Rhizophora

Of the three species of this genus, two of them, Rhizophora mucronata and R. conjugata, are Asiatic and are unknown in America; whilst the third, R. mangle, was until recently regarded as peculiar to the American and West African regions.

When Mr. Hemsley wrote the Report on the Botany of the Challenger Expedition he remarked (iii, 149) that the American Rhizophora (R. mangle) appeared to be restricted to that region, and he questioned its existence in the Pacific Islands as indicated by Jouan for New Caledonia. The same view was taken by Prof. Schimper in his work on the Indo-Malayan strand-flora published in 1891. There was, in fact, much to support this view, since Dr. Seemann, one of the most accomplished botanists who have explored the Pacific, describes only the Asiatic Rhizophora (R. mucronata) in Fiji, and nothing is said of any other species collected by the United States Exploring Expedition under Wilkes in Fiji and Samoa.

However, in a paper on the flora of Tonga, read before the Linnean Society in 1893, Mr. Hemsley includes the American mangrove, Rhizophora mangle, amongst the collections made there by Mr. Lister; and he refers to its occurrence also in Stewart Island (I suppose in the Solomon Group), but he suggests that it was accidentally introduced with ballast in both these localities. In 1897 I found a species of Rhizophora, to all appearances identical with the American species, in great abundance in the Rewa delta in Fiji. Subsequently the same mangrove came under my notice as the prevailing species in Vanua Levu in the same group; and on sending photographs of a branchlet in flower and fruit and of the germinating fruit to Prof. Schimper he expressed the opinion that they belonged to the typical Rhizophora mangle.

There are four typical mangroves in Fiji, namely (1) Bruguiera rheedii, the “Dongo” proper of the natives; (2) Rhizophora mangle, usually known as “Tiri-wai,” that is to say, the Tiri of the river, or rather of the estuary; (3) Rhizophora mucronata, the “Tiri-tambua” of the Fijian, signifying the Whale’s Tooth Tiri in allusion to the form of its fruit; and (4) a seedless form intermediate between the two species of Rhizophora, which the Fijians designate “Selala,” a name signifying “the tree with empty flowers.”

Bruguiera rheedii and Rhizophora mucronata were alone recorded by Dr. Seemann and his predecessors; but he significantly refers to the natives speaking of four mangroves. Mr. Horne, who spent twelve months in the group some years later, also overlooked the American Rhizophora; but it is apparent that both these botanists were naturally more interested in the vegetation of the inland regions than of the coast swamps, and we have before observed that they failed to record Scirpodendron costatum, a giant-sedge very common and conspicuous in the swamps. It is not easy to understand Dr. Seemann’s remark that “mangroves are restricted to but few parts of the larger islands.” Horne, who was in the islands eighteen years afterwards, makes frequent allusion to them. The natives whom I questioned closely on this subject scouted the idea that any of the four mangroves above named were recent arrivals. The coasts, as they said, had always been extensively fringed by mangroves; and the reader has only to refer to my remarks in the second chapter of my volume on the geology of Vanua Levu to convince himself that mangrove swamps of considerable extent existed in the time of Commodore Wilkes (1840).

The Relative Abundance and Mode of Association of the three Fijian forms of Rhizophora.

Stated in their order of frequency, we have first Rhizophora mangle, the American species, then Rhizophora mucronata, the Asiatic species, and lastly the Selala. The first is equally at home at the sea-border and on the banks of brackish estuaries. The second is, as a rule, more exclusively at home on the sea-coasts; and the same may be said for the Selala. Usually all three kinds occur in the lower part of an estuary; but as we ascend the river and the water freshens, the Asiatic Rhizophora and the Selala disappear, and the American plant is alone found in the higher reaches, where the density of the water ranges according to the state of the tide between 1·000 and 1·010. I examined the distribution of these three forms of Rhizophora in numerous estuaries of Vanua Levu, as well as in the Rewa estuary in Viti Levu; and it was ascertained that in all cases they followed the rule above indicated. When the estuary receives but few streams and the water is mostly salt, the three Rhizophoras may extend miles inland; but when it contains a large body of fresh-water, Rhizophora mangle may be the only form observed from the mouth of the river to the head of the estuary, and it may monopolise the adjacent coasts. On the other hand, Rhizophora mucronata may occupy almost exclusively a long extent of coast; or the Selala may prevail in certain localities, as on parts of the Mathuata coast of Vanua Levu.

The manner of association of these three Rhizophoras is of interest in connection with the origin of the seedless Selala. They very rarely occur mingled together, but grow gregariously in contiguous colonies; and not uncommonly all three may occur on the same line of coast within a distance of a few hundred yards. The colonies pass into each other without a break, and there is no fixed rule of association. Whilst on the south side of Vanua Levu the Selala is generally associated with the American Rhizophora, on the north side it is usually in touch with the Asiatic species. In other localities all three occur in contiguous colonies. The Selala colony may be exposed on the line of a river-bank or along the sea-coast, or it may lie in the heart of an extensive mangrove tract. The most extensive mangrove region in Fiji, that of the Rewa delta, is in great part occupied by Rhizophora mangle; but all three forms grow together in the eastern part of the delta; and here, strangely enough, as at Daku, the Selala may grow sporadically, and all three may grow mixed together with their branches intercrossing.

The Characters of the Selala or Seedless Rhizophora compared with those of the American Mangrove (R. mangle) and the Asiatic Mangrove (R. mucronata).

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
Characters	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.
Characters.	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.
Characters	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.

(1) The circumstance of the anthers bursting in the flower-bud would considerably lessen the chances of cross-fertilisation; but this objection is not insurmountable, since numerous insects, such as flies, ants, and small coleoptera, visit the newly opened flowers, and they might sometimes produce a result. When I made this suggestion to Prof. Schimper he replied that insect-pollination was quite possible after the expansion of the flowers.

(2) If, as seems highly probable, the pollen of Selala is impotent and the ovules fertilisable, then its seedless condition implies not only an incapacity for self-fertilisation, but also for cross-fertilisation; and if Selala with its impotent pollen does not admit of cross-fertilisation, this would still less be expected of Rhizophora mucronata and R. mangle where the pollen is potent and where fertilisation takes place in the bud. I endeavoured to fertilise the Selala flowers with the pollen of the two other species; but there were no results, the flowers falling off in a few days. It may here be remarked that on one Selala tree I found a solitary flower with an enlarged ovary, as if through fertilisation.

(3) It is not easy to explain the gregarious growth of the Selala if it is a seedless hybrid. The colonies could not be renovated by mere intercrossing, especially in places where, as on the north coast of Vanua Levu, the dense belt of mangrove is for many miles composed in mass of Selala trees, with a few trees of the Asiatic and American Rhizophoras growing on the outskirts.

It is obvious that in order to clear the way for considering this problem the means of renovating the Selala colonies should be inquired into. In the first place, whilst seedlings occur in numbers under the trees of the other two Rhizophoras they are never to be found under the Selala trees. The mode of reproduction of the Selala is evidently vegetative, and the question arises as to what mode of vegetative reproduction occurs. The Selala trunks, as already observed, are often inclined, the trunks being supported on trestle-like aërial roots. These trunks send out branches which in their turn drop aërial roots; and when the decay of the parent trunk takes place, the branches are able to live independently. The primary branches in due time send out secondary branches which also let fall aërial roots; and thus the process is repeated indefinitely, the result being a maze of semi-prone trunks, branches, and aërial roots. The first stage of the process ends with the death of the parent trunk, and the primary branch, supported by its own aërial roots, is often all that the observer can distinguish in the centre of a colony. This is evidently the mode by which the Selala colonies are renovated in their interior. One sometimes observes in the midst of one of these colonies extensive bare mud-flats 100 to 500 yards across from which apparently the trees have died off en masse. The natives assert that when part of a Selala tract is cleared the trees never grow again.

But pari passu with this process of vegetative reproduction of the Selala, by which the mass of the colony is preserved and renovated, there is evidently some other process of reproduction in operation amongst the trees of Rhizophora mangle and R. mucronata at the edge of the colony, as a result of which Selala seedlings are produced. Whilst no seedlings are to be observed striking into the mud under the Selala trees, numbers occur, as before observed, under the trees of the other two species. Those under the trees of R. mangle possess in nearly all the cases the distinctive leaf-characters of that mangrove, and would be recognised at once as belonging to that species. On the other hand, those beneath the trees of R. mucronata are of two kinds, some of them being readily recognised by their foliage as of the Selala type others, again, being typical seedlings of R. mucronata. Only those seedlings, or “keimlings” as we might term them, were noted that had dropped plumb from the branches above.

Such were the results of my investigations on Vanua Levu. My field of inquiry was then shifted to the Rewa delta, where, with the assistance of the Daku natives, who, like most Fijians, display a keen interest in matters relating to their plants, I spent a few days in investigating the origin of the Selala trees that grow sporadically in that locality. On pulling up some of the young trees we found that the original radicular or hypocotyledonary portion of the keimling could be still distinguished. My zealous native friends also pointed out to me that though the leaves in form and colour were those of the Selala, the rootstock was reddish like that of R. mucronata, and not white as with R. mangle. The natives averred that the Selala trees are produced in the first place from fruits of R. mucronata. When young, they said, they are Tiri-tambuas (R. mucronata), but when old, Selalas. Yet although R. mucronata may be now regarded as the source of the Selala trees, and my Vanua Levu observations pointed unmistakably in this direction, it could not be definitely settled whether this was the result of a cross with the male element of R. mangle or whether the Tiri-tambua (R. mucronata), in producing two types of seedlings, one fertile with the parent characters and the other seedless of the Selala type, brought about the same end. On the whole I am inclined to the view that the Asiatic Rhizophora presents us in the dimorphism of its seedlings the true explanation.

This inference is supported by the behaviour of Rhizophora mangle on the coast of Ecuador, a subject which is discussed in [Chapter XXXII], and I have given the results of my observations on the Ecuadorian Rhizophoras side by side with those on the Fijian trees in the table before given. There are two very distinct forms of the American Rhizophora (R. mangle) in the swamps of Ecuador. There is the low coast tree, the “Mangle chico” of the Ecuadorians, ten to fifteen feet in average height, which grows on the sea-front of the swamps and has all the general appearance and the more conspicuous characters of the American Rhizophora in Fiji. There is also a tall tree, 60, 80, or even 100 feet high, that forms the great mass of the mangrove swamps. In its inflorescence, in the dark green colour of its foliage, and in other characters, it comes near the Fijian Selala; but it differs in fruiting abundantly. This is locally termed the “Mangle grande,” and its true relation to the Fijian Selala appears to be as follows. Whilst both as regards the flowers approach the Asiatic Rhizophora (R. mucronata), the Fijian Selala resembles the Asiatic tree also in its foliage, whilst the “Mangle grande” or the Ecuadorian Selala more resembles the typical American tree (R. mangle) in its leaves and also in its seedlings. Here in the Ecuadorian swamps there can be no question of crossing, since both, according to Baron von Eggers, belong to one species. Therefore I am inclined to the opinion that whilst the Asiatic Rhizophora displays dimorphism in Fiji, the American Rhizophora displays dimorphism in Ecuador. The reversion on the part of the “Mangle grande” of Ecuador to some of the characters of the Asiatic plant is remarkable, and points to the greater antiquity of the Asiatic R. mucronata as compared with the American R. mangle.

This accords with the opinion expressed by Schimper in his work on the Indo-Malayan strand flora that the American Rhizophora is either a degenerated descendant of the Asiatic R. mucronata or a sister form derived from a common ancestor. America, as we have seen, possesses only one of the three species of Rhizophora, and this is the only representative that it owns of the four Asiatic genera (Rhizophora, Kandelia, Ceriops, Bruguiera) that constitute the tribe Rhizophoreæ. The rule prevailing with current-dispersed plants that America is a distributor and not a recipient evidently does not apply to the Rhizophoreæ; and to explain their distribution we must go back to some epoch very remote from the present. That Fiji derived its representatives of Rhizophora mangle from America by the agency of the currents I do not for a moment admit. The restriction of the species and indeed also of the genus to the Western Pacific is very significant. It is far more likely that, as I have pointed out in the case of Lindenia (see page [396]), the American Rhizophora was once widely distributed over the tropics of the Old and New Worlds, and that it is now on the “down grade” towards extinction. Its survival in the Western Pacific could thus be explained without our being obliged to suppose that the seedlings or keimlings have been carried uninjured across the Pacific Ocean, an ocean voyage for which, as shown in a later page, they are not well fitted.

The Occasional Occurrence of more than one Seed in the Fruits of Rhizophora mucronata and Rhizophora mangle (Polyembryony).

The bilocular ovary contains four ovules, one of which only as a rule becomes a seed. But it is incorrect to say that the fruits are always one-seeded, since two or even three seeds are occasionally produced, and they may all germinate. In November, 1897, I noted eight hundred fruits of Rhizophora mangle germinating on the trees in one of the creeks of the Rewa delta. Out of this number eight fruits had two germinating seeds and one had three, the protruding radicles being in all stages of growth. Just two years afterwards I counted eight hundred more fruits in the same locality, and then observed seven with two germinating seeds and none with three, the radicles protruding in all cases. On another occasion at Wailevu in Savu-Savu Bay I counted four hundred, and none had more than a single radicle protruding. The results appear to vary with the locality, but in the Rewa creek the proportion of fruits in which more than one seed germinated was fairly constant at dates two years apart, namely, about one per cent. Occasionally, however, in particular localities a greater proportion may be noticed. Thus near Daku in the Rewa delta I found that the proportion was between two and three per cent. for the same species (R. mangle), those with three germinating seeds being about half per cent.

The case of more than one seed germinating in the fruits of Rhizophora mucronata never came under my observation; but in one locality, where I examined a considerable number of fruits near the stage of germination, between ten and fifteen per cent. showed two seeds approaching maturity.

Warming thoroughly investigated the polyembryony of Rhizophora more than twenty years ago, seemingly from materials brought to him from the West Indies (Engler’s Botanische Jahrbücher, band iv., 1883). With the usual German thoroughness he deals with the work of earlier observers, and goes back to Piso in the middle of the 17th century. Of the four ovules, he remarks, three usually abort, and only in rare cases are two seeds developed. He quotes Baron von Eggers to the effect that only in three per thousand cases was more than one seedling observed protruding from a germinating fruit. These remarks evidently all apply to the American species. I do not find any reference in my notes to polyembryony in Ecuador, and evidently its occurrence is not so frequent there as in Fiji.

It is frequently apparent in the cases where more than one seed germinates in a fruit that on account of the difference in the length of the protruding seedlings germination does not always begin at the same time. Thus in Fiji the difference in the length varied between one and three inches, an amount representing at least from ten to twenty days’ growth, as will be subsequently pointed out. Warming in one of his figures gives a fruit where an interval of some months seems to be indicated, since one of the seedlings has fallen out and the other is protruding less than an inch. By cutting across a fruit containing two seeds one may sometimes observe one seed quiescent and the other beginning to germinate. The significance of this occasional interval between the germination of seeds in the same fruit will be referred to in a later page.

The Seasons of Flowering and Fruiting of the Species of Rhizophora in Fiji.

The Selala flowers all the year. With the two American and Asiatic species there are considerable variations between different localities. Generally speaking, they flower and fruit all the year through; but the flowers are usually less abundant in the warm season from December to February, and the germinating fruits which are to be observed on the trees every month of the year are more numerous in that season.

The History of the Reproductive Process in Rhizophora from the Fertilisation of the Ovule to the Falling of the Plantlet or Seedling from the Tree.

I devoted great attention to this subject in the instance of Rhizophora mangle, being desirous of determining two points, in the first place as to whether there was any period of rest between the maturation and germination of the seed, and in the second place as to the period that elapsed between the commencement of germination and the fall of the seedling.

The principal change in the ovary for the first three or four weeks after fertilisation is shown in its increased breadth. The increase in height is but slight during this period; and in fact after thirty days the ovary only added 2 millimetres to its original height of 3 millimetres. After this the growth of the fruit proceeds until the tip of the radicle pierces its summit, the fruit being then about eleven lines (2·8 cm.) long. From the date of fertilisation to the time the radicle pierces the top of the fruit a period of about fifteen weeks elapses. (The fruit, it should be here remarked, continues to grow in length and breadth after the radicle has protruded, attaining a length of thirteen or fourteen lines [3·5 cm.] when the seedling or “keimling” is ready to fall.)

By referring to the table below it will be observed that there is no period of rest in the growth of the fruit up to the date of the protrusion of the radicle. It will now be shown that there is normally no pause between the epoch of the maturation of the seed and the beginning of germination, or, in other words, that from the time of the fertilisation of the ovule to the onset of germination there is no cessation in the process of growth of the embryo. That period of dormant vitality which almost all seeds pass through forms no normal feature in the life-history of this species of Rhizophora.

Rhizophora mangle and R. mucronata.

Rhizophora mangle.
Growth of fruit in height.
Lines or tenths of an inch (millimetres in brackets).	Number of days since fertilisation.
2 (5)	30
3 (7·5)	42
4 (10)	50
5 (12·5)	61
6 (15)	67
7 (17·5)	74
8 (20)	83
9 (22·5)	92
10 (25)	100
11 (28)	105
	107 { Protrusion of the hypocotyl
Growth of the protruding hypocotyl.
10 (25)	127
20 (50·5)	141
30 (76)	151
40 (101·5)	160
50 (127)	167
60 (152)	175
70 (177·5)	185
80 (203)	202
90 (228)	222
	229 { Fall of the seedling

Explanation of the Table.

We have here shown the period between fertilisation and the fall of the seedling from the tree.

This period divides itself into two parts, the first being concerned with the continuous growth of the fruit and of the inclosed embryo until the tip of the hypocotyl appears through the apex of the fruit, the second being indicated by the growth of the protruding hypocotyl until the fall of the seedling.

The height of the fruit is measured from the base of the calyx-lobes, and the length of the hypocotyl at first from the apex of the fruit and afterwards from the edge of the protruding neck of the cotyledonary body. The height of the ovary at the time of fertilisation is about 3 millimetres; and from that time onward it is to be regarded as a fruit.

[To face page [452].

FIGURES ILLUSTRATING THE DEVELOPMENT OF

THE SEED AND THE GERMINATING PROCESS

OF RHIZOPHORA AND BRUGUIERA

(Natural size. Drawn for convenience of description in the erect position.)

1. Rhizophora mucronata

Fruit 3¹⁄₃ lines (8-9 mm.) high, six to seven weeks after fertilisation. The micropyle is but slightly dilated, and is occupied by a small plug of endosperm.

2. Rhizophora mucronata

Seed of fruit represented in Fig. 1.

3. Rhizophora mucronata

Fruit 5 lines (12 mm.) high, eight to nine weeks after fertilisation. Germination is about to begin. A large plug of endosperm now protrudes through the dilated micropyle, but still covers the lengthening hypocotyl.

4. Rhizophora mucronata
5.

Seed of fruit represented in Fig. 3. In Fig. 4 the plug of endosperm is shown on the upper end of the seed; whilst in Fig. 5 it has been removed, exposing the tip of the hypocotyl.

6. Rhizophora mucronata

Fruit 7¹⁄₂ lines (18 mm.) high, eleven to twelve weeks after fertilisation.

7. Rhizophora mucronata

Fruit, seventeen or eighteen weeks after fertilisation.

8. Rhizophora mucronata

Full-grown fruit with upper portion of seedling just before detachment from the tree. The long tapering plumule is here inclosed in the cotyledonary body. The * indicates the point of detachment of the seedling.

9. Rhizophora mucronata

The cotyledonary body of Fig. 8.

9A.
9B. Rhizophora mucronata
9C.
9D.

Illustrating different stages in the development of the plumule and of the neck of the cotyledonary body resulting finally in the expulsion of the plumular end of the seedling from the fruit cavity as in Fig. 8. (See page [458].)

10. Rhizophora mucronata

Fruit with two seeds.

FIGURES ILLUSTRATING THE DEVELOPMENT OF

THE SEED AND THE GERMINATING PROCESS

OF RHIZOPHORA AND BRUGUIERA—(continued)

(Natural size. Drawn for convenience of description in the erect position.)

11. Rhizophora mangle

Fruit, six weeks after fertilisation.

12. Rhizophora mangle

Seed with plug of endosperm, as shown in Fig. 11.

13. Rhizophora mangle

Fruit, eight weeks after fertilisation. The tip of the hypocotyl is now piercing the plug.

14. Rhizophora mangle

Embryo (enlarged) shown in Fig. 13.

15. Rhizophora mangle

Fruit, ten weeks after fertilisation. The growing hypocotyl has now pierced the plug.

16. Rhizophora mangle

Embryo shown in Fig. 15.

17. Rhizophora mangle

Fruit, nearly sixteen weeks after fertilisation.

18. Rhizophora mangle

Full-grown fruit, just before the detachment of the seedling from the tree. The long tapering plumule is inclosed in the tube of the cotyledonary body. The point of detachment of the seedling is indicated by *.

19. Rhizophora mangle

The cotyledonary body of Fig. 18.

20. Rhizophora mangle

Fruit with two seedlings in different stages of growth (given in the first plate).

21. Bruguiera Rheedii

Fruit, about four weeks after fertilisation. (The shaded portion is the calyx-tube or cup, in the midst of which rises the style.)

22. Bruguiera Rheedii

Germinating seed.

23. Bruguiera Rheedii

Germinating fruit, about eight weeks after fertilisation.

24. Bruguiera Rheedii

Germinating fruit, about ten weeks after fertilisation. Here the growing hypocotyl, carrying the style with it, has pushed upwards the lining membrane of the floor of the calyx-tube, which has ruptured and forms a cap on its extremity.

25. Bruguiera Rheedii

Germinating fruit, thirteen or fourteen weeks after fertilisation.

26. Bruguiera Rheedii

Fruit with full-grown seedling just before its detachment from the tree.

[To face page 453.