Comparative Wood Anatomy

Stems that develop secondary vascular tissue (i.e. xylem and phloem derived from the vascular cambium) have unique demands on transport owing to their mass and longevity. Transport of water and assimilates must occur over long distances, while the increasing physical separation of xylem and phloem requires radial transport. Developing secondary tissue is itself a strong sink positioned between xylem and phloem along the entire length of the stem, and the integrity of these transport tissues must be maintained and protected for years if not decades. Parenchyma cells form an interconnected three-dimensional lattice throughout secondary xylem and phloem and perform critical roles in all of these tasks, yet our understanding of their physiology, the nature of their symplasmic connections, and their activity at the symplast-apoplast interface is very limited. This review highlights key historical work as well as current research on the structure and function of parenchyma in secondary vascular tissue in the hopes of spurring renewed interest in this area, which has important implications for whole-plant transport processes and resource partitioning.

Symplasmic networks in secondary vascular tissues: parenchyma distribution and activity supporting long-distance transport

Plant anatomists are generally agreed on the histological products of successive cambia: bands of secondary phloem and associated secondary xylem (“vascular increments”) embedded in a background of conjunctive tissue (parenchyma, sometimes fibers). Interpretations have varied widely on the ontogeny of this plan. Studies have usually involved one or a few species. The study of numerous taxa, especially in centrospermoid families, leads to the conclusion that there is a common plan, although variations on it are manifold. A master cambium produces secondary cortex externally and, internally, rays, conjunctive tissue, vascular cambia, secondary phloem, and secondary xylem. Secondary phloem and secondary xylem are formed from the vascular cambium in each vascular increment. Vascular cambia function indefinitely, so that a master cambium and a series of vascular cambia (each in a vascular increment) function indefinitely. The master cambium either remains active as long as an axis is actively growing ...

https://bioone.org/journals/the-journal-of-the-torrey-botanical-society/volume-134/issue-2/1095-5674(2007)134[301:SCROHD]2.0.CO;2/Successive-cambia-revisited-ontogeny-histology-diversity-and-functional-significance/10.3159/1095-5674(2007)134[301:SCROHD]2.0.CO;2.pdf

Successive cambia revisited: ontogeny, histology, diversity, and functional significance

Atriplex halimus L.: Its biology and uses

Background 
Secondary growth by successive cambia is a rare phenomenon in woody plant species. Only few plant species, within different phylogenetic clades, have secondary growth by more than one vascular cambium. Often, these successive cambia are organised concentrically. In the mangrove genus Avicennia however, the successive cambia seem to have a more complex organisation. This study aimed (i) at understanding the development of successive cambia by giving a three-dimensional description of the hydraulic architecture of Avicennia and (ii) at unveiling the possible adaptive nature of growth by successive cambia through a study of the ecological distribution of plant species with concentric internal phloem.
Results 
Avicennia had a complex network of non-cylindrical wood patches, the complexity of which increased with more stressful ecological conditions. As internal phloem has been suggested to play a role in water storage and embolism repair, the spatial organisation of Avicennia wood could provide advantages in the ecologically stressful conditions species of this mangrove genus are growing in. Furthermore, we could observe that 84.9% of the woody shrub and tree species with concentric internal phloem occurred in either dry or saline environments strengthening the hypothesis that successive cambia provide the necessary advantages for survival in harsh environmental conditions.
Conclusions 
Successive cambia are an ecologically important characteristic, which seems strongly related with water-limited environments.

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Successive cambia : a developmental oddity or an adaptive structure?

Serial sections of young and old woody stems of Atriplex halimus L. (Chenopodiaceae) were studied with the light microscope and analyzed cinematographically to search for possible connections between the cambia occurring in the primary vascular bundles and the successive cambia. The first extrafascicular cambia initiated from cambial bands which develop continuous with the still active intrafascicular cambia of the primary vascular bundles. The additional successive cambia also appear to start developing continuous with preceding still active cambia. The primary vascular bundles and the vascular strands of the secondary increments are all interconnected.

/pdf/development-of-the-successive-cambia-in-atriplex-halimus-128yvh03nv.pdf

A. Fahn

Papers

Development of the Successive Cambia in Atriplex halimus (Chenopodiaceae)