How Well Understood Are the Processes that Create Dendroclimatic Records? A Mechanistic Model of the Climatic Control on Conifer Tree-Ring Growth Dynamics - Dendroclimatology - page 49

cell in the file. The simplest explanation for the IAA pattern is that the expanding
cells that have left the cambial zone use up the amount of IAA produced within the
cambial zone by the dividing cells. This implies that IAA mediates the positioning
signal from the cambial zone to the enlargement zone. IAA is also under the higher-
level control of the shoot (and root) meristems, and so it can be the mediator of
the external control of cell growth within the cambial and enlargement zones. The
main differences between earlywood and latewood formation are related to seasonal
changes in illumination, temperature, and water supply, so changes in hormone
gradients between the early and later parts of the growing season indirectly show
the results of environmental control of wood formation, although the main patterns
remain largely similar through the season (Uggla et al. 2001 , Fig. 5). Quantitative
variations are primarily related to the width of the cambial zone and the position
of cells. This relationship indirectly supports the hypothesis defining xylem forma-
tion as a partially independent system after the external signal from higher levels of
regulation has been accepted.
3.4 Cell Wall Thickening
The last stage of differentiation of the xylem elements that form the water-lifting
system of the plant is characterized by completion of a rigid secondary wall with
the consequent autolysis of the protoplasm. The secondary wall contains cellulose
microfibrils, xylan, protein, and lignin. They provide a strictly ordered structure
on the exterior of the cell membrane. The three main layers in the secondary wall
are distinguished by the orientation of cellulose microfibrils (Preston 1974 ) . First
there is S1, then a main, much thicker layer, S2, in which cellulose microfibrils
are oriented along the axis of the cell and will frequently display spiral structures.
Then comes S3, a layer that is absent in compression wood. The completion of
the secondary wall involves a complex of intracellular processes and systems: the
endomembrane system for transport, specialization of certain areas of the cyto-
plasmic diaphragm and elements of the cytoskeleton, expression of new genes,
activation of numerous enzymes, biophysical processes connected to between-cell
gradients, and properties of membranes and organization of the cell wall (Catesson
1994 ; Demura and Fukuda 1994 ; Fukuda 1994 ; Savidge 1996 ) .
The completion of the secondary wall with its consequent lignification can be
considered as the final stage in the biogenesis of the cell wall, which happens con-
tinuously during the closing stage of tracheid differentiation. Actually, the tangential
and radial walls of cambial cells represent two levels of the process of maturation
of primary cell walls (Catesson 1990 , 1994 ) . Tangential walls have a more rigid
polysaccharide matrix in comparison with radial walls, the chemical composition
and ultrastructure of which arise from the mechanical properties radial walls need
for subsequent radial growth (Roland 1978 ; Catesson and Roland 1981 ) . The ini-
tial heterogeneity of cambial cell walls disappears during the first stage of tracheid
maturation, when the radial growth of cells is completed. It is supposed that the
ratio of synthetic rates of different types of polysaccharide and their selective inclu-
sion in radial and tangential walls predetermines the fate of the cells (Catesson