Problems of Land Plants, Telome Theory, and Rhynia

THE TELOME CONCEPT

When we speak of evolution occurring within a group of plants, we
usually imagine a mutation producing a new structure and that structure
being selected for within the population. It is easy to believe that the
mutation affected a gene on a chromosome and that "some way or the
other- that genetic change got expressed into a different, observable.
character. In reality, however, plant structures are produced by cell
divisions and differentiation of specific cells into specific
structures. Therefore, a plant growth region (we call these meristems)
receiving a new or different set of instructions from a mutated or
rarely selected for gene, must have a mechanism for producing that new
structure, or the change will not occur or disrupt the plant to the
point of being lethal.

One scientist, a German named Walter Zimmermann, believed that most
evolution in plants was the selective and cumulative result of simple
processes which occurred normally in those plants. He described those
simple processes in 1952 and called them collectively the Telome Theory.
This theory consisted of six simple processes: Overtopping, Reduction
Planation, Webbing, Syngenesis, and Recurvation. These are illustrated
in the attached diagrams, which have been taken from Stewart (1983).
A,Telome is defined as an axis which is dichotomously branched. A series
of such axes on one plant is a telome truss. A very brief description of
the six processes follows:

1. Overtopping.... normally an apical meristem divides, into two. equal
parts (in primitive plants like Rhynia) producing dichotomous branches.
These branches, being identical halves, would normally grow equally. For
his first process, Zimmermann merely suggested that a simple hormonal
stimulant sent to one half of a newly divided apex could cause it to
divide and elongate its cells more rapidly than the other half, thus
producing differential growth, resulting in overtopping of one side of
an axis by the other.

2. Reduction if the same situation occurred as the branching given in
process one above, but instead of a.hormonal signal to increase the rate
of growth being sent, a growth block were sent to one portion of the
axis, then that side would experience slower than normal growth, or a
reduction in growth.

3. Planation The primitive plant (Rhynia -type) is known to branch in
three dimensions. That is, the plane of division of the apical meristem
is not fixed or regular. Hence you end up with a 3-D plant. Planation
suggests that there is a genetic fix which determines that all branching
divisions of the apical initial occur in one plane (at least for one
organ or for one particular time). That means that the resultant
structure is flattened in one plane or planated. This is a particularly
important step in understanding the evolution of large, flat leaves such
as those on the plane trees on the library lawn.

4. Webbing As 'studied in another part of the course, parenchyma cells
are virtually unspecialized and capable of further division and
differentiation (the classical experiments where entire carrot plants
were grown from carrot plugs with hormone added is a good example).
Hence Zimmermann used this process to suggest that axes in proximity to.
one another-could dedifferentiate parenchyma cells and with further cell
divisions "fill in the gaps" between branches, giving the appearance
that adding webbing might between digits on an animal or bird. This
process would be a logical follow-on in the evolution of a leaf from a
planated telome.

5. Syngenesis This process involves the simple failure of the
development of epidermal cells in regions of conunon contact, thus
allowing ultimate lateral fusion of similar tissues. Ziroanermann argued
that if a series of branches were formed, and did not grow apart, then
.epidermal tissue would not likely develop on the inner surfaces and
hence a multi bundled vascular system could be developed within a single
stem. This syngenesis (or fusion) of axes is of critical importance in
later understanding the evolution of the siphonostele in higher plants.

6. Recurvation Zimmermann observed that there were several important
structures in higher plants that' consistenly showed curvature of axes
or portions of them. These include the young leaves ("fiddleheads") of
ferns, the megasporangia of angiosperms and indeed the normal response
of some plants in bending or growing toward the light. It was indeed
this last phenomenon, which Zimmermann knew to be accomplished by
differential cell division and elongation rate on one side on and
branching structure, that suggested to him that such, a process could
easily be under genetic control rather than environmental. This would
result in the regular recurving of certain structures in certain plants.