Laterals

As discussed in relation to shoots and roots, the distinction between axes and laterals is, in evolutionary terms, rather arbitrary. Beerling (2005) has reviewed the evidence that the morphogenetic capacity for leaf production (and, one may suppose, foliar senescence) was present in the earliest vascular land plants but megaphylls did not appear for 40 to 50 million years. It is suggested that the decisive factor in relieving this constraint was the fall in the concentration of atmospheric CO2 during the late Palaeozoic. In the tradition of Goethe and Troll (Kaplan 2001), Agnes Arber (1950) wrote “the leaf is a partial-shoot, revealing an inherent urge towards becoming a whole shoot, but never actually attaining this goal, since radial symmetry and the capacity for apical growth suffer inhibition”. Although Arber’s later writings on plant morphogenesis were philosophical and even mystical in origin, her insights are in tune with modern ideas of developmental genetics (Kirchoff 2001).

Plant architecture is dependent to a large extent on the pattern of growth, rest and death of lateral and apical buds. For example differential growth and abortion of buds accounts for sympodial and monopodial modes of branching. The genes determining plant architecture are highly conserved across the range of angiosperms (Wang and Li 2006) and are likely to be ancient, contributing to the evolution of three-dimensional branched forms from the thalloid construction of the earliest land plants (Hagemann 1999).

References

• Arber A (1950) The natural philosophy of plant form. Cambridge University Press.
• Barthélémy D, Caraglio Y (2007) Plant architecture: a dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny. Annals of Botany 99: 375-407.
• Beerling DJ (2005) Leaf evolution: gases, genes and geochemistry. Annals of Botany 96: 345-352.
• Hagemann W (1999) Towards an organismic concept of land plants: the marginal blastozone and the development of the vegetation body of selected frondose gametophytes of liverworts and ferns. Plant Systematics and Evolution 216: 81-133.
• Kaplan DR (2001) The science of plant morphology: definition, history, and role in modern biology. American Journal of Botany 88: 1711-1741.
• Kirchoff BK (2001) From Agnes Arber to new explanatory models for vascular plant development. Annals of Botany 88: 1103-1104.
• Wang Y, Li J (2006) Genes controlling plant architecture. Current Opinion in Biotechnology 17: 123-129.

Links and further reading

• A very interesting Master's thesis by K Roskoski, University of Minnesota (2005) describes the modelling of leaf, bud, and branch survival in Populus using the ECOSYS computer simulation of tree growth.