OrchidAngiosperms (Flowering Plants)

(SL 5)

Classification/Diagnostic characteristics
All angiosperms share certain traits derived from a common ancestor. These traits include flowers, fruit, and a modified leaf called a carpel that encloses the plant's ovules and seeds. These types of plants are distinguished by their ability to perform double fertilization, produce reduced gametophytes, and germinate pollen on a stigma. Unlike gymnosperms, angiosperms have nutritive tissue called endosperm and phloem with companion cells. An example of a flowering plant, or angiosperm, is the orchid.

There are two major angiosperm clades: monocots and eudicots. Monocots have only one cotyledon, an embryonic storage organ, whereas eudicots have two. Magnoliids make up the sister group of these two main clades.

Kingdom: Plantae (Plants)
Subkingdom: Tracheobionta (Vascular Plants)
Superdivision: Spermatophyta (Seed Plants)
Division: Magnoliophyta (Flowering Plants)
Class: Liliopsida (Monocotyledons)
Subclass: Liliidae
Order: Orchidales
Family: Orchidaceae (Orchid Family)
(SL 8)

Relationship to humans
Humans are one of the animals Angiosperms use to transport their seeds. In their efforts to protect their seeds, Angiosperms have provided humans with one of the staples of human diet: fruit.

Angiosperms serve as the major source of food both directly and indirectly for humans, and are a fertile source for consumer such as: building materials, textile fibers, spices, herbs, and pharmaceuticals. Angiosperms have a multitude of uses as food such as: grains, sugars, vegetables, fruits, oils, nuts, and spices. These plants can also serve other needs including: dyes, fibers, timber, fuel, and medicines. Angiosperms also convert energy from the sun into starch, which is the energy-abundant storage form of sugar. This is then stored in the endosperm of the seed until the seedling germinates and grows. Ultimately, the most important relation between humans and angiosperms is the reliance of humans on angiosperms for food and other consumer products (3). (AG)

Habitat and niche
Angiosperms are found across the globe, but are more densely populated in warm regions. The fruits produced by these plants have diverse characteristics, making the plants common in many types of climates and habitats. Some fruits are woody and hard, allowing them to survive a drought. Others are lightweight and can be spread by the wind.

Most orchid genera live in tropical zones, like those in South America (212 to 250 genera), southeast Asia (260-300 genera), and central Africa (230-270 genera). Only about one tenth to one fifth as many genera of orchid occur in temperate areas, like North America (20-26 genera) and Europe (40-60 genera) as in these tropical zones. (YR) (16)

Orchids have a very extensive range of habitats around the world. (YR) (17)

Predator avoidance
The animals that eat the fruits and nectar from angiosperms are more of an advantage to these plants and are not considered predators.

Like many plants, the biggest predators to orchids are insects, such as aphids and roaches. Aphids are small insects that get their food from sucking on the phloem sap of orchids. They eat the sap of the plants, which prevents the orchids from getting their important sugars and nutrients. Aphids also carry viruses that can be harmful to orchids and they leave behind honeydew that allows fungi to grow that harms the orchids even further. They produce asexually and sexually and their populations grow extremely rapidly. When orchids are infested with aphids, they begin to wilt and even droop. Roaches feed on orchid plants and they are nocturnal so the damage to the plant is usually only noticed after the fact. Unfortunately, in these scenarios it's mostly humans that have to treat orchid plants. (9) (10) (LK)


Aphids are very harmful to orchids, ranging from 1-10 millimeters long. Not all 4,400 species of aphids are pests for orchids, but once they inhabit a plant, it is very hard to get rid of them because of their ability to rapidly reproduce (18). (SP)

Nutrient acquisition
Like most plants, angiosperms obtain their nutrients from the ground through their roots and fox carbon dioxide through photosynthesis.

Angiosperms are vascular plants, containing xylem and phloem in various bundled patterns. They have true roots for holding the plants firm as well as for taking in water and nutrients, true stems, for support and for moving materials up and down, and they have leaves, usually flat, for most of their photosynthesis. Leaves and stems have pores called stomates that can be opened or closed. When open, carbon dioxide for photosynthesis enters air spaces inside the leaf through which it gets to photosynthetic cells, but water is also lost (this is the main source of transpiration from angiosperms). By controlling these pores, a plant can get enough carbon dioxide for its needs and minimize water losses. Clusters of xylem and phloem plus a reinforcing sheath to help support the leaf are called veins. (2) (NU)

water plant.jpg

(SL 7)

Reproduction and Life Cycle

As in gymnosperms, pollination is the first step in seed formation. Next, a pollen tube grows before fertilization occurs. Almost all angiosperms have two male gametes that combine their sperm with either the egg or two other haploid nuclei from the female gametophyte. As a result, a diploid zygote and a cell with a triploid nucleus are formed. These two products give rise to the endosperm, a triploid tissue that will nourish the developing embryo. This process, specific to angiosperms, is known as double fertilization.

In typical sexually reproducing angiosperms, the flower's pollen acts as the sperm cells and are located at the stamen of the flower; the female reproductive organs of a flower are located in the pistil. However, orchids have a unique anatomy. Rather than having stamen and pistils, orchids combine both male and female reproductive organs into one area on the flower called the gynostemium. The anther lies at the top of this column-like structure and contains the pollen, and the sticky concave area below the anther is the stigma, the site for fertilization. While some species have male and female flowers used in sexual reproduction, other species combine the sex organs for self-pollination. Shown below is an anatomical diagram of a Cattleya orchid, a type of orchid which combines its sex organs. (AC) (21)

A Cattleya orchid, which exhibits the unique anatomical feature of the orchid known as the gynostemium. (AC) (21)

Growth and Development
Together the pollen, the male gametophytes, and the ovules, the female gametophytes, form a zygote. This zygote has a backbone, stem or root, and two cotyledons, or seed leaves. The ovule then develops into a seed that contains a diploid zygote and a triploid endosperm. The seedling then become a flower and repeats this process.

Even though most angiosperms tend to be dicots, Orchids, on the other hand, are monocots, where seedlings typically have one cotyledon (seed-leaf), in contrast to the two cotyledons typical of dicots. Triploid endosperm is produced during the the life cycle of most angiosperms as a result of double fertilization, where sperm nucleus fuses with two polar nuclei to form nourishing tissue inside the seeds; however, orchids lack an endosperm around their embryo. Like all seed plants, orchids are heterosporous, and their ovules are contained within carpels rather than being exposed on the surface of scales. Male gametophytes are the pollen grains that find their way to pollinate another flower. When pollination occurs, the zygote develops into an embryo, which consists of an embryonic axis and one cotyledon (seed leaf). Because orchids are angiosperms, the ovule develops into a seed containing the products of double fertilization. During germination, orchids have a single embryonic cotyledon that becomes the embryonic first leaves of the seedling. The root system originates in a short-lived embryonic root called the radicle. Monocots form a fibrous root system composed of many thin roots that originate from the stem at ground level.
Orchids can be either monopodial, meaning one stem grows from a single bud and leaves grow from the apex with the stem growing accordingly (e.g common monopodial orchids are phalaenopsis and vanda), or sympodial, where a few stems grow, they bloom and then die (e.g Cymbidiums and Dendrobiums). (SM) [11,12]


The name Angiosperm translated to “enclosed seeds”. The ovules and seeds in these plants are enclosed in a modified leaf called a carpel. This covering helps protect the ovules and seeds and interacts with incoming pollen to prevent self-pollination.

Orchids are monocots because they only have one leaf that emerges from the seed (13, 14). The seed of a monocot has 1 cotyledon (14). Orchids have flower parts that are grouped in three or six, and it is unusual with is 3 sepals that organize between its 3 petals and under them. Also orchids have bulbous roots (13). The sperm and male gametophytes are encased in a protective cover called a pollen grain. As it grows the protective cover shrinks. The ovary of an orchid is consists of one or more carpels, which is fused or not fused. Each carpel contains ovules, immature seeds. The entire female reproductive system is made up of the gynoecium, which is made up of the ovary, the stigma, and a pistil. The pistil is a long tube leading from the stigma to inside the ovary. The stigma is a sticky surface where pollen grains adhere (13). (SM)


Animals tend to pollinate angiosperms due to the attractive quality of their flowers. Many flowers provide food rewards for animals such as nectar or pollen grains. In the process of obtaining nectar or pollen, animals transfer pollen from one plant to another, contributing to the plant population’s genetic diversity.

After fertilization the ovary and the seed inside develop into a fruit. This can promote seed dispersal through animal consumption.

Sensing the Environment
In order to prevent self-pollination in perfect flowers some angiosperms have stigma that act as screens so that the animals that spread their pollen cannot come in contact with the plant's anthers. Later the next pollinators do come in contact with the anthers, thus leading to fertilization.

Orchids, like many other plants, can sense the environment and adjust their behavior accordingly. Orchids need to have a bright light, although they cannot have too much direct sunlight because it will dry out the leaves. Blooming will only occur when there is sufficient sunlight because the plant is receiving optimal nutrients and can maintain a flower. In addition, orchids respond well to nights where the temperature is cool, between 10 degrees Celsius and 15 degrees Celsius. This helps induce blooming because the orchids will store the nutrients acquired during photosynthesis in the daytime rather than metabolizing them. Orchids are quite sensitive and a number of factors could cause buds to fall off before flowering, which is a problem for many gardeners. Humidity of the environment must be kept between 50% and 70% for orchids to thrive. They should be grown with a second pot beneath the initial pot with a layer of water to ensure the soil will be kept moist. If the roots of the orchids directly touched this water, the orchid would drown, as the capillaries are only one cell thick. (19) (20) (RS)

Gas exchange
Processes involving gas exchange include photosynthesis and respiration. Angiosperms also transport gases through their internal circulatory systems using their xylem and phloem.

Orchids, like all types of flowering plants, perform gas exchange primarily through the stomata. During the day, when light is available for photosynthesis, the plants perform both photosynthesis and respiration. However, orchids will perform photosynthesis at a much higher rate which requires a large amount of carbon dioxide. To solve this problem, plants will release oxygen and obtain carbon dioxide through the stomata (as well as acquire oxygen for photosynthesis and dispose of carbon dioxide produced during respiration), or a pore located on the epidermis of the leaf and kept open with pressured filled guard cells. During the night, when there is no light for available for photosynthesis, the guard cells on the orchid will deflate (lose turgor pressure) causing the stomata to close and preventing water loss. This process also prevents the plant from acquiring carbon dioxide, which is not detrimental due to the lack of light with which to perform photosynthesis. Plants only perform respiration during the night using oxygen which diffused into the cell. Roots and stems can also exchanges gases using pores called lenticels. (SF)(1)

Waste removal
Like other plants, angiosperms remove their waste through respiration, in the form of carbon dioxide.

Angiosperms store their wastes in vacuoles to be excreted from the plant. (SL)

Environmental physiology (temperature, water and salt regulation)
The characteristics of the fruits of some plants differ depending on the environment. For example some fruits are woody and hard, allowing them to survive a drought. Angiosperms that live in warmer climates tend to have much more colorfula nd exotic characteristics.

There are some species of orchid that have the ability to undergo Crassulacean acid metabolism, or CAM photosynthesis, that allows them to survive in particularly dry environments such as deserts. Normally, plants have their stomata open during the day to take in carbon dioxide and close them at night, which results in a large loss of water because it is hotter during the day. With CAM photosynthesis, the orchid opens the stomata at night instead when it is cooler and fixes the carbon dioxide during the day. In some extreme cases, the orchid can enter a CAM-idle state that occurs when conditions are extremely arid. During this state, the plant can leave the stomata closed for extended periods of time during which the oxygen given off in photosynthesis is used in respiration and the carbon dioxide given off in respiration is used in photosynthesis. (4) (CM)

Internal circulation
Angiosperms are also distinguished by their internal water circulation system. In their xylem these plants contain vessel elements, specialized water-transporting cells that connect with one another and allow easy water movement.

external image pressureflow.gif

Plants move nutrients around through vessels called xylem and phloem. Xylem moves water while phloem moves food. Nutrients move because of a food gradient. One cell that produces sugar must give its sugar to a cell with low glucose levels. This relationship is called source to sink. The source cell will create sugar, which, through active transport, will move into the phloem. Because of an increase in solutes, the phloem becomes hypertonic to the xylem. This results in water moving into the phloem through osmosis. The solutes (glucose), looking for a hypotonic environment, will move to the sink cell and, through active transport, leave the phloem. The water, now in a hypotonic solution, will undergo osmosis and return to the xylem (15) RG

Chemical control (i.e. endocrine system)

Respiration and photosynthesis are used to regulate waste and oxygen.


(SL 6)

Review Questions

1. How do orchids work in their ability to regulate their internal environment? (SM)
2. Explain the reproduction cycle of a normal orchid. (SM)
3. What feature or process makes an orchid unique with respect to other angiosperms? (SS)
4. How do orchids utilize xylem and phloem to transport food and water? (JM)
5. How do orchids use their stomata in gas exchange? (JM)
6. How have orchids adapted to be able to withstand extreme environmental conditions? (CM)

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3. www.britannica.com/EBchecked/topic/24667/angiosperm/73095/Signifance-to-humans
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16. __http://www.rainforest-alliance.org/kids/species-profiles/orchid__
17. http://upload.wikimedia.org/wikipedia/commons/5/50/Orchidaceae.png
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19. "Temperature." Orchid Karma. N.p., n.d. Web. 25 Nov. 2013.
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