The margins of foliose lichen thalli comprise individual lobes which grow radially and divide. This results in a complex marginal structure in which lobes differing in morphology, state of division, and growth pattern are crowded together. This article reviews various aspects of the biology of these lobes including their carbohydrate supply, morphology, pattern of division and branching, the effect of lobe overcrowding and interactions between neighbouring lobes. The objective was to determine the extent to which processes affecting individual lobes determine the growth of the thallus as a whole. As the thallus grows lobes become increasingly crowded together and this restricts further lateral growth. Restriction of lobe width may be responsible for the changes in radial growth rate (RGR) with size observed in foliose species. Various aspects of the biology of lobes may be responsible for lobe growth variation including the physical independence of lobes from their neighbours, the genetic origin of the lobes, and the pattern of lobe branching. In addition, overall symmetry of a thallus is maintained by a fluctuating pattern of growth of individual lobes in successive months together with competition for space at the margin.
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