Lodoicea

Lodoicea, commonly known as the sea coconut, coco de mer, or double coconut, is a monotypic genus in the palm family. The sole species, Lodoicea maldivica, is endemic to the islands of Praslin and Curieuse in the Seychelles, and was historically found on the neighboring small islets of St Pierre, Chauve-Souris, and Ile Ronde. The species has the largest seed in the plant kingdom.

Taxonomy

The name of the genus Lodoicea was given by Philibert Commerson, without any explanation for its etymology. It may be derived from Lodoicus, a Latinised form of Louis, in honour of King Louis XV, while other sources say that Lodoicea is from Laodice, the daughter of Priam and Hecuba.
Lodoicea belongs to the subfamily Coryphoideae and tribe Borasseae. Borasseae is represented by four genera in Madagascar and one in Seychelles out of the seven worldwide. They are distributed on the coasts surrounding the Indian Ocean and the existing islands within. Borassus, the genus closest to Lodoicea, has about five species in the Old World, one species in Africa, one in India, South-East Asia and Malaysia, one in New Guinea and two species in Madagascar.

Description

It generally grows to tall. The leaves are fan-shaped, 7–10 m long and 4.5 m wide with a 4 m petiole in mature plants. However, juveniles produce much longer petioles, up to long. It is dioecious, with separate male and female plants. The male flowers are arranged in a catkin-like inflorescence up to long which continues to produce pollen over a ten-year period; one of the longest-living inflorescences known. The mature fruit is 40–50 cm in diameter and weighs 25–45 kg, and contains the largest seed in the plant kingdom. The fruit, which requires 6–7 years to mature and a further two years to germinate, is sometimes also referred to as the sea coconut, love nut, double coconut, coco fesse, or Seychelles nut.
While the functional characteristics of Lodoicea are similar to other trees of monodominant forests in the humid tropics, its unique features include a huge seed, effective funnelling mechanism and diverse community of closely associated animals. These attributes suggest a long evolutionary history under relatively stable conditions. Of the six monospecific endemic palms in Seychelles, Lodoicea is the "only true case of island gigantism among Seychelles flowering plants, a unique feature of Seychelles vegetation". It holds a number of botanical records:

It produces the largest wild fruit known, weighing up to, although domesticated pumpkins and watermelons can be much heavier.
The fruit is composed of three carpels which are the largest of any flowering plant.
These fruit are the slowest to mature, requiring 8 to 10 years.
The mature seeds weighing are the world's heaviest.
The seed upon germinating, produces the longest known cotyledon, up to. and on occasion as long as.
It is the slowest growing of all large trees, although some small to medium-sized desert trees are slower. At the Peradenaya Royal Botanic Gardens, it grew an average of per year over a period of 40 years.
The female flowers are the largest of any palm, up to diameter.
The male catkins, up to in length, are the longest known.
The sepals, which grow with the fruit, are the largest known; up to long by wide.
The leaves of Lodoicea have the longest lifespan of any monocot, nine years to develop in the terminal spike, and then nine more years as a fully functioning leaf. However, adult Lodoicea can have as many as twenty leaves with a potential lifespan of 24 years.
Finally, Lodoicea is the most efficient plant known at recovering nutrients from moribund leaves.

Of the six endemic palms in Seychelles, it is the only dioecious species, with male and female flowers on different plants.

Habit

L. maldivica is robust, solitary, up to 30 m tall with an erect, spineless, stem which is ringed with leaf scars. The base of the trunk is of a bulbous form and this bulb fits into a natural bowl, or socket, about in diameter and deep, narrowing towards the bottom. This bowl is pierced with hundreds of small oval holes about the size of a thimble with hollow tubes corresponding on the outside through which the roots penetrate the ground on all sides, never, however, becoming attached to the bowl; they are partially elastic, affording an almost imperceptible but very necessary "play" to the parent stem when struggling against the force of violent gales.

Leaves

The crown is a rather dense head of foliage with leaves that are stiff, palmate up to 10 m in diameter and petioles of two to four metres in length. The leaf is plicate at the base, cut one third or more into segments 4–10 cm broad with bifid end which are often drooping. A triangular cleft develops at the petiole base. The palm leaves form a huge funnel that intercepts particulate material, especially pollen, which is flushed to the base of the trunk when it rains. In this way, L. maldivica improves its nutrient supply and that of its dispersal-limited offspring.

Flowers

The clusters of staminate flowers are arranged spirally and are flanked by very tough leathery bracts. Each has a small bracteole, three sepals forming a cylindrical tube, and a three-lobed corolla. There are 17 to 22 stamens. The pistillate flowers are solitary and borne at the angles of the rachis and are partially sunken in it in the form of a cup. They are ovoid with three petals as well as three sepals. It has been suggested that they may be pollinated by animals such as the endemic lizards that inhabit the forest where they occur. Pollination by wind and rain are also thought to be important. Only when it begins to produce flowers, which can vary from 11 to 45 years or more old, is it possible to determine the sex of the plant visually. The nectar and pollen are also food for several endemic animals e.g. bright green geckos, white slugs and insects.

Inflorescence

The inflorescences are interfoliar, lacking a covering spathe and shorter than the leaves. The staminate inflorescence is catkin-like, one to two metres long by about in width and produces pollen over a period of 8 to ten years. These catkins are generally terminal and solitary, but sometimes two or three catkins may be present. The pistillate inflorescences are also one to two metres long, unbranched, and the flowers are borne on a zig-zagging rachilla.

Fruit

The fruit is bilobed, flattened, 40 to 50 cm long ovoid and pointed, and contains usually one but occasionally two to four seeds. The epicarp is smooth and the mesocarp is fibrous. The endosperm is thick, relatively hard, hollow and homogenous. The embryo sits in the sinus between the two lobes. During germination, a tubular cotyledonary petiole develops that connects the young plant to the seed. The length of the tube is reported to reach about four metres. In the Vallée de Mai, the tube may be up to 10 m long.
L. maldivica was once believed to be a sea-bean or drift seed, a seed evolved to be dispersed by the sea. However, it is now known that viable nuts are too dense to float, with only rotted out nuts being found floating on the sea, explaining why the trees are limited in range to just two islands.

Habitat

Lodoicea maldivica inhabits rainforests where there are deep, well-drained soils and open exposed slopes; although growth is reduced on such eroded soils.

Phylogeny

Despite the proximity of the Seychelles to Africa, the broader diversity of palm life on the islands are considered to be slightly closer phylogenetically to that of south Asia; with members of the palm subtribe Oncospermatinae occurring both in the Seychelles group and in the Mascarene Islands, Sri Lanka, Borneo, the Malay Peninsula, and the Philippines. A genetic sequencing study of Lodoicea and other palms showed similarity between south Asiatic palms and Lodoicea. Lodoicea is one of four genera in the Lataniieae subtribe of the tribe Borrassae, and sequencing found them to be very closely related to Borassus and Borassodendron, although notably the phylogenetic placement of Lodoicea was among the least confident. The genera Borassus and Borassodendron together include species in Southeast Asia, Malaysia, India, New Guinea, and Madagascar; thus this study provided genetic evidence for the suspected close relationships between Lodoicea and south Asian palms.

Dispersal

Genetic similarity between Lodoicea and south Asian palms, despite their geographical distance, raises questions about ancestral historical dispersal of Lodoicea to the Seychelles; and this natural history of Lodoicea is further obscured by the geology of the Seychelles, as the entirety of the archipelago is composed of non-fossiliferous rock. As such, the prehistoric origins of Seychelles flora is inferred using circumstantial geological and botanical evidence.

Geologic drift-dispersal hypothesis

The ancestral dispersal to the Seychelles may have occurred as Tertiary palm relatives native to Gondwanaland rode the Indian subcontinent during its northward continental drift, whereupon populations were deposited on the modern day Seychelles. This hypothesis derives from the geologic formation of the Seychelles themselves, and offers a strong explanation of the modern day relationship of Lodoicea to Asiatic palms. The granite which forms the majority of the Seychelles archipelago was formed within the Indian subcontinent, and was deposited in its modern-day location in the Indian Ocean following the detachment and northern drift of the subcontinent from Gondwana, before colliding with modern-day south-Asia ~50 mya. Divergence between the palm populations would then follow from the isolation of the archipelago from the rest of Gondwonaland. Evidence suggests at least a proportion of the diversity of Flora on the islands are of "very ancient origin", perhaps being evidence of the persistence of some aboriginal Indian subcontinental species, of which the ancestors of Lodoicea may have been a member. The plausibility of this hypothesis is dependent on whether the Seychelles remained at least in part above sea-level for the duration of their formation, as a complete inundation with sea water at any point during the formation of the islands would have killed any aboriginal flora. Whether such an inundation ever occurred during the formation of the Seychelles is unknown.