Modular growth and function in the vascular cryptogam Lycopodium annotinum

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Abstract
Lycopodium annotinum L. is a long-lived plant that consists of a series of annually produced segments, which can be aged; a 20-year life cycle can be synthesized from historical records of the plant's past growth. Detailed analyses of the modular construction of the plant show elements of deterministic and opportunistic growth which provide a framework in which physiological processes can be interpreted. A balance between deterministic and opportunistic growth is achieved by the relationships in terms of structure, function and longevity, between a series of horizontally growing modules and a series of more ephemeral vertically oriented modules. Each annual horizontal segment has an increasing probability of mortality as it ages, yet the segments are organized into units with indefinite apical growth and proximal senescence. In contrast, each vertical module has the inverse relationship between age and the probability of mortality: the youngest segments always die first. The result is a plant with fixed structures for photosynthesis, spore production and nutrient and water uptake, but with a highly organized and mobile transport system. This pattern of growth results in a foraging behaviour that enables the plant to harvest patchy resources but also allows opportunistic escape when adversity is encountered. During foraging and escape, the youngest horizontal modules, which have the lowest water potentials, preferentially receive water from roots attached to older horizontal modules and receive nutrients recycled from senescing vertical modules and old horizontal modules. This results in the subsidized growth of the youngest horizontal modules. When the subsidies become no longer possible, for example when the distance between the youngest horizontal module and the nearest root is too great when crossing rocks and so on, apical death occurs. This releases lateral meristems from correlative inhibition and the potential zone of exploitation by the plant is greatly increased as the new horizontal modules proliferate. This increases the chance of a favourable microsite being encountered by at least one branch or module.

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