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THE PIPE MODEL THEORY: BORING TRUTH and A FRUITFUL MISTAKE?
I red your articles, obtained via FTP 192.112.26.10., with interest.
My work in plant physiology started in 1956 with an investigation of
the properties of sun and shade leaves. The assumption was that
photosynthates were responsible for their development. A very
troublesome problem was the large variability in the growth of the
plants, notwithstanding accurate control of all the important
environmental conditions. It turned out that this variability was due
to an insufficient root environment. Variability was greatly
diminished by improving this. Now it became possible to establish that
just the opposite was true: leaf thickness was responsible for the
photosynthetic properties of the leaf. Leaf thickness was a growth
phenomenon, elicited by irradiance and not by photosynthesis.
Therefore, in 1970, I changed over from photosynthesis research
to growth research. As with sun- and shade leaves, the generally
accepted theories about growth did not fit my results. This meant that
I started at level 0 ! Now I have a theory about the growth of leaves
and stem in relation to irradiance, day length, temperature and NO3-
dosage. This theory fits with all my relevant data since 1970.
The BORING 'TRUTH' is that such a long time was needed to
establish that the structure and thus also the morphogenesis of a
plant is relatively simple.
The vascular cylinder consists of a number (2, 3, 5, 8, 13, etc.)
of sympodia. Each sympodium is build up out of 'morphogenetic' units.
The units develop in a two-step process (P.R.Larson). The development
of each unit starts as a procambial strand branching off from a
preceding trace, which feeds the youngest visibly initiated primordium
in the sympodium. This proces takes place in the upper 1 mm of the
apex. The procambial strand develops acropetally through the apical
meristem, initiates a new primordium and develops further in this new
primordium. Ten days after the initiation of the new primordium a
second process starts: a subsidiary vascular system is formed
acropetally into the leaf and basipetally into the stem. In this way
each individual leaf is connected to the soil by a PIPE, with an area
related to the length and thickness of the leaf. Consequently, the
morphogenetic unit can be defined as a leaf with its individual
primary vascular system (passing through 2 or 3, 5, 8, 13, etc.
internodes) and its individual secundary vascular system passing from
the leaf to the soil. A tree with its branches is the sum of all the
present and lost morphogenetic units.
The morphogenetic unit develops according to a pattern, nearly
independent of external conditions; at higher temperature the process
proceeds faster, but lasts shorter. There are only a few ways to adapt
to external conditions. It is clear that 'Evolution' sorted out those
species wich were able to support the crown and to conduct enough
materials. The DEVELOPMENTAL PATTERN is the crucial point.
The combination of my quantitative growth analysis (also on the
anatomical level) and the splendid research of P.R.Larson explains the
bold pipe model theory of Shinozaki.
Shinozaki's theory, however, is the analysis of a stand with the
statistically determined loss of branches and leaves. The area of the
pipes is not a physiological-anatomical property, but a statistical
average. The FRUITFUL MISTAKE is that it is not a plant physiological
analysis. In winter deciduous trees lose their leaves and ridicule the
pipe-model theory. There must be a relation between the area of wood
below the crown and the number and size of buds, breaking in the next
summer season.
As you do I also admire the analysis of Shinozaki c.s.
Dr G.A.Pieters
Department of Plant Physiology
Agricultural University Wageningen
Arboretumlaan 4
6703 BD WAGENINGEN
The Netherlands
Tel: 31 8370 82806
fax: 31 8370 84740
BITNET PLAFYS@RCL.WAU.NL
Albert Pieters@algem@PF.WAU
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