|
The Plant
Coconut (Cocos nucifera) is a monocotyledon belonging to the order Palmae.
It is the sole species of the genus Cocos. (Cocos = monkey face, nucifera
= nut bearing) Trees are generally unbranched, erect, cylindrical, pillar-like stem
reaching up to 25-30 m in height. Trunks may reach a diameter of 50 cm. The trunks
are ringed with scars where old leaves have fallen. The top of the trunk is crowned
with a rosette of leaves and a branched inflorescence enclosed in a sheath collectively
known as the spadix.
The stem
The stem is usually unbranched, light grey, smooth and erect or slightly curved
Stem rises from a swollen base, the bole. Growth of stem originates from one terminal
bud in the center at the top of the stem. The first years after germination, only
very short internodes are developed from which sprout many adventitious roots. Only
when the full width of the stem has been reached (after 4 years for talls and 2-3
years for dwarfs) elongation of the stem begins, and the stem emerges from the ground.
As the stem has no outer cambium it does not grow laterally and wounds in the stem
will always remain visible as they will not be covered by new growth. Steps cut
into the stem for climbing will remain for as long as the palm lives. Under normal
conditions, the stem diameter remains the same. Under unfavourable conditions the
diameter of the stem will decrease.
The branching of stem results from slight damage of the growing point, or from severe
stress . The stem consists of a central cylinder surrounded by a narrow band of
tissue, the cortex, called 'bark', about 1 cm thick, somewhat thicker at the base
of the stem. The central cylinder consists of a parenchymatic tissue enclosing vascular
and fibrous bundles. The vascular bundles at the centre are more widely spaced than
at the periphery.
The surface of the cortex shows a pattern of triangular-shaped leaf scars, marking
the stem where former leaves have been attached. Between the leaf scars there are
unscarred areas, the internodes. The distance between the leaf scars shows the growth
rate of the palm. For instance, if a coconut palm produces about 14 leaves per year,
the total distance between 14 leaf scars represents 1 year of stem growth.
The leaf
Every tree has a rosette of leaves at the top of the trunk consisting of opened
leaves and those in the bud in various stages of development. The length of a leaf
depends on variety, growing conditions and age of the palm. It may reach up to 7
metres in tall palms, with a leaf area of up to about 10 m2, and weighing up to
20 kg when green.
Leaf primordia are differentiated about two years before appearing as a central
unopened spear. The phase of elongation within the spear takes about 6 months and
mature leaves may remain on the palm for about three years, sometimes even longer.
The total number of unfolded leaves usually varies between 30 and 40, depending
on growing conditions and variety. Mean annual leaf production is about 14-16 for
mature tall palms and 21 for dwarf palms.
All leaves originate from the growing point at the apex of the stem. The youngest
leaves are folded and wrapped together. They form the central spear, or heart of
the palm. The central cylinder of the spear, is white and soft. It is considered
as a delicacy and is called 'millionaire's salad'. From the spear, the leaves gradually
unfold and expand. They are arranged in spirals, running clockwise in some palms,
anti-clockwise in others. The crown of a mature coconut palm growing under favourable
conditions forms an almost full sphere. The oldest leaves are found in the lowest
whorls, and a few withering leaves may hang alongside the stem before dropping.
The length of the petiole is about one-quarter of the total leaf length, but the
ratio differs with variety. The petiole continues as the mid-rib of the leaf. Weak
leaves may collapse by the weight of heavy fruit bunches resting on the leaf stalk.
The petiole is attached to the stem by means of a sheath in the form of a bracket
firmly clasping the stem with its wings almost around it.
The leaflets growing out from both sides of the mid-rib have different lengths.
The first leaflets at the base are short, the following leaflets gradually increasing
in length, reaching a maximum of about 130 cm at about one-third of the mid-rib,
gradually becoming smaller again towards the tip of the leaf. The smallest leaflets
at the tip may be only 25 cm long. The longest leaflets of dwarf palms may not be
longer than 80-110 cm. Length and width of leaflets vary with variety. The total
number of leaflets of a mature palm leaf varies from about 200 to 300. Thick cuticles
cover the leaf blades. The lower epidermis is thinner than the upper epidermis.
Stomata are located in the lower epidermis.
Flowers
The inflorescence of the coconut palm is a spadix that develops in the axil of each
leaf. Thus, the number of leaves produced annually also determines the number of
spadices developed. Initiation of the spadix begins about 33 months before its opening.
Differentiation of the sheath takes place about two years before its opening and
differentiation of the spikes about six months later. Flower primordia are initiated
about one year before opening.
Before opening, the spadix has a spear-like shape, with a length of about one metre.
The inflorescence is enclosed within the sheath, branches or spikes of the inflorescence
lying close to the main axis. When the inflorescence develops within the sheath,
the latter finally splits open over its entire length, about three months after
its appearance, the inflorescence emerging and unfolding. The spadix consists of
a main axis with 20-65 branches, or spikes, bearing flowers. Female flowers are
located at the base of the spikes, the rest of the spike being fully covered by
male flowers. These flowers are sessile. Each spike may have one or more female
flowers and generally about two to three hundred male flowers.
The total number of female flowers per inflorescence usually varies between twenty
and forty. Dwarf palms generally carry a higher number of female flowers in the
spadix. There is a great variability in this characteristic which, apart from being
determined genetically, is also strongly influenced by growing conditions. First
spadices usually have fewer female flowers per inflorescence than spadices in mature
palms. Palms producing large nuts usually have fewer female flowers than palms producing
smaller nuts. Abnormalities, such as inflorescences with only male flowers also
occur.
The female flower is much larger than the male flower and its shape resembles a
very small coconut. Its size continues to enlarge between the opening of the sheath
and receptivity. Receptivity in tall palms begins about 22-24 days after the opening
of the sheath, in most dwarf palms this period is only 2-7 days and in D x T hybrids
and some green dwarfs it is about 16-21 days. At that stage the diameter of the
female flower is about 3 cm. The ovary is tricarpus and each carpel has a single
ovule, of which normally only one develops while the other two degenerate. But bicarpelate
and tricarpelate nuts can sometimes be found. When the female flower becomes receptive,
it opens at the apex and the three sessile stigmas protrude from it like a three-pointed
star. Necter is produced at their bases and at the three pores on the pericarp towards
the top of the ovary. Receptivity of a female flower of a tall palm lasts about
three days and of a dwarf palm about two days, after which the stigmas turn from
white to pinkish-brown. The female phase, or the period between the beginning of
receptivity of the first female flower and the end of receptivity of the last female
flower lasts about 5-7 days in tall palms and in some green dwarfs. Generally, the
first male flower opens immediately after the splitting of the sheath, starting
with the male flowers located at the sides of the female flowers and those at the
tips of the upper spikes. Opening of the flowers continues from the tips of the
spikes towards the base. After shedding their pollen, male flowers wither and fall
off, usually within two days following their opening. Duration of the male phase
of a coconut palm, that is the period between the opening of the first male flower
and the shedding of the last, is about 18-22 days, depending on variety and growing
conditions
Fruit
Coconuts are large, dry drupes, ovoid in shape, up to 40 cm long and 30 cm wide.
The exocarp or skin is green, yellow, or bronze-gold, turning to brown, depending
on cultivar and maturity. The mesocarp is fibrous and dry at maturity; the product
coir is derived from this layer. The endocarp is the hard shell enclosing the seed.
Seeds are the largest of any plant, and have a thin brown seed coat. Seeds are filled
with endosperm, which is solid and adherent to the seed coat, and also in liquid
form. Copra is derived from the solid endosperm. The seed has 3 germ pores in the
proximal end. Two pores are generally plugged, and the embryo lies beneath the third
functional pore. A "blind coconut" has all 3 pores plugged. The embryonic root and
shoot emerge through the functional pore, while the cotyledon forms a spongy mass
of tissue within the seed cavity, absorbing the endosperm, which fuels initial growth.
Seeds germinate within intact coconuts, and for up to a few months, young palms
can remain rooted within the mesocarp. Dwarf cultivars are precocious and begin
fruiting in 3 years, whereas tall cultivars require 6-10 years. Fruit require about
one year to mature.
The root system
Being a monocotyledon, the coconut has no tap root. Adventitious roots develop from
the bole. The first root is developed at germination and may reach a length of about
5 cm in the first week of growth, during which time also one or more rootlets may
have developed. Rootlets are produced in quick succession, radiating in all directions,
and breathing roots or pneumatophores are also found on the first root. The second
root is formed about 10 days after the first. Six weeks after germination, when
the shoot becomes visible above the husk, the seedling may have three roots, the
longest measuring about 20 cm. Under conditions of high humidity and wounding of
the stem, roots may also develop from the stem above the bole, even at great height.
When covered with soil, these roots grow out normally and develop secondary roots
and rootlets. Old roots die and new roots develop continuously.
The number of primary roots increases with the age of the palm. The total number
of primary roots depends on the variety and health of the palm, as well as on soil
conditions such as fertility, humidity and cultural practices. Primary roots are
about 1 cm in diameter and of uniform thickness. There is a continuous production
of secondary roots. The root colour varies from almost white at the growing point
through cream colour to scarlet, brown and dark brown on the oldest part. This process
takes about 15 years. The roots have no root hairs. Their cortex is covered by a
thick, hard impervious layer, the hypodermis , which does not allow penetration
of water or air. Water and minerals are absorbed only in the white zone about 2-5
cm behind the tip of the root. The tip is covered by a root-cap.
Top
|