This order, judging from the writings of Hegelmaier, Schenck, and Hemsley, is represented by one or other of the common species, Lemna minor, L. gibba, L. polyrrhiza, in various Atlantic islands, as in the Bermudas, the Azores, Madeira, the Canary Islands, and St. Helena; but doubts frequently arise as to their being truly indigenous. Lemna trisulca is regarded by Hemsley as indigenous in the Bermudas. Lemna minor has been introduced in recent years into Hawaii, where I observed it flowering and sometimes fruiting abundantly in the heated waters of the ponds. Two species found in other regions were recorded by Seemann from Fiji, and I have come upon few other records of the occurrence of the order in the tropical islands of the open Pacific. I am inclined to the opinion, based not only on the facts of distribution, but also on the results of numerous experiments on the means of dispersal, that this order has in most cases reached oceanic islands with man’s assistance.
Some years ago I made a systematic study of the habits of the British Lemnæ, most of the results being published in the Linnean Society’s Journal (vols. xxix and xxx), as far as concerned Lemna minor, L. gibba, and L. polyrrhiza. During this inquiry I ascertained that with these species, as well as with L. trisulca, the chances of a bird’s carrying their fronds uninjured in its plumage over a wide extent of ocean were small. None of them survived twenty-four hours’ drying in fine weather, whether in the sun or in the shade; but in rainy weather they withstand an exposure of one or two days. It is, therefore, unlikely, even if the fronds were entangled by their rootlets in a bird’s feathers, that they would be able under ordinary conditions to reproduce the plants after a day’s flight of some five hundred miles across the sea. It must also be remembered that the drying capacity of the air when a bird is in full flight in ordinary weather would be that displayed during a gale of wind with a velocity of at least thirty to forty miles an hour. For this reason I do not think with Kerner that under usual conditions drops of water would be a factor of importance in causing the adherence of minute seeds of any kind to birds’ plumage. Where the seeds are not available, it is most probable that birds disperse the duckweeds by their fronds over short distances, but not across broad seas. This would certainly apply to temperate latitudes, where these plants rarely seed. Thus with Lemna, as with Ceratophyllum, it would seem that the dispersal of the seeds by birds takes place normally only in warm latitudes. Those of the duckweeds could be transported in adherent mud over land-areas.
According to Hegelmaier, the two species of Lemna found in Fiji are L. paucicostata, an Asiatic species, and a variety of an Australian species, L. oligorrhiza, possessing dark root-sheaths. These plants mostly came under my notice in the Rewa delta. They were rarely seen in Vanua Levu, where in one locality I found the typical Lemna minor. The first species is also Samoan.
In 1897 and in 1899, in a pool near Notho in the Rewa delta, in Viti Levu, Fiji, I found a great abundance of a species of Wolffia, specimens of which were sent to Prof. Schimper with my mangrove collections, but his death intervened, and I have not been able to follow up the matter. On comparing the specimens with Hegelmaier’s descriptions and plates, it would seem that the species is near W. arrhiza and W. brasiliensis, but differs from both in the greater length of the fronds. As concerning the means of dispersal of the genus, I may add that the fronds were killed after being allowed to dry for eighteen hours.
Marsilea (Marsileaceæ)
A species of this genus, apparently near Marsilea villosa, was common in the ditches and ponds around Notho, in the Rewa delta, Fiji, in 1897-99. The genus is included by Horne in his list of Fijian plants; but is not given by Seemann. The villous sporocarps, when dry, are very light and readily catch in cloth and in feathers. Hillebrand includes in the Hawaiian flora M. villosa and M. crenulata. The first-named, which was collected by Chamisso and Gaudichaud, finds (he says on the authority of Braun) its nearest relative in a species from Oregon and California. The other has been collected in the Liukiu Islands, the Philippines, Mauritius, and Bourbon. It is very probable that the occurrence of the genus in oceanic islands is due to the agency of birds.
Summary of the Chapter
(1) We are here concerned with the more restricted distribution of non-endemic tropical genera over the Pacific. The general trend eastward of these genera is well brought out in the fact that whilst Fiji possesses some sixty or seventy genera in common with Tahiti to the exclusion of Hawaii, it does not possess a score in common with Hawaii to the exclusion of Tahiti. The grasses and sedges and the mountain genera are not here included; and we are comparing the flora of the Hawaiian lowlands below 4,000 feet with the floras in mass of Fiji and Tahiti.
(2) Hawaii possesses very few genera (less than thirty) that are not found either in Fiji or in Tahiti, or in both; and of these quite a third are to be traced to America.
(3) From two of these genera, Embelia, a land genus, and Naias, an aquatic genus, we obtain two important indications, namely, that specific differentiation has taken place to much the same extent in a water plant as in a land plant, whether in a continent or in an island. In other words, new species have been developed or are developing independently of the immediate environment and of isolating influences.