Mottled Leaves

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Mottled Leaves

Palms are normally thought of as being relatively colourless and plain in terms of their leaves. The majority of them are mid to dark green with some developing a waxy glaucousness where the leaf has to be protected from dryness, excessive temperature variations or wind. A few have silvery-lepidote undersides to the leaves, such as Kerriodoxa elegans from Thailand or Mauritiella and Astrocaryum from the Neotropics. Most palms are of course rainforest plants and don't need to be glaucous so have mid green leaves which are suited to catching as much light as possible.

There are, however, a number of species with curious and very attractively mottled leaves. These plants are often termed variegated, and in reality the mottling is a form of variegation, but the term conjures up visions of the mutant striped foliage of plants which have an inability to produce chlorophyll evenly in the tissues as they form in the meristem. There are palms with such variegation, but these are abnormal within the population of that species and normally experience such disadvantages, due to their lack of normal vigor, that they do not persist in the wild. Such variegated palms are therefore rare and can only be maintained under cultivation.

Of the mottled palms, the variegation is a quite different form. Rather than occurring in streaks which are really chlorophyll-free tissues that form as the new growth is "extruded" from the growing point, and are therefore parallel to the venation of the plant, mottled palm leaves show a very organised patterning of darker tissue areas surrounded by lighter tissues, or in some, by patterns of surface texture which refract the light, giving a silvery, almost metallic appearance. The feature is most common in juvenile plants, many of which lose the mottling as they mature, but some retain it into adulthood, like Pinanga densiflora or Pinanga maculata.

Most mottled palms are shrubs of the rainforest understory where they spend their entire lives in the humid and dark confines under conditions of relatively intense space competition, having tree seedlings, other palm seedlings and specialised ground-living plants to deal with. They also have to deal with grazing animals such as deer, wild pigs, land snails and insects In some areas of Malaysia the forest fauna contains wild cattle, tapirs and primates, which can be very destructive. Life for the tiny understory palm is not as easy as it looks from a first glance. There may be an explanation for those that have developed mottling in this fact.

Apart from a species of Geonoma mottling of understory palm leaves never became widespread in the Americas, despite there being a vast array of shrubby palms there in an even wider array of habitats. For some reason it has not been part of their evolutionary development. Whether this means that the pressures that encourage the development of mottled leaves elsewhere are not present or is a reflection of the genetic background of the plants is hard to say, possibly it is a combination of both factors.

It is in the South-East Asian tropics where leaf mottling has attained such heights of development as to be almost normal amongst at least one genus, Pinanga. This genus has about 120 known species of very specialised plants, some of which are so unlike the generic "norm" that their flowers have to be seen to place them. Most are clump-forming shrubs with pinnate leaves that are variously divided into groups of leaflets rather than being perfectly freepinnate. Many have completely undivided leaves or leaves with a mere two or three major leaflet assemblages along the rachis.

A few are tall trees which compete in the lower canopy level but many are tiny plants of the leaf litter and ground herbage zone. It is amongst these latter that mottling is conspicuous, or rather not conspicuous at all for some are of such unplant-like colours and patterns that they are very well hidden and are difficult to see. P. veitchii is such a palm, the leaves are a brownish colour with the appearance of being dry. There is a certain degree of metallic surface texture which gives the leaf a somewhat velvet-like cast and the mottling breaks up the outline of the leaf very effectively.

It is not just Pinanga that does this, although this genus is the mottle-specialist, because a recently discovered Licuala ( L. mapu) has been found with mottled leaves. The mottling is quite unlike that of Pinanga however, and seems to make the plant more conspicuous. Licuala is exceedingly diverse in SouthEast Asia. With over 108 species, most are in Malaysis, but a secondary area of diversity exists in New Guinea. Borneo seems to have the most interesting array of Licuala species, some of which are considered to be sensational ornamental plants and are now experiencing quite mercenary collecting at the hands of commercial collectors. Some of these highly attractive species occur in very limited populations and have to face habitat loss as well. They probably grow slowly in the wild and have considerable competition to deal with, so excessive seed collecting may be detrimental to them in the long term.

Just why a palm would bother to evolve towards heavily mottled leaves is something of a mystery. Perhaps it is a defence against grazing herbivores who may not recognise the leaf as being fresh, or do not notice its shape and associate it with a tender palm. The same could be said of palms which have brightly coloured new growth, perhaps in an attempt to disguise the soft new growth as something inedible.

Why mottled palms tend to lose this feature as they mature is also something of a mystery. Perhaps the mottling is no longer needed as a defence against herbivores when the plant is large enough to stand out of reach.

A mottling that is very similar to that of most Pinanga occurs in other plants. There are several species of Pandanus in Malaysia and Indonesia, and at least one in Madagascar which display this feature, but again, in the case of the Asian pandans, the mottling is lost as the plant matures. The mottling can be lost much sooner in cultivated plants if nutrients and light management are not carefully aftended to. In the botanic gardens we have found that mottled Pinanga will become a rich, uniform dark green if heavily fertilized and kept in bright light. Plants of Pinanga disticha that were treated in such a way lost their attractive patterning within weeks. Experiments show that growing them in a nutritionally poor soil, such as granite sand with a bit of peat and placed in relatively dark areas will encourage a bright patterning. This poor environment imitates the leached rainforest topsoils that the plants have to deal with in the wild. The plants should be fertilized occasionally with very dilute general fertilizer and kept damp constantly. They will not grow robustly but will make an attractive specimen plant.

Contributed by:

Robert Tucker
Reproduced from Palms & Cycads No. 37, Oct-Dec 1992.

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