The three kinds of Rhizophora, when seen at the same time along a tract of coast, may be readily distinguished by the different shades of green of their foliage, that of Selala being dark green, that of Rhizophora mucronata light green, and that of Rhizophora mangle intermediate in shade. The Selala is usually the tallest of the three, and attains a height of from 20 to 30 feet or even 40 feet and over, the aerial roots dropped from the higher branches giving it a characteristic aspect. Rhizophora mangle is generally the shortest, and at the coast is from 10 to 12 feet high; but where the mangrove vegetation is most luxuriant, as in the great swamps in the interior of the Rewa delta, it forms tall trees as much as 40 feet in height, displaying the aerial roots hanging from the higher branches. Rhizophora mucronata is, as a rule, intermediate in height, and is distinguished by its stout, reddish trunk and reddish aërial roots.
The trunks of Selala are often in an inclined position and supported entirely by the trestle-like aërial roots, the lower end raised some 5 or 6 feet above the ground with the rest of the trunk inclined upwards. They then look like gigantic walking-stick insects. The same habit may be sometimes observed with the larger trees of Rhizophora mucronata, and in fact all three may present at times the same habit of growth. The taller trees of Rhizophora mangle may resemble the Selala in habit, and the smaller trees of the Selala may approach the habit of Rhizophora mangle.
The distinctive characters of the Selala are given in the table opposite. It will be there seen that this form is intermediate between the other two species as regards the form and size of the petioles and peduncles; the size of the bracts and bracteoles; the colour, form, and size of the flowers; and in the length of the style. Its leaves are smaller than in the case of the other two species, but pointed and semi-aristate like those of Rhizophora mucronata. There are, however, two varieties of the Selala, both with larger foliage than that belonging to the prevailing type of the tree, and from 10 to 15 feet in height. In one the flowers are more numerous, each flowering stem branching four or five times and bearing at least twenty-four flowers, the first branch being trichotomous and the rest dichotomous. In the other, which is the prevailing form on the Mathuata coast, there is a nearer approach to Rhizophora mucronata in the rounding of the peduncles and in the length of the style. Then, again, there are divergent varieties of Rhizophora mangle which in the larger bracts and bracteoles and in the greater size, form, and paler hue of the flowers come nearer to the Selala. Taking all the characters together, the Selala, though intermediate between the Asiatic and the American species, comes in the most critical diagnostic points, as in the inflorescence, in the individual flowers, and in the form of the apex of the leaf, nearest to Rhizophora mucronata, the Asiatic species.
The seedless character of the Selala is well known to most Fijians of the coast districts, the native name signifying empty (lala) flowers (se). Now and then they aver that it produces fruit, but the numerous offers of rewards in money never resulted in their bringing me the fruits. During my residence of two years in the group I examined the Selala trees in a great number of localities and never succeeded in finding them in fruit.
With all three kinds the anthers burst in the bud before it begins to open, and we may ask why the process of self-fertilisation, which is effectual with the other two kinds, produces no result with the Selala. In all three cases the flower-buds and expanded flowers hang downwards, and the expanded flowers retain their parts for the first twenty-four hours, the pollen being caught in quantity on the hairy edges of the petals. During the next day the withering stamens fall out, and on the following day the petals fall too. With the Selala, the style soon begins to blacken and wither, and in a few days the flower becomes detached and drops off. With Rhizophora mucronata and Rhizophora mangle, the style preserves its healthy condition, and shortly evidences of fertilisation appear in the altered shape of the ovary. It is apparent, therefore, that in the case of the Selala fertilisation has not occurred, although the mechanical processes connected with it have been carried out. The cause of this is not far to seek.
| Characters | Fiji | ||
|---|---|---|---|
| Rhizophora mucronata. | Selala (a seedless form). | Rhizophora mangle. | |
| Height of tree in feet | 12-20. | 20-40. | 9-12. |
| Colour of foliage | Pale green. | Dark green. | Intermediate shade. |
| Base of leaf | Tapering. | Sub-rounded. | Tapering. |
| Apex of leaf | Acute, and terminating in a twisted point a line (2·5 mm.) long. | Acute, and terminating in a twisted point less than a line (2·5 mm.) long. | Very obtuse, with no twisted point. |
| Leaf-stalk (petiole) | Rounded, 1-12⁄10 inch, (25-30 mm.) long, about as long as the peduncle. | Rather flattened horizontally, 58⁄10th inch (12-20mm.) long, shorter than the peduncle. | Length as in Selala, but flattening very marked. |
| Inflorescence | Branching (dichotomous) two or three times with four to eight flowers. | Branching usually twice, but sometimes three times; first branching trichotomous, rest dichotomous; six to twelve flowers; in one variety, flowers as many as twenty-four. | Usually branching only once (trichotomous) and bearing only three flowers; but sometimes branching again (dichotomous) and bearing then six flowers. |
| Peduncle (lowest flower-stalk) | Rounded. | Flattened above. | Flattening more marked than with Selala. |
| Pedicels | As stout as the peduncle, and rounded. | More slender than the peduncle, and angular. | As in Selala. |
| Bracts and bracteoles | Large, 11⁄2 line (4 mm.). | Small, 2⁄3 line (2 mm.). | Very small or absent. |
| Calyx | Very pale yellow, or dirty white, rounded at base in the bud, lobes 41⁄2-5 lines (11-12 mm.). | As in R. mucronata. | Pale or bright green, angular at base in the bud, lobes 31⁄2 lines (8 mm.). |
| Length of style | 11⁄2 lines (4 mm.). | 1 line (2·5 mm.). | 1⁄2 line (1·5 mm.). |
| Fruit | Ovoid and usually symmetrical, with large persistent bracteoles at base. (Hypocotyl 16 inches.) | No fruits produced. | Conical, somewhat curved, and thus not symmetrical; bracteoles at base very small or absent. (Hypocotyl 9 or 10 inches.) |
| Colour of trunk, rootstock and roots | Reddish. | — | — |
| Characters. | Ecuador. | |
|---|---|---|
| Rhizophora mangle (Mangle chico). | Rhizophora mangle (Mangle grande). | |
| Height of tree in feet | 10-15. | 50-80 and more. |
| Colour of foliage | Pale green. | Dark green. |
| Base of leaf | Tapering or sub-rounded. | Tapering. |
| Apex of leaf | Very obtuse, with no twisted point. | Very obtuse, with no twisted point. |
| Leaf-stalk (petiole) | Flattened above and below, with no median groove, 1⁄2 inch (12 mm.) long; not half as long as the peduncle. | Flat above with a median groove, 1 inch (25 mm.) long, two-thirds the length of the peduncle. |
| Inflorescence | As described under R. mangle of Fiji. | Branching at least three times, sometimes four or five times, trichotomous or dichotomous, twelve to forty-eight flowers. |
| Peduncle (lowest flower-stalk) | Sub-angular. | Rounded. |
| Pedicels | More slender than the peduncle, and rounded. | More slender than the peduncle, and angular. |
| Bracts and bracteoles | Scarcely developed, 1⁄2 line (1 mm.). | Well developed, 1 line (2·5 mm.). |
| Calyx | As with R. mangle of Fiji. | As with R. mucronata and Selala of Fiji, but lobes 4 lines (10 mm.). |
| Length of style | Less than a line (2·5 mm.). | 11⁄2 lines (4 mm.). |
| Fruit | As in R. mangle of Fiji. (Hypocotyl 9 or 10 inches.) | Conical, not symmetrical, and somewhat curved; large persistent bracteoles at base as in R. mucronata. (Hypocotyl 12 to 15 inches.) |
| Colour of trunk, rootstock and roots | — | — |
| Characters | Fiji | Ecuador. | |||
|---|---|---|---|---|---|
| Rhizophora mucronata. | Selala (a seedless form). | Rhizophora mangle. | Rhizophora mangle (Mangle chico). | Rhizophora mangle (Mangle grande). | |
| Height of tree in feet | 12-20. | 20-40. | 9-12. | 10-15. | 50-80 and more. |
| Colour of foliage | Pale green. | Dark green. | Intermediate shade. | Pale green. | Dark green. |
| Base of leaf | Tapering. | Sub-rounded. | Tapering. | Tapering or sub-rounded. | Tapering. |
| Apex of leaf | Acute, and terminating in a twisted point a line (2·5 mm.) long. | Acute, and terminating in a twisted point less than a line (2·5 mm.) long. | Very obtuse, with no twisted point. | Very obtuse, with no twisted point. | Very obtuse, with no twisted point. |
| Leaf-stalk (petiole) | Rounded, 1-12⁄10 inch, (25-30 mm.) long, about as long as the peduncle. | Rather flattened horizontally, 58⁄10th inch (12-20mm.) long, shorter than the peduncle. | Length as in Selala, but flattening very marked. | Flattened above and below, with no median groove, 1⁄2 inch (12 mm.) long; not half as long as the peduncle. | Flat above with a median groove, 1 inch (25 mm.) long, two-thirds the length of the peduncle. |
| Inflorescence | Branching (dichotomous) two or three times with four to eight flowers. | Branching usually twice, but sometimes three times; first branching trichotomous, rest dichotomous; six to twelve flowers; in one variety, flowers as many as twenty-four. | Usually branching only once (trichotomous) and bearing only three flowers; but sometimes branching again (dichotomous) and bearing then six flowers. | As described under R. mangle of Fiji. | Branching at least three times, sometimes four or five times, trichotomous or dichotomous, twelve to forty-eight flowers. |
| Peduncle (lowest flower-stalk) | Rounded. | Flattened above. | Flattening more marked than with Selala. | Sub-angular. | Rounded. |
| Pedicels | As stout as the peduncle, and rounded. | More slender than the peduncle, and angular. | As in Selala. | More slender than the peduncle, and rounded. | More slender than the peduncle, and angular. |
| Bracts and bracteoles | Large, 11⁄2 line (4 mm.). | Small, 2⁄3 line (2 mm.). | Very small or absent. | Scarcely developed, 1⁄2 line (1 mm.). | Well developed, 1 line (2·5 mm.). |
| Calyx | Very pale yellow, or dirty white, rounded at base in the bud, lobes 41⁄2-5 lines (11-12 mm.). | As in R. mucronata. | Pale or bright green, angular at base in the bud, lobes 31⁄2 lines (8 mm.). | As with R. mangle of Fiji. | As with R. mucronata and Selala of Fiji, but lobes 4 lines (10 mm.). |
| Length of style | 11⁄2 lines (4 mm.). | 1 line (2·5 mm.). | 1⁄2 line (1·5 mm.). | Less than a line (2·5 mm.). | 11⁄2 lines (4 mm.). |
| Fruit | Ovoid and usually symmetrical, with large persistent bracteoles at base. (Hypocotyl 16 inches.) | No fruits produced. | Conical, somewhat curved, and thus not symmetrical; bracteoles at base very small or absent. (Hypocotyl 9 or 10 inches.) | As in R. mangle of Fiji. (Hypocotyl 9 or 10 inches.) | Conical, not symmetrical, and somewhat curved; large persistent bracteoles at base as in R. mucronata. (Hypocotyl 12 to 15 inches.) |
| Colour of trunk, rootstock and roots | Reddish. | — | — | — | — |
Although the ovaries of the Selala contain four ovules, which in size and appearance do not differ from those of Rhizophora mangle and R. mucronata, its pollen when compared with that of the other two forms presents a remarkable difference. The pollen of these three mangroves was examined in five localities far apart from each other, and in all the same results were obtained. The pollen-grains of the Selala are much smaller than those of the other two, and differ much from them in form. They are irregularly oval in outline, and have a shrunken look beside the spherical symmetrical grains of the two species with which they are compared. They are from one-fourth to one-third the size of those of Rhizophora mucronata, and from one-third to one-half the size of those of Rhizophora mangle.
There is much to support the view that the Selala is a cross between the other two species, its intermediate characters and its seedless condition being especially indicative of such a derivation; but there are several difficulties in accepting this explanation.