Chapter 10 The palm – a model for success? A. Windsor-Collins1, D. Cutler2, M. Atherton1 & M. Collins1 1School of Engineering and Design, Brunel University, Uxbridge, Middlesex, UK. 2Royal Botanic Gardens, Kew, UK. Abstract The tree palm provides us with an unusual example of where evolution seems to bring about a reduction in macro structure complexity resulting in a long-lived but not necessarily structurally efficient living system ‘design’. The biological background to these durable and successful plants in terms of evolution has been investigated and the results are related to engineering design. The botanical design rationale is essentially survival, not infrequently in harsh weather conditions, and reproduction. More specifically, the mechanisms of the palm functions described offer potential for interpretation in engineering terms. Thus, the structure and features of the palm are discussed in general terms followed by the more detailed thermofluid aspects of the leaf. 1 Introduction Palms can take many different forms and although they are specialised, their morphology can vary greatly. For ease of reference and purposes of this chapter, the palm shall be divided broadly into three principal areas: the root, the trunk/stem and the crown. Palms derive from a monophyletic group of plants, meaning that they share a single common ancestor. They constitute a family of about 200 genera and 2600 species [1] and they belong to the family commonly named Palmae, but correctly known as Arecaceae. The double coconut (Lodoicea) produces the heaviest seed, which may be up to 20 kg in weight [2]. The fruit has an outer layer of tough, fleshy, fibrous material which protects it from predators and enables the seed to float on the water (one of the methods of seed dispersal). The seed takes about a year to germinate and about another year to form its first leaf. Palms can be found in rainforests, in the under-storey region of forests, at mid-canopy level and in the canopy itself as well as in the mangroves, mountain forest, the desert (e.g. Washingtonia) and river banks making them the most ecologically diverse family of plants [3]. Palms are adapted to a diverse range of environments. However, palms do not match the diversity of form found in dicotyledons (dicots). A large percentage of palms follow the same model in that they have simple flexible stems. Two-thirds of this family are associated with rainforests of Central and WIT Transactions on State of the Art in Science and Engineering, Vol 27, © 2006 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) doi:10.2495/978-1-85312-853-0/10 304 Design and Information in Biology South America and Southeast Asia. Some palm genera dominate large areas of forest (Hyphaene and Metroxylon [1]) while others grow in localised habitats. The longest stem recorded is about 200 m from a species of Calamus (Tomlinson, personal communication) making it twice as long as the tallest dicotyledonous and coniferous trees. These palms are familiarly called rattans and are not self-supporting when adult. However, they grow up into the forest canopy by the use of spines (epidermal emergences). Ceroxylon quinduiense has the longest, self-supporting, un-branched terrestrial stem in the world and is the national tree of Columbia, commonly known as the Andean Wax Palm. Ceroxylon quindiuense can reach heights of up to 60 m [2]. Its trunk is covered in a once commercially important white wax and is known as one of the cold hardiest species. It grows in very large groups as part of the forest and is often left when the forest is cleared because it seems the wood blunts chainsaws almost instantaneously. Although Ceroxylon quinduiense has the tallest free-standing trunk in the world, it is not necessarily the most structurally efficient un-branched stem in the world. Palm leaves vary widely in their shape and size as well as function. Raphia, a climbing palm, has the longest leaves which can be up to 25 m in length (Tomlinson, personal communication). Corypha umbellata however is a species of fan (palmate) palm that produces the world’s largest leaf [4] and can be up to 5 m wide. As palms are usually un-branched, they do not have as many points of attachment for leaves compared with branched trees. Palm leaves tend to be larger than those of other plants. Tree palms often emerge above the forest canopy and spread their leaves ensuring efficient light interception for photosynthesis. As well as having all of these extreme dimensions, palms are among the least branched terrestrial plants in the world. Their energy goes into extending the trunk and producing fruit, leaves and roots rather than lateral branching growth. However, branching occurs in the inflorescences and roots, and some palms branch basally just below the surface of the soil. Palms have survived very long on the evolutionary tree risking the destruction of the one and only apical bud, a kind of design compromise. Despite their apparently simpler structure, palms have survived as a recognisable group through millions of years. 2 Evolutionary theory and complexity 2.1 The simplicity of monocots One evolutionary route is for organisms to become more complex over time. However, there seems to be an anomaly with palms as, at first glance, they appear to have either retained a simple morphology or perhaps have evolved a less complex form; yet they are mostly tree-like. They are described as woody monocotyledons (monocots). They lack the vascular cambium found in most dicots and all gymnosperms and have a different method of stem thickening based on primary growth in thickness near the growing tip. Palms have hardened ground tissue and, as with many other types of trees, have developed to become mechanically strong rather than changing their form. Palm mechanics do not depend on their complexity of having an expanding living cylinder of tissues. They retain functional conducting cells throughout their lifetime. According to Pearson [5], gymnosperms are considered to be the first seed plants to develop on earth, although according to Ennos [6], the seed ferns pre-dated these. The angiosperms are by far the most successful and the most recent of the plant groups. The group contains around 80,000 species of trees compared to only 600 species in the other groups. The monocots arose soon after the dicots from which they ‘branched’ in evolutionary terms. It is thought that the first monocots were aquatic plants with their woody tissue in isolated strands throughout their body. This enabled WIT Transactions on State of the Art in Science and Engineering, Vol 27, © 2006 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) The Palm – A Model for Success? 305 them to resist stretching by water currents [6]. Although it is possible that the early pinnate and palmate leaves formed at the same time on the evolutionary scale, interestingly, pinnate leaves may also have derived from palmate leaves. Conifers are the most successful gymnosperms according to Ennos [6] and their wood contains only tracheids,novessels and minimal parenchyma, so conifer wood appears more uniform than the wood of angiosperms. Angiosperms have vessels which are normally wider in diameter than tracheids, along with numerous fibres. Angiosperm wood seems to be efficient at conducting water, though this may on occasion be at the expense of safety. Vessels are made up of a linear sequence of short vessel elements and the vessel elements have pores in their end walls. Vessels may reach several metres in length. Consequently, angiosperm wood as a whole transport water very efficiently, but is prone to embolism in dry or freezing weather. In other words angiosperm wood is particularly well suited to the climate in tropical rainforests where it is warm and wet. Palms are among these angiosperms. Most palm species do not grow as tall as the larger dicot trees. This may reflect mechanical constraints based on the internal structure of palms. Here it seems, secondary growth in thickness gives the dicots the advantage. This manifests itself by the presence of both niche and generalist palms. Perhaps the structure of the palm provides it with extreme physical flexibility. Adaptation to changing environments is one of the key features of evolution in which selection pressure drives change in form and physiology as well as many other aspects. Retaining a simple un-branched form over millions of years is of significance. It may reflect the efficiency of the early model or it may just represent a constraint that is tolerated while other features permit success. 2.2 Neoteny in palms The meaning of Neoteny means that reproduction occurs while an organism is still in, or maintains many characteristics of, its juvenile stage. An example of this may be in the palm Arenga caudata shown in Fig. 1 where it can be seen that the leaves are very large in comparison with the size of the rest of the palm. Takhtajan [7] suggested that neoteny played a significant part in the evolution of angiosperms. 2.3 Survival and other consequences of lack of branching in palms One of the ways in which the palm differs from dicot trees is with the number of branches it possesses [6]. Palm trees usually have no branches, apart from the root system and inflorescences, although they may be multi-stemmed or have creeping rhizomes which can bud from the base. The palm crown consists of closely packed, overlapping leaves which protect the all-important apical cell-producing meristem of which there is only one for each aerial stem; in this respect palms differ from most other dicot trees. From this single point in non-branched palms, all leaves emerge.