Propagation (seed, cuttings, etc): Hypogeal germination or Skotomorphogenic growth
Started by: John Humphries
A discussion on pre-germination of trillium and certain woodland lilies.Go to latest contribution by John Humphries, 19 November 2006, 00:43. Go to bottom of this page.
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I have often pondered the delayed germination, or double dormancy of certain seed, notably woodlanders. The following discussion took place on Trillium-L( a specialist website for trill-aholics) earlier today where sadly there is no facility for pictures.
I offered to post a few pictures on behalf of John Gyer which serve to illustrate the debate he and Gote Svanholm were having. The debate serves to enlighten keen plantspeople as to the process running to enable these plants to survive and prosper in the conditions to which they have adapted.
The case in hand is Cardiocrinum giganteum, a fabulous scented woodland lily which can reach 13feet if grown from seed, though most of us buy pre grown bulbs for planting in situ which will probably only reach 8-10feet, still one of the most spectacular plants to grow in woodland.
As you would expect from Trillium-L there is a distinct cross reference to Trillium.
Some Liliums (Like Hansonii) require cross pollination - some do not.
Not all seed in a seed pod are viable but in many species you can see the embryo if you look carefully using a lens and light from behind. If you can see the embryo it is probably viable.
Some Liliums - notably the woodlanders have, what for the last sixty years or so been called delayed hypogeal germination. This means that they use the time before the spring to convert the seed into a rooted micro-bulb. This ensures a quick emergence as soon as it is warm enough. The root is already in place and the first true leaf exists in "embryonic" state.
The speed is essential because it enables the young plant to utilise the early spring before the (deciduous) leaf canopy robs them of light and to some extent water. The process is mainly governed by temperatures. This is natural since the purpose is to syncronize the plant with the seasons.
Normally this means that the seed should be sown immediately and kept at room temperature or just below for a few months to enable this rooted micro bulb to form. The micro-bulb will then rest until it has been subjected to low temperatures for a few months. It will quickly sprout above ground as soon as tempertures again rise.
Typically, the hypogeal germinators use the cotyledon as a kind of umbilical cord and not as a leaf. The first leaf above ground is a real leaf not the cotyledon.
On the other hand, the epigeal germinators first send up a green cotyledon above ground (sometimes with the seed coat as a cap on top).
Darkness does not seem to be a requirement. I fail to see why it should be called scotomorphic until some one has shown conclusively that all hypogeal germinators (and there are many) need darkness. Even so I fail to see why a new word should be invented when the literature has been using another for so long time.
Cardiocrinum is sometimes stated to have delayed hypogeal germination but in my experience this is not always the case. Some lilium can be quite variable and one factor might be the treatment of the seed in the first few weeks.
I would sow most of the seed very soon but divide the seed. One part to be kept at room temperature for three months ( 20°C should be OK) and then cold for three months. One part to be kept cold until spring. Cold does not mean freezing it means a few degrees above freezing (4°C should be OK). One part I would keep dry and cool until early spring and sow then.
First skotomorphogenic growth as I have defined it (see detailed discussion in Harold Holmes web site - www.trilliumresearch.org ) does not REQUIRE darkness. For trillium, and I think for the Cardiocrinum, It can proceed in light as well. The defining characteristic is that the ENDOSPERM IS STORED SUNLIGHT that feeds the growth of the seedling independent of the ambient light conditions. In my garden both Trillium and Cardiocrinum undergo their skotomorphogenic growth phase in relative darkness about half a centimeter underground. I reject the term "hypogeal" for it describes only a position relative to the soil surface. It does not speak to the growth process that happens before the cotyledon (in the case of trillium and cardiocrinum which is under discussion) emerges at the surface as a photosynthetic organ and begins the photomorphogenic growth phase (growth dependent directly on sunlight as an energy source.)
I have not sectioned a cardiocrinum seedling so I can not speak to its internal structure and areas of starch storage. The external structures were named in reference to structures in trillium seedlings that I have studied more closely. Thus the hypocotyl is the stem-like structure that extends from the microbulb mentioned by Gote up to the "hook" or curved area of the cotyledon petiole. In Trillium the hypocotyl contains starch that provides the energy that drives the elongational growth of spring that brings the cotyledon into the light. The hypocotyl region in cardiocrinum appears to serve the same function. The cotyledon is suctorial in both trillium and cardiocrinum. It dissolves the endosperm and absorbs it (in trillium through the tip area only) for distribution to the growing plant. The microbulb in cardiocrinum is barely thicker than the end of the hypocotyl itself. It is seen mainly as an opacity a mm or two above the root. This opacity seems to have a slight swelling that will become the bulb scales and a central opacity that will become the base plate area. This is analogous to the bud meristem and rhizome in trillium. Cardiocrinum has a "transition zone", a term derived from Esau's plant physiology text, that connects the microbulb to the root. The root has a dense tuft of root hairs along its entire about 1 cm length. Somewhere I've read that these are not true root hairs, but serve as a sort of anchoring structure for the seedling. Which ever is the case they form a dense matrix that occludes a quantity of soil.
Here for comparison Trillium grandiflorum seedling.