find that they present a motley group. It has been already remarked that this is the group of shore plants that derives most recruits from the inland flora, and that it is in this group that the differences between the shore-floras of tropical regions find their expression. Yet a very odd collection of plants is here exhibited. Sometimes the beach-flora is composedin great part of these plants; and a sorry spectacle is presented by a beach possessing such plants as Gossypium tomentosum, Heliotropium anomalum and H. curassavicum, Lipochæta integrifolia, Tephrosia piscatoria, Tribulus cistoides, &c. Yet to the student of plant-distribution such a motley collection would be full of suggestiveness. From the circumstance that species of Cuscuta, Jacquemontia, and Lipochæta, that are peculiar to the Hawaiian Islands, have made their homes on the beach, he would infer that since Nature has been compelled to borrow from the endemic inland flora, there has been some difficulty in stocking the beaches with their plants. The occurrence of endemic species amongst the strand-plants would be viewed by him as especially indicating incapacity on the part of the ocean currents.
Yet in the quantities of drift timber, showing evidence of many months and probably even of years of ocean-transport, to be seen stranded on the weather coasts of these islands, the observer discerns undoubted evidence of the efficacy of the ocean currents. But what he finds are huge stranded pine logs of “red-cedar” and “white-cedar” from the north-west coasts of America. He may search the drift for days together, as I have done, and discover no tropical fruits or seeds except such as could be supplied by the present Hawaiian flora. The subject of this drift is especially discussed inNote 30; and it need only be mentioned here that it is not improbable that, as shown in the next chapter, some drift may reach Hawaii from tropical America under exceptional conditions, and that its presence is masked by the Oregon drift.
The agency of the drifting log in carrying small seeds in its crevices would be effectual in the instance of plants from the temperate coasts of North America. For example, the nutlets of Heliotropium curassavicum, which have no buoyancy, might easily be washed, together with sand, into the cracks of a pine log stranded temporarily on the Oregon coast where this plant occurs. Themodus operandiwas brought home to me when examining the drift brought down by the Chancay River on the coast of Peru. Here I found this species of Heliotropium growing on the margin of a swamp near some stranded logs, that would probably be carried out to sea when the river was next in flood.
It is probable, I may add, that the seeds or fruits of some of the plants of the non-buoyant group of the Hawaiian littoral flora may be dispersed in birds’ plumage. For instance, the spiny fruits of Tribulus cistoides sink in sea-water; but they are well suited for entangling themselves in birds’ feathers.
It is possible that the hairy seeds of Gossypium tomentosum may have been thus distributed; but there is much that is enigmatical about this plant (seeChapter XXVI).
The Inland Extension of the Beach Plants of Hawaii.—When we regard the inland extension of littoral plants in Hawaii, we get fresh indications of the meagreness of the strand-flora. Several of the species, as Cæsalpinia Bonducella, Cassytha filiformis, Tephrosia piscatoria, &c., show themselves only occasionally on the sandy beaches, though they are common enough on the old scantily vegetated lava-flows near the coast and are often found miles inland. Indeed, Dr. Hillebrand not infrequently in describing the station only gives prominence to the situation of the plants away from the beaches, and places most of them on the old lava plains that extend inland from the coast. It is only by a detailed examination of extensive coast lines in these islands that I have succeeded in preserving to a small degree their reputation as beach plants. A few of them behave somewhat strangely in their inland station. Thus, the seeds of Cæsalpinia Bonducella obtained from various localities showed no buoyancy in my experiments; and had I not found a solitary buoyant seed in the stranded drift I should have inferred that this was a rule without exception.
It is to be remarked that whilst some plants like Scævola Koenigii occasionally stray a few hundred yards inland on the surface of the old lava-flows, others like Ipomœa pes capræ and Vitex trifolia, that are spread far and wide over the inland plains of Fiji, are confined in Hawaii to the beaches and their immediate vicinity. Some of the plants like Hibiscus tiliaceus, Morinda citrifolia, and Pandanus odoratissimus, that are regarded as having been introduced by the aborigines, behave exactly like indigenous plants in the inland plains; but this is not necessarily an indication of an indigenous plant in this group, since the Cactus (Opuntia Tuna) and the Castor-Oil Plant (Ricinus communis) have spread all over the drier lower regions of the islands, whilst Aleurites moluccana, the Candle-Nut Tree, which has no means of reaching these islands without man’s agency, now forms entire woods on the mountain slopes, usurping the place often of the original forests.... Further details relating to this subject are given inNote 31.
The principal points in the foregoing discussion of the strand-flora of Hawaii may be thus summed up:—
(1) The indigenous, that is, the pre-aboriginal, strand-flora of this group lacks not only the mangroves and their associated plants, but also most of the characteristic beach-trees of the SouthPacific, which are known to owe their wide distribution in tropical regions to the currents.
(2) The meagreness of the littoral flora is intensified by the tendency of some of the plants to extend inland and to desert the coasts, and by the occurrence on the beaches of peculiar species not found outside the Hawaiian Islands.
(3) The absence of the mangrove formation and of so many of the typical beach trees of the Pacific cannot be attributed either to the lack of suitable stations, or to climatic conditions, or to deficient floating power of the seed or fruit.
(4) As in the case of Tahiti, the mangroves and their associated plants are lacking because the floating seedlings of Rhizophora and Bruguiera, the pioneer plants of a mangrove-swamp, have failed to reach Hawaii in a fit condition for establishing themselves. The numerous plants that accompany a mangrove-swamp have thus been unable to find a home, though the buoyant powers of their fruits or seeds are often great.
(5) With the missing beach-trees, however, which possess fruits that can float for years unharmed in sea-water, no such incapacity is suggested. Most of them have large fruits, which could only reach Hawaii through the currents. This absence from the Hawaiian indigenous strand-plants of most, if not all, of the large-fruited species, where on account of size the agency of birds is absolutely excluded, is very remarkable; and it at first seems to throw grave suspicion on the efficacy of the currents for the whole strand-flora.
(6) It is, however, to be noticed that these large-fruited beach trees have not only failed to reach Hawaii but have also failed to reach America. The question thus acquires quite a different aspect, and America becomes the possible source of most of the Hawaiian plants with buoyant seeds or fruits.
(7) This subject is discussed in the next chapter; but it is here shown that at their best the currents have taken but a secondary part in stocking the Hawaiian beaches with their plants, since many of the plants have non-buoyant seeds or fruits.
(8) The drift stranded on the shores of the Hawaiian Islands is composed of logs from the north-west coast of North America. No drift from the south has been discovered; but it is not unlikely that future investigators will find some seed-drift from tropical America.
THE WORLDSHOWINGOCEAN CURRENTSJohn Bartholomew & Co., Edinr.
THE WORLDSHOWINGOCEAN CURRENTSJohn Bartholomew & Co., Edinr.
THE WORLDSHOWINGOCEAN CURRENTSJohn Bartholomew & Co., Edinr.