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Maki provided a good material on Ochid and Fungus. I typied the pictures to script as below, hope it helps.
Orchids and Fungus
Of all of the flowering plants, orchids are considered the most highly evolved. These plants have developed very specialized requirements for survival. One of those requirements is a cooperative association with a kind of fungus.
Before leaves or roots can develop from an orchid seed, a relationship must be formed with specialized fungi called mycorrhizae that are found in the soil. These mycorrhizal fungi actually penetrate the cells of the seed and assist in supplying mineral nutrients to the developing plant. As the plant develops, the fungus spreads, penetrating more and more cells and delivering nutrients in the process. Without this help, the development of roots and leaves could not take place. The development of an orchid seed and the establishment of the relationship with the fungus are slow process. It may take several years before an orchid plant appears above ground – as many as ten for some species.
The relationship between orchid and fungus benefit both and is not abandoned once the plant emerges from the soil. When the plant is exposed to sunlight and is carryout photosynthesis, it is probably supplying some of its photosynthetic products such as sugars and starches to sustain the fungus. Some terrestrial orchids lack chlorophyll altogether. These species depend entirely on the fungus to provide their nutrition.
Many orchids can remain underground for several years. Building the food reserves and energy that will be needed for flowering and reproduction. After this large expenditure of energy, the plant will retreat underground for several more years. The orchid will re-emerge again when nutrient reserves are sufficient for another attempt at reproduction. These underground phases in the orchid life cycle when it is cut off from sun would be impossible where it not for the help received from the fungus.
As I said few moments ago, orchids are highly developed plants. Each species has had time to adapt to a specific niche in the environment. They are typically associated with a specific set of environment conditions such as nutrient availability, sunlight and water. For example, some orchid species need a certain amount of calcium and soil with a high ph to survive. It is reasonable to assume that this sensitivity may reflect the specific conditions needed to form the critical association between the orchid and its fungus partner.
Q1.What does the professor say about orchid seeds? (click on 2 answers)
(1) Most of them never develop into plants
(2) They cannot absorb nutrients by themselves (O)
(3) They require special nutrients to develop
(4) They can remain underground for long periods. (O)
Q2 Why is the fungus so important to orchid plant?
(1) The plants feeds on fungus.
(2) The fungus keeps animals from eating the seeds.
(3) The fungus allows the plant to grow in almost any kind of soil.
(4) The fungus supplies nutrients to the seed. (O)
Q3. The professor mentioned some types of orchids that (click on 2 answers)
(1) spend their entire lives underground
(2) do not depend on fungus to survive
(3) cannot make food using sunlight (O)
(4) only emerge from the ground to reproduce (O)
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发表于 2003-7-29 09:17:00
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