The shoot then lignifies, precluding its further elongation. They will mature in the next growing season as the bud elongates into a shoot. Most will become embryonic leaves or scales which will overwinter, in many species of temperate regions, within a bud. New primordia form successively above the last initiated, in the phyllotaxy characteristic of the taxon. Primordia are initiated by local cell division and enlargement on the shoot apical meristem (Wardlaw 1968). Here, we focus on the shoot apical meristems that form subsequently and that comprise the expanding, then declining, population of apical meristems active during a tree’s lifetime. Stem units are anatomically associated with leaves and provide the organic connection between leaves and the stems bearing them.Ī tree’s first shoot apical meristem is that of its embryo, and its first leaf primordia become the cotyledons (Foster and Gifford 1974). The concept of leaf primordium initiation as a driving force for plant growth has been raised with regard to wheat (Triticum aestivum L., Hay and Kemp 1990) but appears to be a novel explanation for a complex woody plant.įormation of stem units following primordium initiation Precise applicability to other taxa remains to be determined. Much of the supporting evidence cited below originated from studies in Pinus because pines are the most studied of trees. Even determinate processes that are predictable in their outcome are influenced by environmental variation, historical events in a tree’s long developmental time frame, and the unique genetic legacies of each taxon. The determinate nature of this synthesis does not exclude familiar sources of variability. Once begun, those growth activities proceed to completion, unless prevented by disturbance or resource depletion. This may occur immediately or after a period of dormancy. I propose here that the initiation of leaf primordia on shoot apical meristems that sets in motion the annual suite of tree growth activities. How then are primary and secondary growth initiated and controlled to yield the uniform product of each year’s growth cycle? What provides the stimulus that drives the growth of apical and lateral meristems when internal and external conditions are conducive to biological activity? Moreover, it replicates the distinctive species phenotype, which results from the interplay of genome, and environment broadly construed. It also allows the tree to achieve a size and physiological stage of maturity that supports sexual reproduction. This deposition of new growth keeps the tree alive, as no tree can persist without renewing its tissues. 3).Įach year, a tree makes itself anew by adding modules of primary and secondary growth to the surface of the previous year’s crown, stem, and roots. I have become increasingly convinced that this cycle is an entity so well integrated that it seems hardly possible to investigate or survey any of its parts successfully if they are separated from the whole (Sarvas 1974, p.
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