How Do Trees Grow in Girth? Controversy on the Role of Cellular Events in the Vascular Cambium

Abstract

Radial growth has long been a subject of interest in tree biology research. Recent studies have brought a significant change in the understanding of some basic processes characteristic to the vascular cambium, a meristem that produces secondary vascular tissues (phloem and xylem) in woody plants. A new hypothesis regarding the mechanism of intrusive growth of the cambial initials, which has been ratified by studies of the arrangement of cambial cells, negates the influence of this apical cell growth on the expansion of the cambial circumference. Instead, it suggests that the tip of the elongating cambial initial intrudes between the tangential (periclinal) walls, rather than the radial (anticlinal) walls, of the initial(s) and its(their) derivative(s) lying ahead of the elongating cell tip. The new concept also explains the hitherto obscure mechanism of the cell event called ‘elimination of initials’. This article evaluates these new concepts of the cambial cell dynamics and offers a new interpretation for some curious events occurring in the cambial meristem in relation to the radial growth in woody plants.

Intrusive growth of initials does not affect cambial circumference in Robinia pseudoacacia

“…In summary, our results support a view that intrusive growth of initial cells occurs between tangential cell walls of neighbouring initials and their immediate derivatives 16 , 28 . Thus, it can be stated that this type of growth is associated with the rearrangement of cambial cells 6 , 28 , and not with the increase in cambial circumference 29 .…”

“…The role of intrusive growth of initial cells, which occurs independently of periclinal divisions of the mother cells, would be universal for both types of cambia and result in a gradual rearrangement of the initial surface 22 , 24 – 26 . A current arrangement of the cambial initials is constantly recorded in the newly formed layers of cambial derivatives deposited on the xylem or phloem side 6 , 29 .…”

“…Together with the progression of the intrusive growth of an initial cell, separated tangential cell walls between which intrusive growth occurs first become obliquely and then radially oriented. At the stage of transformation of tangential walls into oblique ones characteristic slants are visible in the cross-sections of the vascular cambium 6 , 15 , 29 (Fig. 4 ).…”

“…If it was the case that intrusive growth of a cambial initial contributes to circumferential enlargement of the vascular cambium, one should be able to observe an increase in tangential dimension of rearranging cells (tangential dimension of a growing initial cell together with tangential dimension of two neighbouring radial files would be larger than tangential dimension of these two files before occurrence of intrusive growth). Such analysis is justified because the spatial arrangement of cambial derivatives constitutes a historical record of events that took place in the initial layer in the past 6 , 15 , 24 , 25 , 28 , 29 . Thus, the arrangement of cells in layers located farther from the initial layer (on the xylem or phloem side), is a record of cellular events that took place in the initial layer further in the past.…”

“…Cells deposited by cambial initials, i.e. cambial derivatives, may undergo tangential, radial, and axial growth (depending on the type of cell) and differentiation followed by the maturation process 3 – 6 . As a consequence, secondary vascular tissues are formed.…”