The two growth phases separated by a chilling period that I have proposed to describe what I've seen in germination experiments are not intended to be separate types of germination. In trillium they are part of a growth process that happens in BOTH germinating seeds and in adult plant growth. That is why I have proposed to call them growth phases. The skotomorphogenic phase of seedling growth is always dependent on endosperm for energy. This is followed by a chilling requirement the length and intensity of which is species dependent. This appears to change the hormonal balance in the seedling from one that involves cell proliferation and differentiation during the skotomorphic phase to one that involves elongation of the hypocotyl and cotyledon in the first photomorphic phase. I think of the two phases as an oscillation - like that of a pendulum. How you define the cycle depends on where you start. I prefer to consider germination finished when the seedling has formed by skotomorphogenic growth. Hence there is no photomorphic germination - merely the normal spring growth of a trillium plant that begins the oscillation between the two phases that will continue for over 40 years under favorable growth conditions.
In evolutionary terms plant families that are considered basal - primitive when I first started learning botany- are those with lots of endosperm relative to the embryo volume. In dicots the Magnolia family and in monocots the lilies. Gradually embryos developed more completely into miniature seedlings and the importance of endosperm decreased until in beans there is essentially no endosperm and the entire seed is a miniature seedling plant. Since evolution does not go backwards - or at least I've never seen arguments that support such regression - plants with lots of endosperm and undeveloped embryos - like Trillium - began their evolution with much the same seed structures as those we see today. They have survived because these structures are adapted to the vagaries of the environment where they now grow.
In the case of Trillium I believe that this adaptation involves high seed moisture content. Trillium seeds - particularly seeds of the sessile species - seem able to lose nearly 50% of their weight and, if slowly rehydrated, begin skotomorphic growth. This ability to lose and gain moisture adapts them to survive relatively dry summers. (Pedicillate species have much smaller seeds and seem to have less tolerance for drought. This corresponds to their more moist and cooler habitats relative to sessiles.) The initial skotomorphic growth of the embryo within the seed takes about 80 days - longer if some moisture loss suspends the growth for a time. This puts germination (radicle emergence) near the end of summer when fall rains and lower temperatures increase soil moisture. The skotomorphic growth then continues in this more moist environment. Although its energy is derived from the endosperm, the primary root does have root hairs and these may well absorb both moisture and nutrients that are in short supply in the endosperm. I suspect that phosphorus and nitrogen are the most likely nutrients to be absorbed. If the seed does not ripen until late, there is not enough time - even for an "active" embryo to complete skotomorphogenic growth before winter. In this case skotomorphic growth is delayed until the year after the seed is shed. Embryo dormancy produced toward the end of the seed ripening process becomes a mechanism to that increases survival by delaying skotomorphic growth until soil conditions are favorable.
This outline does not require two types of germination - just one - skotomorphogenic germination. After that the pendulum-like cycle between skoto and photo growth begins and continues monotonously for the life of the plant.
From Trillium-L, January 7, 2010.
A preliminary version of this was given at the Mt. Cuba Trillium Symposium, 2008.
See also: Holmes Trillium Research site.
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