For most of us the mention of Palm tree’s conjures up the image of Coconut fringed white sandy beaches, warm sunshine, gentle trade winds and crystal clear warm blue ocean. Hardly ever is it appreciated that Palms belong to a very large family with around 3,000 species which are scattered throughout the warmer parts of the world from rain forests to cooler mountain areas, from arid deserts to tropical beaches.
Palms have been used extensively in tropical settlements and holiday resorts where they are favoured for there exotic forms, there very predictable size, low maintenance and providing welcome shade from the hot tropical sun. However, outside of the tropics in more temperate climates they have not been used to such an extent except in only a few places, for example Los Angeles or the french Riviera. In such locations, Washingtonia and Phoenix Palms have been used extensively on the streets and boulevards so creating the perception that palms are large plants more suited to wide open spaces than to smaller private gardens.
There is an extremely diverse number of species of palms of different sizes, colours, shapes and growing conditions. Some grow best in places that have hot, fairly dry summers and yet can tolerate winters with frequent frosts ( temperate ). Others are more suited to places where temperatures are relatively even with regular rainfall ( tropics ). Many species that are native to cooler climates will not grow in the tropics, rotting in the wet season when conditions are hot and very humid.
There are about 200 genera of palms with almost 3000 different species, mostly they are confined to moist tropical areas between 20 deg North & 20 deg South with at least 500 mm of rain per annum.The northernmost palm is the Mediterranean Fan Palm ( Chamaerops humilis ) which grows around the Mediterranean in Europe and North Africa; the southernmost is the Nikau Palm ( Rhopalostylis Sapida ) from New Zealand.
Some genera cover a very broad range outside of the tropics, examples of these are Sabal, Syagrus and Livistona, but generally most palms are more restricted in there range. Some may only occur on specific islands and are found no where else, examples of these are Junia, Hedyscepe, Hyophorbe and Brahea.
Palms have a very distinctive form which is easily identified from other plant families. There trunks can be singular or multiple, short or very tall, some are subterranean, others very slim and some are bottle shaped. Some palms retain there old fronds for a long time while others shed them readily leaving a smooth trunk.
Palm trunks do not have bark and do not develop true wood with growth rings as seen in most other trees. There trunks consist of bundles of conducting vessels scattered throughout softer tissue. This structure gives the palm great strength and flexibility allowing it to bend in strong winds.
The root system consists of numerous small roots. Many of the understory and rain forest palms produce most of there roots near the surface of the soil with some growing beneath the humus across the forest floor. Palms which occur in open forest or in areas with seasonal rainfall send most of there roots downward to anchor the palm and to search for moisture.
Fronds take on only two basic forms either PALMATE, that is fan shaped, or PINNATE, that is feather shaped fronds. Only one genus of palm has pinnate fronds which are further divided, this is Caryota, the pinnate fronds are divided again with the wedge shaped segments of the leaflets resembling the tail of a fish. These fronds are termed as being BIPINNATE .
Within both of the frond types there is considerable variation in size, shape and amount of division of the frond into segments. This variation is unmatched by any other plant family. Palmate fronds can be big or small, entirely without a segment as is the case with the tropical palm Licuala grandis, or divided with distinct segments, as is the case with Rhapis excelsa. Intermediate forms also occur and examples of these are Livistona and Pritchardia.
In some palmate fronds, the petiole does not terminate at a place on the base of the blade, but enters it so that the frond appears to be intermediate between palmate and pinnate. Fronds such as these are referred to as being COSTAPALMATE, some of the Sabal species are examples of this.
Pinnate fronds are divided into leaflets, each one being positioned along the frond stem or rachis. These leaflets can be narrow or wide, linear or wedge shaped. Some species such as Chamaedorea ernesti-augusti , the pinnate fronds are not divided but are entire. In others that are divided , the leaflets can be regularly arranged as is the case with Howea forsteriana or irregularly arranged as with Syagrus romanzoffiana, in this instance the leaflets are arranged in different planes which gives the frond a plumose or feather like appearance.
Mature palm fronds can vary in colour from deep green to pale green and from pale blue to silver or burgundy.The emerging new frond on Chambeyronia macrocarpa will be deep red, this will eventually change to the colour of the older fronds. In some populations of Archontophoenix the emerging frond can be bronze in colour, also later changing to be the same as the older fronds.
Palms usually produce very small and insignificant flowers. The inflorescences emerge from within, below or above the crown of fronds and for some species on lower parts of the trunk. These flowers can be unisexual or bisexual and the arrangement of the flowers can be of three basic sexual forms;
- MONOECIOUS – this form is where male and female flowers are present on the same or separate inflorescences but on the same tree.
- DIOECIOUS – this form is where male and female flowers are on individual inflorescences and on separate tree’s.
- POLYGAMOUS – where bisexual and unisexual flowers are combined in various ways on the same inflorescence. if the unisexual flowers are absent, the arrangement is termed to be HERMAPHRODITE and the tree has only bisexual flowers.
Once pollinated the fruits that form usually only contain one seed. The seeds are usually either ovoid ( egg shaped ), ellipsoid ( solid ellipse ), or globose ( almost spherical ) in shape and range vastly in size from a few millimetres, as with Washingtonia robusta to almost 500 mm as is the case with the ‘Double Coconut’, Lodoicea maldivica from the Seychelles Islands.