Axes

In evolutionary terms, the distinction between laterals and axes is an arbitrary one since, according to current thinking, leaves are derived from megaphylls which in turn evolved from the three-dimensional lateral branches of early vascular land plants (Beerling and Fleming 2007). It is not surprising, therefore, that common mechanisms are recognisable in the senescence patterns of different morphological elements of angiosperms. Furthermore, distinctions between laterals and axes are even more artificial in cases of morphological analogues, such as the leaf-like cladodes of species such as Opuntia and Ruscus.

The root axis is simpler in anatomy and morphology than the shoot, but roots carry out a wider range of functions than stems, including anchorage, water and nutrient acquisition, storage and perennation and a range of biotic interactions, for example with mycorrhiza and rhizobia. According to Raven and Edwards (2001) identifiable rhizoids evolved in the sporophytes of at least two distinct lineages of vascular plants in the Early Devonian era (c. 400 million years ago). The totipotency and morphogenetic plasticity of plants allows roots to redifferentiate into shoots and vice versa (Ozawa et al. 1998), indicating that the potential for expressing the mode of senescence of both kinds of axis is a systemic property of each structure.

References

• Beerling DJ, Fleming AJ (2007) Zimmermann's telome theory of megaphyll leaf evolution: a molecular and cellular critique. Current Opinion in Plant Biology 10: 4-12.
• Ozawa S, Yasutani I, Fukuda H, Komamine A, Sugiyama M (1998) Organogenic responses in tissue culture of srd mutants of Arabidopsis thaliana. Development 125: 135-142.
• Raven JA, Edwards D (2001) Roots: evolutionary origins and biogeochemical significance . Journal of Experimental Botany 52: 381-401.

Links and further reading

• All about types of shoots and roots on the very useful UCLA General Botany website.
• Friedman WE, Moore RC, Purugganan MD (2004) The evolution of plant development. American Journal of Botany 91: 1726-1741.
• Sugiyama M (1999) Organogenesis in vitro. Current Opinion in Plant Biology 2: 61-64.