There are actually three questions here, all dealing with Rhynia (Rhynia is known by two names, the diploid life cycle "Rhynia major", and the haploid life cycle "Rhynia gwynne-vaughanii"). I thought it would be a good idea to put these 3 questions together because they all deal with the same general ideas.
1) HOW HAS THE FERN IMPROVED OVER RHYNIA RELATIVE TO THE WATER-RELATED PROBLEMS ASSOCIATED WITH MOVING TO THE LAND?
2) BRIEFLY DISCUSS HOW RHYNIA SOLVED 7 OF THE FOLLOWING PROBLEMS THAT LAND PLANTS FACE BY LIVING ON LAND.
a. Get water - accomplished by roots. Hardest part of move from water to land habitat was that organisms had to live in a medium that has more than one state (like wind, rain, dark, different temperatures). Specialization is the solution. Species look different because they learned how to specialize differently
b. Move water - accomplished by xylem; transports water.
c. Retain water - accomplished by cuticle on leaf; keeps water in.
d. Nutrients - accomplished by roots
e. Maximizing light - accomplished by leaves and growing tall to get to sun
f. Gas exchange - accomplished by stomates
g. Competition - plants defended themselves with thorns and toxins
h. Support - accomplished by xylem and roots. Unlike in a water environment, plants need support in gas environment
j. Guarantee next generation - accomplished by having lots of sperm and pollen
k. Maximize variation - accomplished by outbreeding
l. Need water for sex - we will talk about this later
m. Conserve energy - accomplished by reducing energy during life cycle
Thanks for your help!
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
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. An
s series of such axes on one plant is a telome truss. A v very brief description of the six
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 o there
half, thus producing differential growth, resulting in overtopping of one side of an axis by
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
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' consistently 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.
This solution addresses problems related to land plants, the Telome Theory and Rhynia.