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Orchid Morphology

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This page is about Orchid Morphology which refers to the outward appearance (shape, structure, colour, pattern) of orchids and their component parts.

Stem shapeEdit

All orchids are perennial herbs and lack any permanent woody structure.

SympodialEdit

Oncidium concolor plate

Sympodial growth demontrated by Oncidium concolor

Orchids with sympodial growth have a specialized lateral growth pattern in which the terminal bud dies.

The growth continues by development of new shoots sprouting from or next to those of previous years (such as in the genus Cattleya or Cymbidium). Pertumbuhan ini terus berlanjut dengan pengembangan tunas-tunas baru tumbuh dari atau di samping orang-orang dari tahun sebelumnya (seperti dalam ===Cattleya atau Cymbidium ).

The base of the stem of sympodial epiphytes, or in some species essentially the entire stem, may be thickened to form what is called a pseudobulb, a storage organ derived from the part of a stem between two leaf nodes.

Dasar batang epifit pucuk, atau dalam beberapa spesies dasarnya seluruh batang, dapat menebal untuk membentuk apa yang disebut pseudobulb, penyimpanan organ yang berasal dari bagian dari batang antara dua node daun. The pseudobulbs contains nutrients and water for drier periods. The pseudobulbs mengandung nutrisi dan air untuk musim kemarau. Pseudobulbs themselves are relatively short lived (1-5 years), but are continually produced from the growing tip of the rhizome. Pseudobulbs sendiri tinggal relatif pendek (1-5 tahun), tetapi terus diproduksi dari ujung tumbuh dari rimpang. These contain food reserves for drier periods. Ini berisi cadangan makanan untuk musim kemarau. At their end appear one or two leaves, or sometimes four or more. Pada akhirnya mereka muncul satu atau dua daun, atau kadang-kadang empat atau lebih.

The pseudobulb has a smooth surface with lengthwise grooves and can have different shapes, often conical or oblong.

Its size is very variable; in Bulbophyllum it is no longer than two millimeters, while in the largest orchid in the world, Grammatophyllum speciosum (giant orchid), it can reach three meters. Some Dendrobium have long, canelike pseudobulbs with short, rounded leaves over the whole length, some other orchids have hidden or extremely small pseudobulbs, completely included inside the leaves.

With aging the pseudobulb sheds its leaves and becomes dormant.

At this stage it is often called a backbulb. A pseudobulb then takes over, exploiting the last reserves accumulated in the backbulb, which eventually dies off. In warm and humid climates, many terrestrial orchids do not need pseudobulbs.

MonopodialEdit

Aerangis ellisii plate

Monopodial growth demonstrated by Aerangis ellisii

Monopodial orchids have growth habits grow upward from a single point. They add leaves to the apex each year and the stem grows longer accordingly.The word Monopodial is derived from Greek "mono-", one and "podial", "foot", in reference to the fact that monopodial plants have a single trunk or stem.

Orchids with monopodial growth often produce copious aerial roots that often hang down in long drapes and have green chlorophyll underneath the grey root coverings which are used as additional photosynthetic organs. They do not have a rhizome or pseudobulbs so species adapted to dry periods have fleshy succulent leaves instead. Flowers generally come from the stem between the leaves. With some monopodial species, the stem (the rhizome) might fork into two, but for all monopodial orchids this is not necessary for continued growth, as opposed to orchids with sympodial growth.


KeikisEdit

Keiki, the Hawaiian word for a "baby" pron. (Kay-key), refers to a plant produced asexually by an orchid plant, usually used when referring to Dendrobiums, Phalaenopsis, or Vandaceous orchids. The baby plant is an exact clone of the mother plant, sometimes flowering while still attached to the mother plant. They occur through the accumulation of growth hormones at a specified point.

Kekis come in two forms a regular and a basal keiki. The regular keiki is a small plant growing from one node along the flower stem, instead of a branch. This is induced by the accumulation of growth hormones at that point, either naturally or by the application of keiki paste, a cytokinin hormone which induces growth in the node of an orchid inflorescence. The basal keiki is a baby plant growing from the base of the mother plant. Sometimes keikis bloom while still attached to the mother plant.

LeavesEdit

Like most monocots, orchids generally have simple leaves with parallel veins, although some Vanilloideae have a reticulate venation. They may be ovate, lanceolate, or orbiculate and very variable in size. Their characteristics are often diagnostic. They are normally alternate on the stem, often plicate, and have no stipules. Orchids leaves often have siliceous bodies called stegmata in the vascular bundle sheaths (not present in the Orchidoideae) and are fibrous.

The structure of the leaves corresponds to the specific habitat of the plant. Species that typically bask in sunlight, or grow on sites which can be occasionally very dry, have thick, leathery leaves and the laminas are covered by a waxy cuticle to retain their necessary water supply. Shade species, on the other hand, have long, thin leaves.

The leaves of most orchids are perennial, that is they live for several years, while others, especially those with plicate leaves, shed them annually and develop new leaves together with new pseudobulbs, as in Catasetum.

The leaves of some orchids are considered ornamental. The leaves of the Macodes sanderiana, a semiterrestrial or lithophyte, show a sparkling silver and gold veining on a light green background. The cordate leaves of Psychopsiella limminghei are light brownish green with maroon-puce markings, created by flower pigments. The attractive mottle of the leaves of Lady's Slippers from temperate zones (Paphiopedilum) is caused by uneven distribution of chlorophyll. Also Phalaenopsis schilleriana is a lovely pastel pink orchid with leaves spotted dark green and light green. The Jewel Orchid (Ludisia discolor) is grown more for its colorful leaves than its fairly inconspicuous white flowers.

Some orchids, as Dendrophylax lindenii (Ghost Orchid), Aphyllorchis and Taeniophyllum depend on their green roots for photosynthesis and lack normally developed leaves, as of course do all of the heterotrophic species.

PhotosynthesisEdit

The majority of plants in the Orchid family uses Crassulacean acid metabolism or CAM photosynthesis to fixate carbon dioxide. Plants open their stomata during the cooler and more humid night-time hours, permitting the uptake of carbon dioxide with the minimum water loss. During the day shut their stomata and concentrates CO2 around the enzyme RuBisCO increasing its efficiency.

Other orchid plant use C3 carbon fixation which converts carbon dioxide into 3-phosphoglycerate. These plants are found in areas with more moderate temperatures and bright lighting.

CAM

C3

PigmentsEdit

RootsEdit

Roots

aerial roots from epiphytic orchid

Orchids exhibit the root stricture of monocotyledonous plants and have no 'primary roots' (main central roots from which secondary roots grow), but only 'secondary roots', which start directly from the stem or from other secondary roots. Often the roots store nutrients and water, helping to retain nutritional substances that deposit on their bases. In some cases, the roots have chlorophyll organs that can sustain photosynthesis when the plants lose their leaves. The roots vary in thickness, from very thin to very thick. Root structure varies considerably among orchid genera, depending on how and where they grow.[1]

Terrestrial orchids may be rhizomatous or form corms or tubers. These may be spherical or elongated cylinders, and they store water and nutrients, replacing the role of pseudobulbs in epiphytic species. Occasionally, these tubers split from the mother plant, giving rise to new plants. Some Australian species of the subtribus Caladeniinae have almost no roots, only a small ovoid tuber. At the other extreme, the tubers of the Brazilian Cleistes are thin and delicate, measuring more than one meter, spreading in several directions. Collecting this species is almost impossible, for their tubers break easily, causing the plant to die.[2] As in the Cypripedium species, terrestrial orchids can have extensive underground rhizomes that vernalize and produce new growth in the following year [3].

The root caps of terrestrials are smooth and white. Sympodial terrestrials, such as Orchis and Ophrys, have two subterranean tuberous roots. One is used as a food reserve for wintery periods, and provides for the development of the other one, from which visible growth develops.

Epiphytic orchids have modified aerial roots that can sometimes be a few meters long. They generally have robust roots that are cylindrical when aerial and which become flatter after attachment to the substrate. They are often covered by the vellamen, a thick spongy tissue that helps orchids to quickly absorb water from rainfall, and humidity from the air. Some species, particularly those of the subtribus Catasetiinae, behave as if they were pneumatophorous, with many thin roots growing up forming a sort of 'wig' that catches leaves and sediments that are shed from above during the rains. It is made of dead cells and can have a silvery-grey, white or brown appearance.[2] The cells of the root epidermis grow at a right angle to the axis of the root to allow them to get a firm grasp on their support. Nutrients mainly come from animal droppings and decaying matter on their supporting tree.

The root's life time varies according to environmental conditions, and generally is less than the stem life time. New roots usually shoot during or at the end of each vegetative growth period. Although not the primary nutritional source of orchids, they usually benefit from a kind of symbiosis with a fungus (Mycorrhiza) that is lodged on the vellamen exterior cells of their roots, and which excretes nutrients that are absorbed directly by the roots.[2]



FlowerEdit

Fruits and seedsEdit

Orchid seeds

Orchid seeds from Beitrage zur morphologie und biologie der familie der orchideen

The seeds are generally almost microscopic and very numerous, in some species over a million per capsule. After ripening they blow off like dust particles or spores. They lack endosperm and must enter symbiotic relationship with various mycorrhizal basidiomyceteous fungi that provide them the necessary nutrients to germinate, so that all orchid species are mycoheterotrophic during germination and reliant upon fungi to complete their lifecycle.

As the chance for a seed to meet a fitting fungus is very small, only a minute fraction of all the seeds released grow into an adult plant. Germination can take up to fifteen years.

Horticultural techniques have been devised for germinating seeds on a nutrient-containing gel, eliminating the requirement of the fungus for germination, greatly aiding the propagation of ornamental orchids.

ReferencesEdit

  1. Phillip Cribb (2001) Morphology of Orchidaceae. Pridgeon AM, Cribb PJ, Chase MW, Rasmussen FN eds., Genera Orchidacearum vol. 1. Oxford University Press, Oxford, UK ISBN 0198505132.
  2. 2.0 2.1 2.2 Joseph Arditti (1992) Fundamentals of Orchid Biology. Wiley & Sons. ISBN 9780471549062.
  3. Selbyana, 18(2): 172-182, 1997

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