Define Epigeal and Hypogeal

J.E. Shields
Mon, 04 Jan 2010 14:48:46 PST
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."

John Gyer, working with Trillium, has gotten around this profusion of 
confusion with epigeal/hypogeal by coining two new terms, which refer 
specifically to physiology, whether the growth phase involves 
photosynthesis or not:

photomorphogenesis -- growth process involving photosynthesis

skotomorphogenesis -- growth process not involving photosynthesis.

(if I have understood his use of these two terms correctly.)

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.  This 
germination and development process is first skotomorphogenic and only 
later becomes photomorphogenic.

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.

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 

Do these descriptions fit monocot reality?  I take it as given that the 
described growth processes do fit lilies, regardless of what we name 
them.  I think they also fit some other geophytes.  Some species of 
Scadoxus, certain Hymenocallis, etc., based on my own observations, do go 
through an initial skotomorphogenic phase 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, but are consistent with our discussion so far.  They 
persist in the literature of plant physiology; and in horticulture and 
plant physiology, the terms clearly are not intended to have a trivial 
meaning, so what do they mean?  I didn't make them up.

This is an interesting discussion, and I wonder what plant physiologists 
currently working on germination, differentiation, and development would 
say about these terms?  I'm simply trying to find a good definition of them 
for the glossary, so that I won't just add fuel to the fire of 
confusion  when I try to use these words later on.

Jim Shields

Jim Shields             USDA Zone 5             Shields Gardens, Ltd.
P.O. Box 92              WWW:
Westfield, Indiana 46074, USA
Tel. ++1-317-867-3344     or      toll-free 1-866-449-3344 in USA

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