Fig. 6

A perspective proposed by Tomescu and Groover [45] (top panel) regards vascular cambial growth as a complex modular developmental feature that is the sum of multiple component processes, each controlled by an independent regulatory module. In this perspective, component processes are deployed in a mosaic pattern among plant lineages, and their different combinations result in as many distinct modes of secondary growth. If each component process leaves a structural fingerprint in the anatomy of secondary tissues, the combinations of component processes can be inferred for the modes of secondary growth observed in the fossil record. This perspective allows for a basic set of component processes that could have defined a hypothetical single common origin of secondary growth across tracheophytes (or across euphyllophytes), underpinned by a basic toolkit of corresponding regulatory modules representing a deep homology (sensu Shubin et al. [99]) in the clade. In the traditional perspective on secondary growth (bottom panel), the implicit assumption was that of vascular cambial growth as a unitary developmental feature that was assembled de novo in each taxonomic group that evolved secondary growth independently and in parallel with other groups