7.1.4 Meristems responding to their environment

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Success of plants in colonising terrestrial environments is achieved partly by elaborating extensive structures to acquire and concentrate inorganic nutrients. In shoots this involves a rigid scaffolding of branches and stems on which leaves are deployed to optimise light interception and CO2 assimilation. Roots explore the soil systematically to extract scarce nutrients and water. Calculations on rye roots revealed these astonishing statistics: a mature plant had 600 km of root length, 650 m2 of root surface area and 13 million root tips (Dittmer 1937). A large proportion of the mass of a mature plant, especially in perennials, is committed to structural roles, leaving localised zones of meristematic activity to generate new structures. Strategically located meristems, such as the vascular cambium of trees, adjacent to supporting structures facilitate efficient growth, transport and cell specialisation. The indeterminate nature of plant meristems confers the capacity to grow continuously and to adapt to changes in the environment. For example, exaggerated stem elongation in response to low light levels enables lower storey climbers to intercept enough light to sustain growth. The response of meristems to low light levels is accompanied by production of shade leaves by plants with dense canopies or growing in shaded locations. This alternative leaf morphology harvests light efficiently because of changes in cell organisation initiated at the meristem. For example, in eucalypts, thin leaves from shaded regions in the tree canopy have fewer layers of mesophyll cells and shorter, more loosely packed palisade cells. Together with increases in chlorophyll concentrations and other changes to the photo-synthetic apparatus, shade leaves allow plants to exploit low light environments.

Similarly, indeterminate growth of root apices allows exploitation of soil resources such as immobile phosphate residues, generating an extensive network of lateral roots around the inorganic resource. Typical examples of local nutrient enrichment arise from ungerminated seed, decaying fauna and superphosphate granules. Receding water tables can also be tapped through preferential root growth, reflecting again the ability of roots to respond morphogenetically to their environment.