7.1.3 Meristems as templates for morphogenesis

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The location and activity of individual meristems give rise to the diverse morphologies we recognise within the Plant Kingdom. Palms and grass trees have a distinctive morpho-genesis with the entire shoot canopy produced from the activity of a single apical meristem. Removal of the crown of a coconut palm inevitably kills the whole plant. The roots of palms and grass trees are also extraordinary in that they grow and senesce in a seasonal pattern which confers tolerance to poor soils and fire.

In contrast, woody trees produce complex shoot morphologies through combined activity of terminal and lateral apices. We see the product in the height and diverse branching pattern of large trees. Australian eucalypts show a diversity of shoot forms, ranging from the single slender trunk of a mountain ash or karri, topped by a branched canopy, to the multiple trunks of mallee eucalypts. The branching form of mallee species is determined by simultaneous activity of many apical meristems. Similarly, excavation of roots of large trees has often revealed complex branching patterns which enable effective exploration of large volumes of soil and extraction of water and nutrients. In the case of Eucalyptus marginata (jarrah; see Figure 3.3), a root system arises from strong meristematic activity in the surface levels of the root system as well as proliferation of deep sinker roots. These dual root morphologies, also found in Banksia (see Figure 3.2), are impressively adapted to the poor lateritic soils on which this species grows. Further complexity in root morphogenesis is illustrated by the proteoid (cluster) roots of members of the Proteaceae (see Section 3.1). Although the trigger for meristematic activity which leads to intensive local branching is unknown, cluster roots probably confer a competitive advantage in nutrient-poor soils, possibly through enhancing phosphate acquisition. Grasses have a distinctive morphology which arises from the local activity of intercalary meristems. These meristems give rise to semi-autonomous plants called tillers which comprise leaves, stems and reproductive parts and are subtended by nodal roots. Cell divisions within the intercalary meristem are developmentally responsible for the characteristic morphology of grasses, a family that is well adapted to herbivory.