Roy Reynolds
Cycads are xeromorphic plants that look like palms, but are actually grouped with gymnosperms. Today, they are not a significant group of plants, but they were a dominant part of the vegetation during the Mesozoic Era. Within plants that produce seeds, cycads are unique in that they produce motile sperm. They are an ancient plant group and may be considered the sister group to all other seed plants. “The group is thought to have arisen from within the ancient seed ferns” (Cycad History). Cycads are found today in the tropical, subtropical, and warm temperate regions of the earth. This paper will describe the adaptations that have enabled cycads to out live the dinosaurs.
Cycads have three different types of roots: primary, coralloid, and adventurous.
The primary roots provide anchorage, uptake water and nutrients from the earth, and have the ability to contract. The main primary roots are thick, fleshy, and have the capabilities to store starch.
All known cycads have erect roots that look like coral and are known as coralloid roots. Within these coralloid roots there is a mutualistic relationship going on between the cycad and nitrogen fixing cyanobacteria. “Cycads provide fixed carbon and a stable environment to the cyanobacteria in exchange for fixed nitrogen” (Medeiros and Stevenson). When colonies of cyanobacteria are established their growth and development are controlled by the cycad. “The duration of a single cyanobacterial colony is entirely dependant on the life of the coralloid root within it is contained” (Medeiros and Stevenson). The relationship between the cycad and the cyanobacteria is focused on balancing the growth of the partners so an efficient exchange of nutrients can take place.
Adventurous roots can arise on the lower side of trunk offsets and swollen bubils. These roots grow downward thru the air in close proximity to the trunk and also often through the cortex.
Cycad trunks vary from being subterranean to a large columnar structure. The pachycaulos trunks are succulent like in having a well-developed pith and cortex with a minimal amount of secondary wood. The trunk also stores an enormous amount of starch. “Secondary growth and thickening in cycad stems occur by successive development of cambia” (Jones 39). “Branching mechanisms are not well developed in those cycads which have aerial trunks” (39). Auxiliary buds do not exist in cycads, instead bubils can develop on their trunk, which may develop into branches. “Undeveloped bubils may act as a fall back mechanism if the major growing point(s) on the trunk are damaged or destroyed” (39). Damaged cycad stems can produce roots and buds, which can develop into new plants.
Leaves terminate the trunks of cycads. Large growing cycads produce leaves in a flush while low growing species produce a single leaf at a time. “Cycads which have subterranean trunks usually have erect or obliquely spreading leaves” (40). In arborescent trunked species as new leaves are produced, older leaves are pushed out and downward. Most cycads have pinnate leaves. To limit transpiration thru the leaves cycads have thick cuticles and sunken stomata. The rhachis may bear spines or surface prickles that help protect the leaves from being ate by herbivores. The petiole may persist on the stem after the leaf has died and act as a protective mechanism. “Cataphylls are scale-like structures which are actually reduced leaves and may be referred to as scale leaves” (41). Cataphylls form a protective covering on the trunk.
Cycads are dioecius and form cones instead of flowers. The cones are composed of sporophylles. The male cones, called microsporphyll, grow on their underside microsporangium, which contain microspores that develop into pollen. The female cones, called megasporophyll, develop ovules on their lower half that will after pollination become viable seeds.
Pollination in cycads was thought to be effected only by the wind. “More recent investigations have suggested that beetles, especially weevils, and small bees may make a more important contribution to the transfer of pollen” (Binney 39). “Studies have also shown that some cycads at least will produce heat or odors to attract these animal vectors” (39).
The fleshy sarcotesta, which surrounds the slerotesta, contains chemical inhibitors that delay germination and may assist in maintaining moisture of the kernel. The sclerotesta is a very hard woody coat that surrounds and protects the kernel, which consists of the embryo and its food supply. Cycad seeds are large and often brightly colored. This bright coloration attracts birds and mammals that disperse the seeds.
Many cycads have developed mechanisms of root and stem contraction. The contractile roots and stem pull the plant underground. The collapsing of cells in the cortex and pith of the stem and roots cause the contraction. Contraction of the taproot seems to be significant for the establishment of cycad seedlings by pulling the susceptible parts underground were they are protected until the plants are large enough to develop protective leaf bases.
“Many
species of cycads occur naturally in areas where fires are of frequent
occurrence, often occurring annually” (Jones 65). Cycads cope fairly well with fire because
leaf bases on the trunk protect the internal growing apex from damage. Leaves, cones, and exposed seedlings are
destroyed by the blaze. “Those species
with subterranean stems are well protected by the soil.” “Soon after the fire new leaves are produced
and the cycad plant quickly recover.”
“In the savannahs of
“All species of cycad contain virulent toxins which can cause severe debilitating symptoms or even death if ingested in sufficient quantities” (52). These toxins are suspected to have evolved during the Mesozoic Era and have been retained as a protective mechanism against predation. The two major toxic compounds in cycads are cycasin and macrozamin. These toxins have been suspected of causing neurological disorders in animals. When ingested in sufficient quantities these toxins are toxic to the liver and may cause cancer.
These adaptations have enabled cycads to outlive many of the prehistoric species that lived on the Earth and hopefully they will allow them to survive the destruction we have imposed on them in their native environment.
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<http://plantnet.rbgsyd.gov.au/PlantNet/cycad/ethn.html>
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<http://plantnet.rbgsyd.gov.au/PlantNet/cycad/foss.html>
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and Nitrogen Fixation” (Colonization). Accessed
<http://plantnet.rbgsyd.gov.au/PlantNet/cycad/Nitrogen/colonization.html>
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Assed
<http://plantnet.rbgsyd.gov.au/PlantNet/cycad/Nitrogen/nfixintro.html>