Curious seed-structures of this kind must have their significance in the history of the plant; and on returning to England I looked a little further into the matter. To follow up this kind of inquiry, however, would carry me far beyond the limits prescribed for this note, and I have only treated it here in a tentative fashion. Different botanists of eminence have paid attention to this subject, amongst them Roxburgh, Thomson, and Miers (see Dr. T. Thomson in Journ. Linn. Soc. Bot., vol. ii., p. 47, 1858, and Mr. J. Miers in Trans. Linn. Soc. Bot., vol. i., 1880). It would appear that the seed-structure of Barringtonia is also found in Careya, a genus of the same Myrtaceous tribe, and in Garcinia and other genera of the Guttiferæ, as well as in other inland plants.
Mr. Miers, after reviewing the opinions of his predecessors, gives the results of his own investigations. The solid embryo found in Barringtonia and many other genera consists, he observes, (a) of an external portion, the “exorhiza,” which nourishes the germinating seed and then dies away; (b) of an internal portion, the “neorhiza,” which, growing at each end, forms the central portion of the stem and root; and (c) the “medullary sheath” of Mirbel, that lies between the two, and is composed of elementary vascular tissue, which ultimately gives origin to the wood, bark, and leaves of the stem and yields woody fibre to the root. The exorhizal portion in some cases, as in Barringtonia acutangula, splits into four parts during germination. Mr. Miers compares this seed-structure with that of Rhizophora, employing the same terms, “neorhiza” for the internal portion which forms the seedling, and “exorhiza” for the external portion which merely nourishes it. However, I may add that the exorhizal portion in Rhizophora, as shown in [Chapter XXX.], is now regarded as formed by the coalesced cotyledons, and is termed the “cotyledonary body”; so that by implication the corresponding part of a Barringtonia seed should be regarded from the same standpoint.
It may be apposite to notice here that Barringtonia racemosa displays one capacity which does not appear to belong to B. speciosa. The branches stuck in wet soil throw out roots and establish themselves. This capacity of vegetative reproduction is turned to account by the Fijians, who make “live-fences” of this tree in wet localities.
NOTE 51 (page [135])
On a Common Inland Species of Scævola in Vanua Levu, Fiji
This is a tall shrub, or small tree, nine or ten feet high, which corresponds with S. floribunda, Gray, as far as Seemann describes it. It has small, black, juicy drupes, well suited for dispersal by birds, having no “suberous” mesocarp as in the shore species (S. Kœnigii), and no capacity for dispersal by currents. It grows, much like the Hawaiian inland species, in exposed situations where there is plenty of light, as on mountain-peaks, at the borders of forests, in open-wooded districts, and in the plains, and is to be found at all elevations from near the sea up to the highest mountain summit (3,500 feet) when the station is suitable. I noticed it on the higher slopes and frequently on the tops of nearly all the principal mountains that I climbed. It is evident that birds carry the “stones” from one mountain-peak to another, and no doubt they explain the presence of the species in Tonga. Dr. Seemann speaks of it as a beach plant in Viti Levu. The plant familiar to me in Vanua Levu is only on very rare occasions to be seen as an intruder in the beach-flora.
NOTE 52 (page [137]).
On the Capacity for Dispersal by Currents of Colubrina oppositifolia, an Inland Hawaiian Tree
The seeds in my experiments sank within ten days; but they are not readily detached from the fruit, as in the case of the buoyant seeds of the littoral species (C. asiatica). The fruits, which may float for a week or two, break down, as Hillebrand observes, tardily and imperfectly, and could give but little assistance to dispersal by water.
NOTE 53 (page [141])
On the Genus Erythrina
We have in E. indica a widely distributed littoral species, ranging from India through Malaya to eastern Australia, and over nearly all the groups of the Pacific, reaching to Tahiti and the Marquesas, but not occurring in Hawaii. It is associated in Fiji and Tonga with another shore-species, E. ovalifolia, Roxb., found also in India and Malaya. I did not come on the second species in Fiji, and according to Seemann it is rare. It is possible that there is a genetic connection between the two; and it is noteworthy that in one case Seemann was uncertain (p. 426) whether the species was E. ovalifolia or only a variety of E. indica.
In different parts of their areas both these species may be found inland. This no doubt is to be connected with their occasional cultivation. The Polynesians who esteem E. indica for its handsome scarlet flowers and its scarlet seeds often plant it near their houses; but it is curious that if we look at the pages of Seemann, Horne, and one or two other botanical authors who have written on the Pacific, we find no reference to its littoral station, the first-named botanist merely characterising it in Fiji as occurring “wild or planted.”