The cambial zone includes a tier of meristematic cells between the developing wood (xylem) towards the inside of the stem and bark (phloem) tissue towards the outside (Figure 7.19). Cell divisions in this region either occur in parallel to the stem axis (periclinal) adding cells to a radial file of cells, or perpendicular to the stem axis (anticlinal) adding new cells to the initiating layer and thereby allowing for the establishment of new radial files. The cambium proper is formed by a layer or layers of initiating cells (cambial initials) that undergo mostly periclinal, but also anticlinal division to give rise to radially aligned files of secondary tissue. These initials are enclosed on both radial walls by xylem and phloem mother cells, respectively. Mother cells have a higher differentiation state than initials as they are destined to become either xylem or phloem elements, whereas true cambial initials retain the ability to produce both. Xylem and phloem mother cell divisions make up the majority of periclinal divisions in this zone and cambial initials replenish mother cells when those are lost from the cambial zone. Most anticlinal divisions are contributed by cambial initials resulting in the formation of new radial files and supporting the increase in girth of the growing tree stem.
Meristematic cells in the cambium include fusiform initials and ray cell initials, which are responsible for the formation of specific cell types in both xylem and phloem tissues (Figure 7.19b). Fusiform initials give rise to longitudinally aligned cells including vessels, tracheids and fibres towards the inside of the stem (in gymnosperms only tracheids are formed) and sieve tube elements towards the outside. They are large (compared to ray initials), longitudinally elongated and highly vacuolated. Ray initials on the other hand give rise to transversely aligned cells, primarily ray parenchyma cells, and occur as scattered aggregations within the cambial zone. The ratio of fusiform to ray initials can vary within the cambial zone depending on plant species, age, environmental factors and growth rates. For example, in angiosperms, fusiform initials on average make up 60-90% of the initials within the cambium. While in some species the cambium is comprised entirely of fusiform initials in others they make up as little as 25%.
Fusiform initials in the cambium occur either in a storied arrangement, where cells are laterally aligned, or non-storied, where they are arranged irregularly and often overlap. Divisions in a storied cambium generally occur synchronously resulting in lateral alignment and are typical of some small woody plant species, whereas divisions in a non-storied cambium occur asynchronously and are typical in trees. A storied cambium is believed to represent a more advanced evolutionary arrangement. Fusiform cell derivatives undergo variable amounts of cell expansion during differentiation with radial width often doubling and longitudinal length remaining more or less similar except for some longitudinal extension at the tip (expansion zone in Figure 7.19b). Longitudinal extension, particularly in xylary elements, occurs in the form of intrusive (or sliding growth) which involves the extension of tapered end walls of derivatives into the available space around them. Consequently, the end walls intertwine with end walls of other cells forming strong bonds that increase the mechanical strength of the stem.
The arrangement of rays in angiosperms is highly variable and can be uniseriate (1 cell wide), biseriate (2 cells wide) or multiseriate (3 or more cells wide) while in gymnosperms rays are exclusively uniseriate. Ray initials do not undergo much elongation during differentiation and have been observed to unite frequently, both vertically and laterally to form aggregate rays as well as to split by cell invasion of fusiform initials. The size of ray initials tends to remain constant throughout the life of the plant but their derivatives show a gradual increase in size with age and distance from the pith. Trans-differentiation from a fusiform initial to ray initials can occur in the cambium to maintain a balance between derivatives. This takes place either by segmentation or by progressive shortening of a fusiform initial, whilst trans-differentiation from a ray to a fusiform initial involves elongation via intrusive growth. The mechanisms that drive this process are largely unknown but proximity to other ray initials as well as the effects of plant hormones such as auxin and ethylene have been suggested.