Axillary Meristems and the Development of Epicormic Buds in Wollemi Pine (Wollemia nobilis)

Down in the wood something stirs

It is not surprising that when we travel from continent to continent we will encounter very different floras. However, one feature that may surprise us is the different architecture of particular groups of plants. Thus, I was amazed when I stepped off a plane in Australia for the first time, at the different shapes of many of the trees, compared with those common in Europe and North America. Burrows et al., from Wagga Wagga and Mount Annan, NSW, Australia (pp. 835-844) remind us that 23 different architectural types have been reported amongst woody plants. They work specifically with Wollemia nobilis, a member of the Araucariaceae that exhibits atypical architecture. Most members of this family possess a single trunk with plagiotropic branches. If the main trunk is badly damaged it can be replaced by a bud outgrowth from lower down. However, W. nobilis exhibits both the formation of new branches from the base (coppicing) and from pre-existing stems and branches in the absence of damage, giving it a very different appearance from other species in the family. The authors have thus investigated the distribution of axillary meristems from top to bottom of small (2-m high) W. nobilis trees. Near the stem apex, the meristems are identified as clusters of dividing or potentially dividing cells but there is no differentiation of bud primordia or of vascular connections. However, the further down the stem the meristems are situated, the more they are differentiated, even though they are buried deeper beneath the bark. Thus, the meristems are not only maintained in the older parts of the stem but they are more and more able to grow into shoots, thus enabling the formation of new branches. The interesting question now is what stimulates the outgrowth of these branches. Is it just reduced apical dominance or is there also a specific signal?