Define Epigeal and Hypogeal

Kenneth Hixson
Mon, 04 Jan 2010 19:27:06 PST
Dear members, I have the uncomfortable feeling that this discussion
is proceeding from "First, let us define our terms" rather than
"Here is something I don't understand, do our definitions fit?"
I don't think that is what was intended.

	(I wrote this before reading Jim McK's last post, so it says
many of the same things in slightly different words)

	My understanding of seed germination is a little different
than much of what has been discussed so far, and hopefully some comments
will add to the understanding of this process--and it is a process, not
a single event.

1)  Germination--occurs not when a seed or root emerges, but when
the seed changes from a resting state to an active state--usually
after absorbing moisture, and often after certain other changes,
which may involve after-ripening of the seed, whether from maturing of
the seed, or caused by temperature changes, leaching of inhibitors,
exposure to smoke chemicals, etc.  In some instances, a seed germinates
without ever emerging from the original seed--and when and how leaves
and bulb are formed is an afterward-occurring part of the process of
plant formation.

> If we take the words literally as used for dicots, they simply do not apply 
> usefully to monocots, it seems to me.  By the way, I didn't make up the 
> definitions cited; they are Ed McRae's in his book, "Lilies."
> What occurs in some plants is that the germinating seed first produces a 
> plant without involving photosynthesis.  First, a bulb, root, or rhizome is 
> formed underground -- regardless of where the seed was lying.  Then, 
> sometime later, that bulb or rhizome produces a green leaf -- hence a true 
> leaf, not a cotyledon regardless of what shape the leaf has.

2)	The way I read this, a crinum seed which germinates on the seed/
flower stalk, is not a bulb?  The same for a seed which falls to the
ground, is not covered up, but still forms leaves, roots, and what
appears to be a bulb?  In neither case is the plantlet underground.
Yet, they form leaves, bulbs, and exhibit photosynthesis.  Further, it
has often been observed in lily seed exposed to light, (such as has been
held in a plastic bag of moist media), normally hypogeal germinating
seed will proceed to grow without exposure to the normally required cool
resting period.   Such light-exposed seed grown bulbs may also turn
green where exposed to light--with no leaves present, although leaves
develop later.  These plantlets grown in plastic bags are not--have
never been--underground, so are they supposed to be considered epigeal
despite how they formed bulbs, roots, leaves?

> A different sort of plant produces a green leaf or cotyledon shortly after 
> germinating, and probably before the first rudimentary bulb has started to 
> form.  This germination is "photomorphogenic" from almost the beginning of 
> the process.  This first green leaf could maybe be a cotyledon, but I don't 
> know whether or not it ever is in monocots.

3)  This presumably corresponds to what is usually called epigeal or
immediate epigeal, lily seed.  The term hypocotyl has been used for this
"cotyledon", but the definitions I've seen are confusing.  Think of an
onion seedling.

> In Ed McRae's definitions, the epigeal lilies produce a green leaf sometime 
> during the first season of growth,  while the hypogeal lilies produce the 
> first leaf only after a period of cold dormancy, so in the second season of 
> growth.
> As Jim McK. defined "epigeal germination" and "hypogeal germination" -- 
> where the seed germinates rather than how -- the terms are trivial and 
> almost meaningless, 

4) Ed McRae was trying to provide advice on how beginners could produce/
propagate lilies.  He didn't try to provide all the exceptions to the
rules, and there are numerous exceptions.  Jan de Graff, Ed McRae's
first employer, gave more complex rules, and listed immediate and
delayed epigeal germination,  immediate and delayed hypogeal
germination, then proceeded to list L. brownii australe, L. parryi, and
L. speciosum as lilies which normally form bulblets before forming
leaves (hypogeal germination), but which without ever taking a rest
period or needing exposure to cool temperatures, would proceed to form
leaves reasonably prolifically the first growing season.

5)  Where a bulb forms may be an accident--above ground or under.  How
a bulb forms is more diagnostic.  Bulbs form in two main ways--either
some growth is sent up immediately, or the seed first forms a bulb, then
sends up growth.  Those lilies which send up a leaf immediately can be
recognized because the leaf is narrow and linear.  They usually have the
seed on the tip of the leaf, and may resemble an onion seedling.
	Lilies which form a bulb first, then send up leaves, have leaves that
are fatter/much broader, not narrow and linear.  The seed does not perch
on the tip of the first leaf to come up.  While some of the lilies in
this class require a warm period to form the bulblet, western american
lilies require a cool period to form a bulblet, but can germinate, form
a bulblet and send up a leaf within about three weeks under the right
conditions.  This would presumably also fit immediate hypogeal
germination.  Since it can happen to seeds lying on top of a seedbed, it
could also be called epigeal germination, even though a bulblet was
formed before a leaf appeared?

Am I confused yet?



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