Readjustments of cambial initials in Wisteria floribunda (Willd.) DC. for development of storeyed structure

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

This study aimed to test the hypothesis whether intrusive growth of initial cells is related to the increase in circumference of Robinia pseudoacacia vascular cambium—both qualitatively and quantitatively. The mode of intrusive growth of cambial initial cells was also studied. Samples collected from tree trunks were examined using series of semi-thin transverse sections. Anatomical reconstructions of radial and tangential planes of analysed fragments of cambial tissue were made. Observations and measurements have shown that the intrusive growth of R. pseudoacacia initial cells does not contribute to an increase in tangential dimension of observed tissue fragments where cell rearrangement occurs. Moreover, initially separated tangential walls of cells (between which cambial initial cell elongates intrusively) are transformed into obliquely oriented walls. These results stand in accordance with a statement that only symplastic growth of initials, not intrusive growth, is responsible for the increase in circumference in all woody plants with the continuous cambial cylinder. Moreover, we managed to capture the moment of transition of initial status from one cell to another for the first time. This phenomenon may be explained on the basis of the system of mechanical stresses operating not only in the secondary meristematic tissue but also in a whole plant organism.

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

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.

The formation of interlocked grain in African mahogany (Khaya spp.) analysed by X-ray computed microtomography

Interlocked grain occurs when the orientation of xylem fibres oscillates, alternating between left- and right-handed spirals in successive wood layers. The cellular mechanisms giving rise to interlocked grain, thought to involve the slow rotation of fusiform initials within the vascular cambium, remain unclear. We suggest that observations of wood structure at the cellular level, but over large areas, might reveal these mechanisms. We assayed timber from several commercially important tropical angiosperms from the genus Khaya (African mahogany) that exhibit interlocked grain using X-ray computed microtomography followed by orthogonal slicing and image processing in ImageJ. Reconstructed tangential longitudinal sections were processed with the ImageJ directionality plug-in to directly measure fibre orientation and showed grain deviations of more than 10° from vertical in both left- and right-handed directions. Grain changed at locally constant rates, separated by locations where the direction of grain change sharply reversed. Image thresholding and segmentation conducted on reconstructed cross sections allowed the identification of vessels and measurement of their location, with vessel orientations then calculated in Matlab and, independently, in recalculated tangential longitudinal sections with the directionality plug-in. Vessel orientations varied more than fibre orientations, and on average deviated further from vertical than fibres at the locations where the direction of grain change reversed. Moreover, the reversal location for vessels was shifted ~400 μm towards the pith compared with the fibres, despite both cell types arising from the same fusiform initials within the vascular cambium. We propose a simple model to explain these distinct grain patterns. Were an auxin signal to control both the reorientation of cambial initials, as well as coordinating the end-on-end differentiation and linkage of xylem vessel elements, then it would be possible for fibres and vessels to run at subtly different angles, and to show different grain reversal locations.

What do we know about growth of vessel elements of secondary xylem in woody plants?

Despite extensive knowledge about vessel element growth and the determination of the axial course of vessels, these processes are still not fully understood. They are usually explained as resulting primarily from hormonal regulation in stems. This review focuses on an increasingly discussed aspect - mechanical conditions in the vascular cambium. Mechanical conditions in cambial tissue are important for the growth of vessel elements, as well as other cambial derivatives. In relation to the type of stress acting on cambial cells (compressive versus tensile stress) we: (i) discuss the shape of the enlarging vessel elements observed in anatomical sections; (ii) present hypotheses regarding the location of intrusive growth of vessel elements and cambial initials; (iii) explain the relationship between the growth of vessel elements and fibres; and (iv) consider the effect of mechanical stress in determining the course of a vessel. We also highlight the relationship between mechanical stress and transport of the most extensively studied plant hormone - auxin. We conclude that the integration of a biomechanical factor with the commonly acknowledged hormonal regulation could significantly enhance the analysis of the formation of vessel elements as well as entire vessels, which transport water and minerals in numerous plant species.

Revisiting the anatomy of the monocot cambium, a novel meristem

The monocot cambium is semi-storied, and its cells do not undergo rearrangement. The monocot cambium is a lateral meristem responsible for secondary growth in some monocotyledons of Asparagales. It is an unusual meristem, not homologous with the vascular cambia of gymnosperms and non-monocotyledonous angiosperms. Owing to the limited information available on the characteristics of this meristem, the aim of this study was to survey the structure of the monocot cambium in order to clarify the similarities and dissimilarities of this lateral meristem to the vascular cambium of trees. Using the serial sectioning analysis, we have studied the monocot cambium of three species of arborescent monocotyledons, i.e., Quiver Tree Aloe dichotoma, Dragon Tree Dracaena draco, and Joshua Tree Yucca brevifolia, native to different parts of the world. Data showed that in contrast to the vascular cambium, the monocot cambium is composed of a single type of short initials that vary in shape, and in tangential view display a semi-storied pattern. Furthermore, the cells of the monocot cambium do not undergo rearrangement. The criteria used in identifying monocot cambium initial cell are also discussed.

Mathematical modeling of intrusive growth of fusiform initials in relation to radial growth and expanding cambial circumference in Pinus sylvestris L

This study on the cambium of Pinus sylvestris L. examines the intrusive growth of fusiform cambial initials and its possible contribution to the tangential and radial expansions of the cambial cylinder. The location and extent of intrusive growth of the fusiform initials were determined by microscopic observations and by mathematical modeling. In order to meet the required circumferential expansion of the cambial cylinder, the fusiform initials grow in groups by means of a symplastic rather than intrusive growth, leaving no room for the assumption that intrusive growth of the initials takes place between radial walls and has a direct role in the increase of the cambial circumference. Therefore, it is postulated that the fusiform initials grow intrusively between the tangential walls of the neighboring initials and their immediate derivatives and not between the radial walls of the adjacent initials as per common belief.