Missing Endosperm
The majority of plants supply their seeds with a food supply so they can
germinate. For instance the part of an apple that you like to eat is the food supply or endosperm
for the apple seeds in the middle of that apple. Orchid seeds are different, they have no food supply,
they have no endosperm.
Orchid Seeds
If you were to break open an orchid seed pod what you see looks like dust because the
average size of an orchid seed is between 0.3 and 0.8 mm in size. The smallest orchid seed is 0.05 mm in
size. All orchid seeds consist of a spindle-shaped seed coat that is just one cell layer thick that encloses
an extremely small and simplified embryo. There is no endosperm that would add weight to the seed which
enables the seed to be easily dispersed by the wind.
How Do They Germinate?
The dust-like and endosperm deficient orchid seed has to take in nutrients
to begin the process of germination, enable protocorm formation and fuel seedling growth. For orchids that are
growing in their natural environment there is only one source of nutrients - mycorrhizal fungi. When the orchid
seed is exposed to moisture it will swell up, however if no compatible mycorrhizal fungi is present the seed
will not germinate. This vital relationship that orchids and mycorrhizal fungi have is called myco-heterotrophy.
Myco-heterotrophy
Myco-heterotrophy is defined as a symbotic relationship between certain kinds of
plants and fungi, in which the plant gets all or part of its food from parasitism upon fungi rather than from
photosynthesis. An orchid seed gets all of its food to start germination, protocorm formation and early
seedling growth from one or more species of fungi. All orchids growing in their natural environment, whether in the
forest, meadow, bog, along a stream or in a jungle start their lives as myco-heterotrophs.
The vast majority of orchids do not continue to rely on mycorrhizal fungi to continue to grow and flower because they develop chlorophyll which enables photosynthesis. In the orchid family, however, there are some 250 species that are myco-heterotrophs throughout their lives.
Orchids in the Supermarket
The orchids that you see today in your local supermarket or nursery
did not start their lives as myco-heterotrophs. These orchids had the nutrients needed to germinate, form
protocorms and fuel seedling growth supplied by an agar gel contained in a sterile flask in a lab that supplied
ideal conditions of temperature and light. This method is called asymbotic seed germination.
More Information
To help you understand the two methods of orchid seed germination, myco-heterotrophy and asymbotic seed
germination you are encouraged to check out the following reference information.
Myco-heheterotrophy
Orchid Seeds: Nature's Tiny Treasures - An article by Wolfgang Stuppy, seed morphologist at the Royal Botanic Garden Kew, in England.
Mycoheterotrophy-The Mysterious World of Plants Living on Fungi - A general article about mycoheterotrophy.
Orchid Mycorrhizas - A short article about orchid mycorrhizas.
Orchidaceous Endomycorrhizas - A more detailed article about orchid mycorrhizas.
Mycorrhizal Types - Did you know that there are a number of types of mycorrhiza? Find out here.
Mycorrhizal Fungi Affect Orchid Distribution and Population Dynamics - This article reviews the research that has been undertaken from the early 1990's to 2017 with regard to orchid mycorrhizal fungi (OMFs) and how they affect the distribution and populations of orchids in their natural environments.Asymbotic Seed Germination
Techniques and Applications of In Vitro Orchid Seed Germination - This is a detailed article describing the process of asymbotic seed germination.
Orchid Media - This is a listing of different orchid media available from Phyto Technology Laboratories LLC. It lists the actual componets of the media, which includes the original media developed by Lewis Knudson in the 1920's.
Laboratory Equipment - Do you need lab equipment like Petri dishes, filter disks, Agar, laminar flow hoods, test tubes, micron filters, alcohol lamps, steroclaves, sterilization indicator strips, sterile culture kits, Erlenmeyer fermentation flasks, or other lab equipment? If so check out the large selection of lab equipment on the Fungi Perfecti website.
Asymbotic Technique of Orchid Seed Germination - This is a link to the Table of Contents of a 145 page book written by Aaron Hicks, Director of the Orchid Seedbank Project. For any one wanting to try to raise orchids from seed or if you already working in a lab and want a reference guide then this book would be worth the cost.