Environmental Control of Xylem Differentiation

6.6 Conclusions and Discussion

A conceptual scheme of the environmental control of xylem formation has been outlined. This has been based on the findings reported in Chaps. 3–5, which point to the the cambial zone as the target of environmental control, and the strong relationships between cell production, expansion, and cell wall thickening.

The basic conceptual scheme of the environmental control of xylem differentiation should have certain characteristics. These include an ability to treat the activities occurring in the cambial zone as the targets of environmental control, albeit within a hierarchical structure. In this structure, signals from a high level, for example the whole tree or the canopy, control the operation of key processes at lower levels, for example the tissues. In this case, we propose that the key process is cell production in the cambial zone. It, in turn, controls the outcome of other processes taking place at the tissue level, for example the extent of cell expansion and cell maturation.

Several examples from very different regions, from the forest-tundra in the North, through the steppe zone of southern Siberia and the semi-arid American southwest to monsoon Asia have been examined in the light of this scheme. In each case, as well as for a number of statistical studies of much wider geographical extent, it seems useful to think in the terms we have described. In particular, it is possible to explain observed patterns of cell dimensions within and between growing seasons rather simply, if one assumes the primacy of the environmental control of cell production over the final products of expansion and maturation.