Define Epigeal and Hypogeal

Jim McKenney
Mon, 04 Jan 2010 13:10:52 PST
I agree with Tim that the meanings of epigeal and hypogeal, at least at a
literal level,   are clear, and that the uncertainty Jim Shields has
expressed is due to his associating the word germination (instead of the
word cotyledon) with the words epigeal and hypogeal. 


I think what Jim Shields has called epigeal germination and hypogeal
germination might better be called epigeal cotyledon and hypogeal cotyledon.
That leaves us with four categories: epigeal cotyledons, hypogeal
cotyledons, epigeal germination and hypogeal germination. Punnett square,


Isn't that what you mean, Tim? 


Lilies germinate with their seeds above the ground (in epigeal fashion) or
with their seeds buried in the ground (in hypogeal fashion). The cotyledons
of lilies are sometimes epigeal (i.e. they appear above ground) and
sometimes hypogeal (i.e. they do not appear above ground). But what is
perhaps more important to our discussion is this: some cotyledons remain
largely within the seed and some elongate to become the first leaf-like
growth of the developing plant. 


Many ranunculaceous plants have both eipgeal and hypogeal germination but
have cotyledons which are hidden: the first leafy growth to appear is a true
leaf, not a cotyledon. 


In cases where the seeds of a given taxon germinate both above ground and
below ground, the terms epigeal and hypogeal can be understood to be purely
descriptive of the individual plant under observation, and not predictive of
the behavior of other members of the same taxon. 


In the refrigerator right now is a pot of germinating Lilium humboldtii. In
the lily literature this is typically described as a hypogeal species. The
seeds were scattered on the surface of the soil and not covered. They are
germinating now; what will become the hypocotyl has emerged from the seed.
As is typical of this species, the end of the growth axis which becomes the
bulb will, as it elongates, push down into the soil, and the incipient bulb
will form underground. In this case, the seeds germinated in epigeal fashion
- i.e. they germinated above ground, uncovered and exposed to air and light.
The bulb will form in hypogeal fashion, and the first true leaf will push up
from below ground. 


If I had covered the seeds with soil when I planted them, the cotyledons
would have always been underground. Since most growers cover seeds, it makes
sense to them to call these seed hypogeal. 


When I was recently taken to task for my use of the word corm to describe
what many others call a tuber, I began to check the literature on this and
related topics. In addition to discovering that there is plenty of
disagreement on this topic, I ran into another problem. Jim Shields alluded
to this when he mentioned the problem of comparing germination in monocots
and dicots. 


So, here's a question for someone out there who really knows what they are
talking about: are the seemingly similar structures (such as stems, leaves,
flowers) of monocots and dicots truly homologous or are they merely
analogous? For instance, given the differences in the anatomy and embryology
of stem tissue in monocots and dicots, can a stem tuber in a dicot truly be
homologous to a tuber in a monocot? They are merely analogous, aren't they? 



Jim McKenney

Montgomery County, Maryland, USA, 39.03871º North, 77.09829º West, USDA zone
7, where the hamamelidaceous shrub Loropetalum chinense is beginning to
bloom in one of my (very crowded) cold frames. 

My Virtual Maryland Garden



Webmaster Potomac Valley Chapter, NARGS 

Editor PVC Bulletin 


Webmaster Potomac Lily Society







More information about the pbs mailing list