What triggers cambium differentiation?
The second hypothesis of the growth layer width being determined by soil water salinity was only partially accepted. The similar width of the growth layers of different Avicennia species together with the influence of radial growth on the formation of a new growth layer is in favour of a mainly endogenous control of cambium differentiation. An additional external influence of tree height and soil water salinity was recorded. Also, in other species, stem radius and number of growth layers are highly related (Ogden, 1981; Ruthrof et al., 2003), but in A. marina the relationship applies even along the different radii of a single asymmetric stem disc. Moreover, the radial increment of 2005–2006 was only slightly related to the width of the corresponding growth layers as compared with trees with one vascular cambium. A strictly controlled growth layer formation results in a regular dispersion of the phloem tissue over the entire stem disc. Within one growth layer, the ratio of xylem to phlo
