Archimedes Plutonium wrote in message ...
25 May 2003 21:13:19 -0700 galathaea wrote:


unanswered. One concerns the evolutionary homologies of the
cotyledons, ie. can cotyledon shape be used for phylogenetic
relationships. Unfortunately, the "ontogeny recapitulates phylogeny"
pattern found in animals does not apply anywhere near as well in
plants. This is particularly true after the seed is formed, were much
separation of form has alreadt occurred. The cotyledon's shape can be
fairly similar across several species of a genera, but it rarely keeps
this similarity over genera. This is because the duties of the
cotyledon often must be structured to particular environments.
However, the cotyledon's role does have a fairly large evolutionary
history in the vascular plants. It was a necessary adaptation to
support plant growth through to the phase where it can produce enough
food on its own by photosynthesis. In fact, the number of them (1 or
2) separates two major evolutionary forms of the vascular plants, the
monocots and the dicots.

I think I can summarize my initial question better today. Often it takes
several days to make a question better.

Most definitely. We always start grasping and fine tune only later.


Question: Why should any plant have its first two leaves very much different from all other successive
leaves? That is the question that is bothering me. Why
should my baby pear tree have its first two leaves so starkly different from
its future leaves. Do these first leaves confer some superior advantage to the plant
or are they different because of the ancient past of the species, the genetic mess
of the species is brought forth in its first leaves.

What I had tried to convey in my first post was the variability of
forms in the cotyledon. This is strong evidence to point to the fact
that they have been an active part of evolution and are not mere
vestiges. They have a purpose separate from those of the mature
leaves of the plant: although they too participate somewhat in
photosynthesis, they are in reality a part of the seed mass and store
starch and other materials to assist in getting the plant growth
started. Making comparisons between cotyledons and mature leaves
would not be beneficial; they are separate organs with separate jobs.

I believe it is a question of whether genetic-mess (like gill slits in humans) or
whether these first leaves confer some survival advantage on the seed growing
to maturity.

My guess is that it is "genetic mess of the past". And that if the plant had a choice of having its
first leaves such as a pear tree, that the plant would have
all of its leaves of one type. That the true pear leaf is superior in every way
such as photosynthesis ability than the cotyledon first leaf.

The reason's cotyledons may not appear in certain plants is because
other seed mass may be sufficient for those plants in the environments
in which they have adapted. Other plants rely heavily on the
cotyledons for their immediate readiness to participate in assisting
the plant's early growth, and photosynthesis, transpiration, and other
tasks normally assigned to leaves may be done by the cotyledons after
first emergence from the soil.

So, basically, what I am trying to show is that the cotyledons should
be seen more as separate organs than as early developmental forms.
They have evolved quite alot and take many forms, and I do not believe
it would be proper either genetically or by form to characterize them
as evolutionary vestiges.

But I do enjoy the question. It is certainly a valid line of
reasoning, and fortunately there is alot known about these fascinating
little cotyledons to provide a fairly good answer. Unfortunately, I
think the answer is most likely negative...