Geometric analysis of intrusive growth of wood fibres in Robinia pseudoacacia

ABSTRACTAll cell types of the secondary xylem arise from the meristematic cells (initials) of the vascular cambium and grow under mechanical constraints emerging from the circular-symmetrical geometry that characterises many tree trunks. The course of intrusive growth of cambial initials has been elucidated, but is yet to be described in the case of xylem fibres. This study explains the geometry of intrusive growth of the secondary xylem fibres in the trunk ofRobinia pseudoacacia.Long series of serial semi-thin sections of the vascular cambium and the differentiating secondary xylem were analysed. Since fibres grow in close vicinity to expanding cells of the derivatives of the vascular cambium, we assumed that they have similar growth conditions. Dealing with the cylindrical tissue of the vascular cambium in a previous study, we used a circularly symmetrical equation for describing the growth mechanism of cambial initials. Like the cambial initials, some of the cambial derivatives differentiating into the various cell types composing the secondary xylem also exhibit intrusive growth between the tangential walls of adjacent cells. As seen in cross sections of the cambium, intrusively growing initials form slanted walls by a gradual transformation of tangential (periclinal) walls into radial (anticlinal) walls. Similarly, the intrusive growth of xylem fibres manifests initially as slants, which are formed due to axial growth of the growing cell tips along the tangential walls of adjacent cells. During this process, the tangential walls of adjacent cells are partly separated and dislocated from the tangential plane. The final shape of xylem fibres, or that of vessel elements and axial parenchyma cells, depends upon the ratio of their intrusiveversussymplastic growths in the axial, circumferential and radial directions.

Growth activity of fusiform initials in storeyed cambium

The intrusive growth of fusiform initials was studied in <i>Entandrophragma</i>, This growth makes possible a change in the position of the cell ends of one storey in respect to those in the neighbouring storey, and this is 'turn leads to changes in the orientation of cambial initials. The growth activities of the oppositely directed ends of the same initial are not the same. Not all cell ends are simultaneously actiye. There are groups of ends distributed alternately in a storey which are active or inactive, so that a pattern of growth activity appears in the cambium. The activity of a particular cell end changes in successive time periods. Owing to this, the rate of creeping of one end past. those of the adjoining storey varies. This phenomenon of changes in the activity of cell ends in time may be referred to the phenomenon of the appearance of the growth activity pattern on the cambium surface under the hypothesis of transverse shifting of the elements of this pattern ("active" and. "inactive" groups) in relation to the cambial initials.

Mechanism of changes in grain inclination in wood produced by storeyed cambium

The changes in cell orientation in the cambium of <i>Entandrophragma</i> producing wood with interlocked type of grain, and in the cambium of <i>Tilia</i> in a spirally girdled stem are traced through serial tangential sections of wood. In <i>Entandrophragma</i> the changes result from the intrusive growth of a fusiform cell whih repeatedly produces a new pointed tip from one side of the existing tip which disappears. This causes a sort of creeping of cell ends of one storey past those of the adjoining storey. The oppositely directed ends of the cells belonging to one storey creep in opposite directions so that the position of the cell centres remains constant and only the angle between the cells and the stem axis changes. The stratification of short rays in <i>Entandrophragma</i> represents an adaptation to the changes in celi orientation involved in the formation of interlocked grain. The mechanism of changes in grain inclination in <i>Tilia</i> is intermediate between that based on the creeping of cell ends and that based on pseudotransverse division and intrusive elongation which is known in non-storeyed cambia.

Cell events i n cambium, connected with the formation and existence of a whirled cell arrangement

The history of cambium development was reconstructed on the basis of serial tangential sections of a woody tumour of <i>Picea excelsa</i> (Lam.) Lk. On the area with whirled arragement of the tracheids intrusive growth was found to occur at the lateral radial and longitudinal edges apart from its normal occurrence at the radial apical edges. Intrusive .growth at the radial lateral edge leads to forking of the fusiform initial in the cambium plane. The growth which appears on longitudinal lateral edges oocurs between the tangential walls of cells in neighbouring storeys. In such places a drastic reconstruction of the cell arrangement in neighbouring annual rings was observed. Intrusive growth. between the tangential walls leads to a whirled arrangement of tracheids.

Upward movement of the domain pattern in the cambium producing wavy grain in Picea excelsa

The cambium which produces wavy-grained xylem in spruce differs from normal cambium by a higher frequency of oblique anticlinal divisions and a higher rate of intrusive growth of fusiform initials. Since the orientation, either to the left or to the right, of the divisions and the overlaps achieved by the growting tips is uniform within the areas called domains, the domain pattern of the cambium is reflected in the pattern of grain undulations in the xy1em. The domain pattern moves longitudinally about 0.7 mm during the production of l mm of xylem. A visible expression of the movement is the obliquity of undulation lines on the radial face of the wavy xylem.